These materials are to be used only for the purpose of individual, private study and may
not be reproduced in any form or medium, copied, stored in a retrieval system, lent, hired,
rented, transmitted, or adapted in whole or in part without the prior written consent of
Jeppesen.
Copyright in all materials bound within these covers or attached hereto, excluding that material
which is used with the permission of third parties and acknowledged as such, belongs exclusively
to Jeppesen.
Certain copyright material is reproduced with the permission of the International Civil Aviation
Organisation, the United Kingdom Civil Aviation Authority, and the Joint Aviation Authorities
(JAA).
This book has been written and published to assist students enrolled in an approved JAA Air
Transport Pilot Licence (ATPL) course in preparation for the JAA ATPL theoretical knowledge
examinations. Nothing in the content of this book is to be interpreted as constituting instruction or
advice relating to practical flying.
Whilst every effort has been made to ensure the accuracy of the information contained within this
book, neither Jeppesen nor Atlantic Flight Training gives any warranty as to its accuracy or
otherwise. Students preparing for the JAA ATPL theoretical knowledge examinations should not
regard this book as a substitute for the JAA ATPL theoretical knowledge training syllabus
published in the current edition of “JAR-FCL 1 Flight Crew Licensing (Aeroplanes)” (the Syllabus).
The Syllabus constitutes the sole authoritative definition of the subject matter to be studied in a
JAA ATPL theoretical knowledge training programme. No student should prepare for, or is
entitled to enter himself/herself for, the JAA ATPL theoretical knowledge examinations without
first being enrolled in a training school which has been granted approval by a JAA-authorised
national aviation authority to deliver JAA ATPL training.
Contact Details:
Sales and Service Department
Jeppesen GmbH
Frankfurter Strasse 233
63263 Neu-Isenburg
Germany
Tel: ++49 (0)6102 5070
E-mail: fra-services@jeppesen.com
For further information on products and services from Jeppesen, visit our web site at:
www.jeppesen.com
JA310101-000
ii
© Jeppesen Sanderson Inc., 2004
All Rights Reserved
ISBN 0-88487-350-1
Printed in Germany
PREFACE_______________________
As the world moves toward a single standard for international pilot licensing, many nations have
adopted the syllabi and regulations of the “Joint Aviation Requirements-Flight Crew Licensing"
(JAR-FCL), the licensing agency of the Joint Aviation Authorities (JAA).
Though training and licensing requirements of individual national aviation authorities are similar in
content and scope to the JAA curriculum, individuals who wish to train for JAA licences need
access to study materials which have been specifically designed to meet the requirements of the
JAA licensing system. The volumes in this series aim to cover the subject matter tested in the
JAA ATPL ground examinations as set forth in the ATPL training syllabus, contained in the JAA
publication, “JAR-FCL 1 (Aeroplanes)”.
The JAA regulations specify that all those who wish to obtain a JAA ATPL must study with a
flying training organisation (FTO) which has been granted approval by a JAA-authorised national
aviation authority to deliver JAA ATPL training. While the formal responsibility to prepare you for
both the skill tests and the ground examinations lies with the FTO, these Jeppesen manuals will
provide a comprehensive and necessary background for your formal training.
Jeppesen is acknowledged as the world's leading supplier of flight information services, and
provides a full range of print and electronic flight information services, including navigation data,
computerised flight planning, aviation software products, aviation weather services, maintenance
information, and pilot training systems and supplies. Jeppesen counts among its customer base
all US airlines and the majority of international airlines worldwide. It also serves the large general
and business aviation markets. These manuals enable you to draw on Jeppesen’s vast
experience as an acknowledged expert in the development and publication of pilot training
materials.
We at Jeppesen wish you success in your flying and training, and we are confident that your
study of these manuals will be of great value in preparing for the JAA ATPL ground examinations.
The next three pages contain a list and content description of all the volumes in the ATPL series.
iii
ATPL Series
Meteorology (JAR Ref 050)
• The Atmosphere
• Wind
• Thermodynamics
• Clouds and Fog
• Precipitation
• Air Masses and Fronts
• Pressure System
• Climatology
• Flight Hazards
• Meteorological Information
General Navigation (JAR Ref 061)
• Basics of Navigation
• Magnetism
• Compasses
• Charts
• Dead Reckoning Navigation
• In-Flight Navigation
• Inertial Navigation Systems
Radio Navigation (JAR Ref 062)
• Radio Aids
• Self-contained and
External-Referenced
Navigation Systems
• Basic Radar Principles
• Area Navigation Systems
• Basic Radio Propagation Theory
Airframes and Systems (JAR Ref 021 01)
• Fuselage
• Windows
• Wings
• Stabilising Surfaces
• Landing Gear
• Flight Controls
• Hydraulics
• Pneumatic Systems
• Air Conditioning System
• Pressurisation
• De-Ice / Anti-Ice Systems
• Fuel Systems
Powerplant (JAR Ref 021 03)
• Piston Engine
• Turbine Engine
• Engine Construction
• Engine Systems
• Auxiliary Power Unit (APU)
Electrics (JAR Ref 021 02)
• Direct Current
• Alternating Current
• Batteries
• Magnetism
iv
• Generator / Alternator
• Semiconductors
• Circuits
Instrumentation (JAR Ref 022)
• Flight Instruments
• Automatic Flight Control Systems
• Warning and Recording Equipment
• Powerplant and System Monitoring Instruments
Principles of Flight (JAR Ref 080)
• Laws and Definitions
• Aerofoil Airflow
• Aeroplane Airflow
• Lift Coefficient
• Total Drag
• Ground Effect
• Stall
• CLMAX Augmentation
• Lift Coefficient and Speed
• Boundary Layer
• High Speed Flight
• Stability
• Flying Controls
• Adverse Weather Conditions
• Propellers
• Operating Limitations
• Flight Mechanics
Performance (JAR Ref 032)
• Single-Engine Aeroplanes – Not certified under JAR/FAR 25
(Performance Class B)
• Multi-Engine Aeroplanes – Not certified under JAR/FAR 25
(Performance Class B)
• Aeroplanes certified under JAR/FAR 25 (Performance Class A)
Mass and Balance (JAR Ref 031)
• Definition and Terminology
• Limits
• Loading
• Centre of Gravity
Flight Planning (JAR Ref 033)
• Flight Plan for Cross-Country
Flights
• ICAO ATC Flight Planning
• IFR (Airways) Flight Planning
• Jeppesen Airway Manual
• Meteorological Messages
• Point of Equal Time
• Point of Safe Return
• Medium Range Jet Transport
Planning
Air Law (JAR Ref 010)
• International Agreements
and Organisations
• Annex 8 – Airworthiness of
Aircraft
• Annex 7 – Aircraft Nationality
and Registration Marks
• Annex 1 – Licensing
• Rules of the Air
• Procedures for Air Navigation
• Air Traffic Services
• Aerodromes
• Facilitation
• Search and Rescue
• Security
• Aircraft Accident Investigation
• JAR-FCL
• National Law
v
Human Performance and
Limitations (JAR Ref 040)
• Human Factors
• Aviation Physiology and Health Maintenance
• Aviation Psychology
Operational Procedures (JAR Ref 070)
• Operator
• Air Operations Certificate
• Flight Operations
• Aerodrome Operating Minima
• Low Visibility Operations
• Special Operational Procedures
and Hazards
• Transoceanic and Polar Flight
Communications (JAR Ref 090)
• Definitions
• General Operation Procedures
• Relevant Weather Information
• Communication Failure
• VHF Propagation
• Allocation of Frequencies
vi
• Distress and Urgency
Procedures
• Aerodrome Control
• Approach Control
• Area Control
Table of Contents
CHAPTER 1
The Atmosphere
Introduction ...................................................................................................................................................1-1
Definition of the Atmosphere.........................................................................................................................1-1
Properties of the Atmosphere .......................................................................................................................1-1
Composition of the Atmosphere....................................................................................................................1-1
Water (H2O) ..................................................................................................................................................1-3
The Water Cycle ...........................................................................................................................................1-3
Particles and Dust.........................................................................................................................................1-3
Carbon Dioxide (CO2) ..................................................................................................................................1-4
Structure of the Atmosphere .........................................................................................................................1-4
Troposphere..................................................................................................................................................1-4
Tropopause...................................................................................................................................................1-5
Stratosphere .................................................................................................................................................1-7
Stratopause...................................................................................................................................................1-7
Mesosphere ..................................................................................................................................................1-7
Mesopause ...................................................................................................................................................1-7
Thermosphere...............................................................................................................................................1-7
International Standard Atmosphere (ISA) .....................................................................................................1-8
ISA deviation.................................................................................................................................................1-8
Jet Standard Atmosphere (JSA) ...................................................................................................................1-9
Answers to ISA deviation questions............................................................................................................1-10
CHAPTER 2
Pressure and Pressure Systems
Introduction ...................................................................................................................................................2-1
Atmospheric Pressure...................................................................................................................................2-1
Measuring Atmospheric Pressure .................................................................................................................2-2
Mercury Barometer .......................................................................................................................................2-2
Aneroid Barometer........................................................................................................................................2-3
Units of Measurement ...................................................................................................................................2-3
Pressure Variation ........................................................................................................................................2-4
Horizontally ...................................................................................................................................................2-4
Diurnally ........................................................................................................................................................2-4
Vertically .......................................................................................................................................................2-4
The Relationship between Pressure and Temperature.................................................................................2-5
Pressure/Height Calculations........................................................................................................................2-6
Pressure Values............................................................................................................................................2-8
QFE ..............................................................................................................................................................2-8
QNH..............................................................................................................................................................2-8
QFF...............................................................................................................................................................2-8
The Standard Pressure Setting.....................................................................................................................2-8
Synoptic Charts.............................................................................................................................................2-8
Pressure Systems.........................................................................................................................................2-9
Depressions ..................................................................................................................................................2-9
Depression Weather ...................................................................................................................................2-10
Anticyclones................................................................................................................................................2-10
Troughs.......................................................................................................................................................2-12
Trough Weather ..........................................................................................................................................2-12
Ridges.........................................................................................................................................................2-13
Ridge Weather ............................................................................................................................................2-13
Cols.............................................................................................................................................................2-13
Col Weather ................................................................................................................................................2-14
Movement of Pressure Systems .................................................................................................................2-14
Meteorology
vii
Table of Contents
CHAPTER 3
Altimetry
Introduction .................................................................................................................................................. 3-1
Pressure Calculations .................................................................................................................................. 3-1
Converting between Height and Altitude ...................................................................................................... 3-2
Converting between Altitude and Pressure Altitude/Flight Level .................................................................. 3-4
Pressure Change ......................................................................................................................................... 3-5
Correcting for Temperature .......................................................................................................................... 3-6
Converting between QNH and QFF ............................................................................................................. 3-8
Mountain Flying.......................................................................................................................................... 3-10
Altimeter Settings ....................................................................................................................................... 3-10
Calculation of Minimum Usable Flight Level .............................................................................................. 3-11
CHAPTER 4
Temperature
Introduction .................................................................................................................................................. 4-1
Temperature Scales..................................................................................................................................... 4-1
Fahrenheit .................................................................................................................................................... 4-1
Celsius ......................................................................................................................................................... 4-1
Kelvin ........................................................................................................................................................... 4-1
Conversion Factors ...................................................................................................................................... 4-1
Measurement of Temperature...................................................................................................................... 4-2
Heating of the Atmosphere .......................................................................................................................... 4-3
Solar Radiation............................................................................................................................................. 4-3
Terrestrial Radiation..................................................................................................................................... 4-4
Conduction ................................................................................................................................................... 4-4
Convection ................................................................................................................................................... 4-5
Latent Heat of Condensation ....................................................................................................................... 4-5
Advection ..................................................................................................................................................... 4-5
Diurnal Variation of Temperature ................................................................................................................. 4-5
The Greenhouse Effect ................................................................................................................................ 4-7
CHAPTER 5
Water in the Atmosphere
Introduction .................................................................................................................................................. 5-1
Water States and Latent Heat ...................................................................................................................... 5-1
Evaporation .................................................................................................................................................. 5-1
Melting ......................................................................................................................................................... 5-1
Sublimation .................................................................................................................................................. 5-2
Condensation ............................................................................................................................................... 5-2
Freezing ....................................................................................................................................................... 5-2
Saturation..................................................................................................................................................... 5-2
Humidity ....................................................................................................................................................... 5-2
Absolute Humidity ........................................................................................................................................ 5-3
Saturation Content ....................................................................................................................................... 5-3
Relative Humidity ......................................................................................................................................... 5-3
Humidity Mixing Ratio .................................................................................................................................. 5-3
Super-saturation........................................................................................................................................... 5-4
Saturation and Dewpoint.............................................................................................................................. 5-4
Condensation Level ..................................................................................................................................... 5-5
Diurnal Variation of Humidity........................................................................................................................ 5-6
Water Vapour Pressure................................................................................................................................ 5-6
Saturation Vapour Pressure Curve .............................................................................................................. 5-7
Measurement of Humidity ............................................................................................................................ 5-8
Psychrometer ............................................................................................................................................... 5-8
Humidity Method .......................................................................................................................................... 5-9
Answers to Exercises................................................................................................................................. 5-10
viii
Meteorology
Table of Contents
CHAPTER 6
Density
Introduction ...................................................................................................................................................6-1
The Ideal Gas Laws ......................................................................................................................................6-1
Boyle’s Law...................................................................................................................................................6-2
Charles’s Law ...............................................................................................................................................6-2
The Gas Equation .........................................................................................................................................6-2
Effect of Water Vapour on Air Density ..........................................................................................................6-3
Variation of Surface Air Density with Latitude ...............................................................................................6-3
Variation of Air Density with Height...............................................................................................................6-3
Variation of Air Density with Latitude and Height ..........................................................................................6-4
Diurnal Variation of Density...........................................................................................................................6-5
Density Altitude .............................................................................................................................................6-5
Calculating Density Altitude ..........................................................................................................................6-6
Effect of Density on Aircraft Performance .....................................................................................................6-7
Answers to Exercises....................................................................................................................................6-8
CHAPTER 7
Stability
Introduction ...................................................................................................................................................7-1
Adiabatic Processes .....................................................................................................................................7-1
The Dry Adiabatic Lapse Rate ......................................................................................................................7-1
The Saturated Lapse Rate ............................................................................................................................7-1
The Environmental Lapse Rate.....................................................................................................................7-2
Summary of Adiabatics .................................................................................................................................7-2
Stability of the Air ..........................................................................................................................................7-2
Absolute Stability ..........................................................................................................................................7-3
Absolute Instability ........................................................................................................................................7-3
Conditional Instability ....................................................................................................................................7-3
Summary of Stability .....................................................................................................................................7-5
Neutral Stability.............................................................................................................................................7-5
Convective or Potential Instability .................................................................................................................7-6
Inversions......................................................................................................................................................7-7
Cloud Formation ...........................................................................................................................................7-8
The Dry Thermal ...........................................................................................................................................7-8
Formation of a Cloud ....................................................................................................................................7-9
Calculating Cloud Base...............................................................................................................................7-10
Forecasting Cloud Formation......................................................................................................................7-11
CHAPTER 8
Clouds
Acknowledgements.......................................................................................................................................8-1
Introduction ...................................................................................................................................................8-1
Cloud Terms .................................................................................................................................................8-1
Cloud Classification ......................................................................................................................................8-2
Layer Clouds.................................................................................................................................................8-2
Clouds of Great Vertical Extension ...............................................................................................................8-2
Low Clouds ...................................................................................................................................................8-3
Stratus, ST ....................................................................................................................................................8-3
Stratocumulus, SC ........................................................................................................................................8-4
Medium Clouds .............................................................................................................................................8-4
Altostratus, AS ..............................................................................................................................................8-4
Altocumulus Castellanus, ACC .....................................................................................................................8-5
Altocumulus Lenticularis, ACL ......................................................................................................................8-5
Meteorology
ix
Table of Contents
CHAPTER 8 (Continued)
High Clouds ................................................................................................................................................. 8-5
Cirrus, CI ...................................................................................................................................................... 8-5
Cirro-Stratus, CS.......................................................................................................................................... 8-6
Cirro-Cumulus, CC....................................................................................................................................... 8-6
Clouds with Great Vertical Development...................................................................................................... 8-7
Cumulus ....................................................................................................................................................... 8-7
Cumulonimbus ............................................................................................................................................. 8-8
Cloud Amounts............................................................................................................................................. 8-9
Cloud Base .................................................................................................................................................. 8-9
Cloud Ceiling.............................................................................................................................................. 8-10
Measuring Cloud Base............................................................................................................................... 8-10
AIREPS...................................................................................................................................................... 8-10
Human Observations ................................................................................................................................. 8-10
Balloons ..................................................................................................................................................... 8-10
Ceilometer.................................................................................................................................................. 8-10
Alidade ....................................................................................................................................................... 8-10
Vertical Visibility ......................................................................................................................................... 8-10
Summary of Cloud Type and Characteristics ............................................................................................. 8-11
CHAPTER 9
Cloud Formation
Introduction .................................................................................................................................................. 9-1
Turbulence ................................................................................................................................................... 9-1
Conditions .................................................................................................................................................... 9-1
Mechanism................................................................................................................................................... 9-2
Cloud Types ................................................................................................................................................. 9-4
Convection ................................................................................................................................................... 9-4
Conditions .................................................................................................................................................... 9-4
Mechanism................................................................................................................................................... 9-5
Advection ..................................................................................................................................................... 9-6
Cloud Types ................................................................................................................................................. 9-6
Orographic Uplift .......................................................................................................................................... 9-6
Conditions .................................................................................................................................................... 9-6
Mechanism................................................................................................................................................... 9-7
Cloud Types ............................................................................................................................................... 9-10
Frontal Uplift............................................................................................................................................... 9-10
Conditions .................................................................................................................................................. 9-10
Mechanism................................................................................................................................................. 9-10
The Warm Front ......................................................................................................................................... 9-10
The Cold Front ........................................................................................................................................... 9-11
Cloud Types ............................................................................................................................................... 9-12
Convergence.............................................................................................................................................. 9-13
Conditions .................................................................................................................................................. 9-13
Mechanism................................................................................................................................................. 9-13
Cloud Types ............................................................................................................................................... 9-13
CHAPTER 10
Precipitation
Introduction ................................................................................................................................................ 10-1
Precipitation Processes.............................................................................................................................. 10-1
Bergeron Theory (The Ice Crystal Effect)................................................................................................... 10-1
Coalescence Theory (Capture Effect))....................................................................................................... 10-2
Intensity of Precipitation ............................................................................................................................. 10-2
Continuity of Precipitation .......................................................................................................................... 10-2
Precipitation Types..................................................................................................................................... 10-3
Hail............................................................................................................................................................. 10-4
x
Meteorology
Table of Contents
CHAPTER 11
Thunderstorms
Introduction .................................................................................................................................................11-1
Conditions ...................................................................................................................................................11-1
Trigger Actions............................................................................................................................................11-1
Thunderstorm Classification........................................................................................................................11-1
Heat/Airmass Thunderstorms .....................................................................................................................11-2
Convection ..................................................................................................................................................11-2
Orograohic Uplift .........................................................................................................................................11-2
Advection ....................................................................................................................................................11-2
Convergence...............................................................................................................................................11-2
Frontal Thunderstorms................................................................................................................................11-2
Identification of Thunderstorms...................................................................................................................11-3
Stages of Development...............................................................................................................................11-3
Growth Stages ............................................................................................................................................11-3
Mature Stage ..............................................................................................................................................11-3
Dissipating Stage ........................................................................................................................................11-4
Supercell Thunderstorms ............................................................................................................................11-5
Movement of Thunderstorms ......................................................................................................................11-5
Squall Lines ................................................................................................................................................11-5
Hazards ......................................................................................................................................................11-5
Turbulence and Windshear .........................................................................................................................11-5
Gust Front ...................................................................................................................................................11-6
Microbursts .................................................................................................................................................11-6
Hail..............................................................................................................................................................11-7
Icing ............................................................................................................................................................11-7
Lightning .....................................................................................................................................................11-8
Static ...........................................................................................................................................................11-8
Water Ingestion...........................................................................................................................................11-8
Tornadoes...................................................................................................................................................11-9
Pressure Variation ......................................................................................................................................11-9
Weather Radar............................................................................................................................................11-9
Avoidance Criteria.....................................................................................................................................11-10
CHAPTER 12
Visibility
Introduction .................................................................................................................................................12-1
Types of Visibility Reduction .......................................................................................................................12-1
Types of Visibility ........................................................................................................................................12-1
Meteorological Visibility...............................................................................................................................12-1
Runway Visual Range.................................................................................................................................12-1
Oblique Visibility..........................................................................................................................................12-2
Measurement of Visibility ............................................................................................................................12-2
Measurement of Runway Visual Range......................................................................................................12-3
RVR Reporting............................................................................................................................................12-3
Visibility While Flying ..................................................................................................................................12-4
Types of Fog ...............................................................................................................................................12-6
Radiation Fog..............................................................................................................................................12-6
Advection Fog .............................................................................................................................................12-7
Steaming Fog (Artic Sea Smoke)................................................................................................................12-8
Frontal Fog..................................................................................................................................................12-8
Hill Fog........................................................................................................................................................12-9
Other Reducers of Visibility.......................................................................................................................12-10
Smoke Fog (Smog)...................................................................................................................................12-10
Dust and Sand ..........................................................................................................................................12-10
Meteorology
xi
Table of Contents
CHAPTER 12 (Continued)
Precipitation ............................................................................................................................................. 12-10
Visual Illusions ......................................................................................................................................... 12-11
Shallow Fog ............................................................................................................................................. 12-11
Rain Showers........................................................................................................................................... 12-11
Layer Cloud.............................................................................................................................................. 12-11
Rain Effects.............................................................................................................................................. 12-11
CHAPTER 13
Icing
Introduction ................................................................................................................................................ 13-1
Conditions .................................................................................................................................................. 13-1
Effects of Icing............................................................................................................................................ 13-1
Icing Definitions.......................................................................................................................................... 13-2
Supercooled Water Droplets ...................................................................................................................... 13-3
Size of Supercooled Water Droplets .......................................................................................................... 13-3
Freezing Process ....................................................................................................................................... 13-3
Types of Icing............................................................................................................................................. 13-4
Clear Ice..................................................................................................................................................... 13-4
Rime Ice..................................................................................................................................................... 13-4
Mixed Ice.................................................................................................................................................... 13-5
Rain Ice...................................................................................................................................................... 13-5
Hoar Frost .................................................................................................................................................. 13-5
Factors Affecting the Severity of Icing........................................................................................................ 13-6
Engine Icing ............................................................................................................................................... 13-7
Piston Engine Icing .................................................................................................................................... 13-7
Jet Engine Icing.......................................................................................................................................... 13-8
Ice Protection ............................................................................................................................................. 13-9
CHAPTER 14
Wind
Introduction ................................................................................................................................................ 14-1
Terms Associated with Wind ...................................................................................................................... 14-2
Forces Acting upon the Air ......................................................................................................................... 14-2
The Pressure Gradient Force..................................................................................................................... 14-3
The Geostrophic Force .............................................................................................................................. 14-3
The Geostrophic Wind ............................................................................................................................... 14-5
The Geostrophic Wind Scale ..................................................................................................................... 14-7
The Gradient Wind ..................................................................................................................................... 14-7
Winds Near the Equator ............................................................................................................................. 14-9
The Surface Wind ...................................................................................................................................... 14-9
Diurnal Variation of the Surface Wind ...................................................................................................... 14-10
Measurement of Surface Wind................................................................................................................. 14-11
Isallobaric Effect....................................................................................................................................... 14-12
xii
Meteorology
Table of Contents
CHAPTER 15
Local Winds
Introduction .................................................................................................................................................15-1
Land and Sea Breezes ...............................................................................................................................15-1
Sea Breeze .................................................................................................................................................15-1
Land Breeze................................................................................................................................................15-2
Operational Implications of the Land and Sea Breezes ..............................................................................15-2
Katabatic and Anabatic Winds ....................................................................................................................15-3
Katabatic Wind............................................................................................................................................15-3
Anabatic Wind.............................................................................................................................................15-4
Foehn Wind/Effect ......................................................................................................................................15-5
Valley/Ravine Wind.....................................................................................................................................15-6
Headland Effect ..........................................................................................................................................15-7
Low-Level Jet..............................................................................................................................................15-7
Nocturnal Jet...............................................................................................................................................15-7
Valley Inversion...........................................................................................................................................15-7
Coastal Jet ..................................................................................................................................................15-8
Low Level Jet in Front of an Extra-Tropical Cold Front ...............................................................................15-8
CHAPTER 16
Air Masses
Introduction .................................................................................................................................................16-1
Origin and Classification .............................................................................................................................16-1
Modification of Air Masses ..........................................................................................................................16-2
Air Masses Affecting Europe.......................................................................................................................16-3
Arctic ...........................................................................................................................................................16-3
Polar ...........................................................................................................................................................16-3
Tropical .......................................................................................................................................................16-5
Air Mass Summary......................................................................................................................................16-5
CHAPTER 17
Fronts and Occlusions
Introduction .................................................................................................................................................17-1
Types of Front.............................................................................................................................................17-1
Warm Front .................................................................................................................................................17-1
Cold Front ...................................................................................................................................................17-2
Quasi-Stationary Front ................................................................................................................................17-2
Pressure Situation at a Front ......................................................................................................................17-2
Semi-Permanent Fronts of the World..........................................................................................................17-3
Arctic Front..................................................................................................................................................17-3
Polar Front ..................................................................................................................................................17-3
Mediterranean Front ...................................................................................................................................17-3
Inter-Tropical Convergence Zone (ITCZ) ....................................................................................................17-4
Characteristics of Fronts .............................................................................................................................17-4
Warm Front .................................................................................................................................................17-4
Cold Front ...................................................................................................................................................17-5
Polar Front Depressions .............................................................................................................................17-6
Weather Associated with the Polar Front Depression.................................................................................17-8
Occlusions ................................................................................................................................................17-11
Meteorology
xiii
Table of Contents
CHAPTER 18
Upper Winds
Introduction ................................................................................................................................................ 18-1
Thermal Wind Component ......................................................................................................................... 18-1
Calculating the Thermal Wind Component................................................................................................. 18-2
Upper Wind ................................................................................................................................................ 18-3
Global Upper Winds ................................................................................................................................... 18-5
Jet Streams ................................................................................................................................................ 18-5
Common Jet Streams ................................................................................................................................ 18-6
Sub Tropical Jet Stream............................................................................................................................. 18-6
Polar Front Jet Stream ............................................................................................................................... 18-8
Winds Around a Polar Front Depression.................................................................................................... 18-9
Clear Air Turbulence .................................................................................................................................. 18-9
Identification of Jet Streams ..................................................................................................................... 18-10
Contour Charts......................................................................................................................................... 18-10
Thickness Charts ..................................................................................................................................... 18-11
CHAPTER 19
Windshear and Turbulence
Windshear .................................................................................................................................................. 19-1
Definitions and the Meteorological Background ......................................................................................... 19-1
Definition .................................................................................................................................................... 19-1
Low Altitude Windshear ............................................................................................................................. 19-1
Meteorological Features............................................................................................................................. 19-2
Thunderstorms ........................................................................................................................................... 19-2
Frontal Passage ......................................................................................................................................... 19-2
Inversions................................................................................................................................................... 19-3
Turbulent Boundary Layer.......................................................................................................................... 19-3
Topographical Windshears ........................................................................................................................ 19-3
The Effects of Windshear on an Aircraft in Flight ....................................................................................... 19-4
Techniques to Counter the Effects of Windshear ....................................................................................... 19-8
ICAO Definitions....................................................................................................................................... 19-10
Nature of Turbulence ............................................................................................................................... 19-11
Turbulence, Meteorological Factors ......................................................................................................... 19-11
Thermal Turbulence ................................................................................................................................. 19-11
Mechanical/ Frictional Turbulence............................................................................................................ 19-11
Mountain Waves ...................................................................................................................................... 19-12
Flight Over and in the Vicinity of High Ground ......................................................................................... 19-12
Conditions ................................................................................................................................................ 19-12
Visual Detection of Mountain Waves........................................................................................................ 19-13
Turbulence ............................................................................................................................................... 19-14
Turbulence at Low and Medium Levels.................................................................................................... 19-14
Turbulence in the Rotor Zone .................................................................................................................. 19-14
Turbulence in Waves ............................................................................................................................... 19-14
Turbulence at High Levels (near and above the tropopause) .................................................................. 19-15
Turbulence Near the Jet Stream .............................................................................................................. 19-15
Turbulence in the Stratosphere ................................................................................................................ 19-15
Downdraughts .......................................................................................................................................... 19-15
Icing ......................................................................................................................................................... 19-15
Flying Aspects.......................................................................................................................................... 19-15
Low Altitude Flight.................................................................................................................................... 19-15
High Altitude Flight ................................................................................................................................... 19-16
Inversions................................................................................................................................................. 19-16
Marked Temperature Inversion ................................................................................................................ 19-16
Reporting Turbulence............................................................................................................................... 19-17
xiv
Meteorology
Table of Contents
CHAPTER 20
Non-Frontal Pressure Systems
Introduction .................................................................................................................................................20-1
Low, Cyclone or Depression, and Trough ...................................................................................................20-1
Low Pressure Types ...................................................................................................................................20-2
Secondary Depression................................................................................................................................20-2
Icelandic Low ..............................................................................................................................................20-3
The Origin Of Low Pressures And Weather ................................................................................................20-5
Orographic or Lee Side Lows or Troughs ...................................................................................................20-5
Thermal Depressions ..................................................................................................................................20-6
Instability Lows............................................................................................................................................20-7
Mediterranean Low .....................................................................................................................................20-7
Polar Lows ..................................................................................................................................................20-8
Baltic Sea Cyclones ....................................................................................................................................20-8
Cells of Cold Air Aloft (Cold Pools) .............................................................................................................20-8
Anticyclone or High, and Ridge or Wedge ..................................................................................................20-9
Nature of a High..........................................................................................................................................20-9
High Pressure Systems ............................................................................................................................20-10
Subtropical Highs (Warm Anticyclones) ....................................................................................................20-10
Continental Highs (Cold Anticyclones) ......................................................................................................20-10
High Pressures And High Pressure Ridges (Or Wedges) In Series Of Travelling Depressions................20-11
CHAPTER 21
Meteorological Observations and Meteorological Services
Types of Service .........................................................................................................................................21-1
Pre-Flight Briefing .......................................................................................................................................21-1
Meteorological Charts .................................................................................................................................21-1
Broadcast Text Meteorological Information.................................................................................................21-2
Special Aerodrome Meteorological Reports (SPECI)..................................................................................21-2
Terminal Aerodromes Forecast (TAF) ........................................................................................................21-2
Special Forecasts and Specialized Information ..........................................................................................21-3
SIGMET Service .........................................................................................................................................21-3
Aircraft Reports ...........................................................................................................................................21-4
Routine Aircraft Observations .....................................................................................................................21-4
Special Aircraft Observations......................................................................................................................21-4
Clear Air Turbulence (CAT).........................................................................................................................21-5
Airframe Icing..............................................................................................................................................21-6
Aerodrome Closure.....................................................................................................................................21-6
In-flight Procedures.....................................................................................................................................21-6
Accuracy of Meteorological Measurement or Observation..........................................................................21-7
Marked Temperature Inversion ...................................................................................................................21-7
Aerodrome Warnings ..................................................................................................................................21-7
Special Facilities .........................................................................................................................................21-8
Windshear Alerting......................................................................................................................................21-8
Windshear Reporting Criteria......................................................................................................................21-8
Observing Systems and Operating Procedures ..........................................................................................21-9
Cloud Height ...............................................................................................................................................21-9
Temperature ...............................................................................................................................................21-9
Horizontal Surface Visibility.........................................................................................................................21-9
Runway Visual Range (RVR)....................................................................................................................21-10
Meteorology
xv
Table of Contents
CHAPTER 22
Meteorological Messages
Introduction ................................................................................................................................................ 22-1
Aerodrome Meteorological Report ............................................................................................................. 22-1
Special Aerodrome Meteorological Reports ............................................................................................... 22-1
Terminal Aerodrome Forecasts.................................................................................................................. 22-1
Actual Weather Codes ............................................................................................................................... 22-1
Identifier ..................................................................................................................................................... 22-2
Surface Wind Velocity ................................................................................................................................ 22-2
Horizontal Visibility ..................................................................................................................................... 22-2
Runway Visual Range (RVR) ..................................................................................................................... 22-3
Weather ..................................................................................................................................................... 22-4
Significant Present and Forecast Weather Codes...................................................................................... 22-4
Cloud.......................................................................................................................................................... 22-5
CAVOK ...................................................................................................................................................... 22-5
Air Temperature and Dewpoint .................................................................................................................. 22-5
Sea Level Pressure (QNH) ........................................................................................................................ 22-6
Supplementary Information ........................................................................................................................ 22-6
Recent Weather (RE)................................................................................................................................. 22-6
Windshear (WS)......................................................................................................................................... 22-6
Trend.......................................................................................................................................................... 22-6
Runway State Group.................................................................................................................................. 22-6
Runway Designator (First Two Digits)........................................................................................................ 22-7
Runway Deposits (Third Digit).................................................................................................................... 22-7
Extent of Runway Contamination (Fourth Digit) ......................................................................................... 22-7
Depth of Deposit (Fifth and Sixth Digits) .................................................................................................... 22-7
Friction Coefficient or Braking Action (Seventh and Eighth Digits)............................................................. 22-8
'Auto' and 'Rmk'.......................................................................................................................................... 22-8
Missing Information .................................................................................................................................... 22-8
Examples of METARS ............................................................................................................................... 22-8
Aerodrome Forecasts (TAF) Codes ........................................................................................................... 22-9
TAF Contents and Format.......................................................................................................................... 22-9
Significant Changes ................................................................................................................................... 22-9
Other Groups ........................................................................................................................................... 22-10
VOLMET Broadcasts ............................................................................................................................... 22-11
CHAPTER 23
The Synoptic Chart
Introduction ................................................................................................................................................ 23-1
The Station Circle Decode ......................................................................................................................... 23-3
Pressure (1 o'clock) ................................................................................................................................... 23-3
Pressure Tendency (3 o'clock)................................................................................................................... 23-3
Past Weather (5 o'clock) ............................................................................................................................ 23-4
Additional Past Weather Symbols .............................................................................................................. 23-4
Low Cloud or Vertical Visibility (6 o'clock) .................................................................................................. 23-5
Vertical Visibility ......................................................................................................................................... 23-5
Dewpoint (7 o'clock) ................................................................................................................................... 23-5
Visibility (9 o'clock Outer Position) ............................................................................................................. 23-5
Present Weather (9 o'clock Inner Position) ................................................................................................ 23-6
Weather in the Past Hour But Not at the Time of Observation ................................................................... 23-7
Surface Air Temperature or Dry Bulb Temperature (11 o'clock) ................................................................ 23-8
Medium Level Cloud (12 o'clock Lower Position)....................................................................................... 23-8
High Level Cloud (12 o'clock Upper Position) ............................................................................................ 23-9
Total Cloud Cover (Shown in the Centre of the Circle) ............................................................................ 23-10
Surface Wind............................................................................................................................................ 23-10
xvi
Meteorology
Table of Contents
CHAPTER 24
Upper Air Charts
Introduction .................................................................................................................................................24-1
Symbols for Significant Weather .................................................................................................................24-1
Fronts and Convergence Zones and Other Symbols ..................................................................................24-2
Cloud Abbreviations ....................................................................................................................................24-2
Cloud Amount .............................................................................................................................................24-2
Clouds Excerpt............................................................................................................................................24-2
Cumulonimbus Only....................................................................................................................................24-3
Weather Abbreviations................................................................................................................................24-3
Lines and Symbols on the Chart .................................................................................................................24-3
Significant Weather Chart ...........................................................................................................................24-4
Upper Wind and Temperature Charts .........................................................................................................24-6
Averaging Wind Velocities ..........................................................................................................................24-8
CHAPTER 25
Climatology – The World Climate
Introduction .................................................................................................................................................25-1
Ideal Global Circulation ...............................................................................................................................25-1
Rotation of the Earth ...................................................................................................................................25-2
Idealised Pressure Zones ...........................................................................................................................25-3
The Earth’s Tilt............................................................................................................................................25-3
Pressure Zones...........................................................................................................................................25-4
Equatorial Low (Trough) .............................................................................................................................25-4
Sub-Tropical Highs .....................................................................................................................................25-4
Temperate Low ...........................................................................................................................................25-4
Polar High ...................................................................................................................................................25-4
Prevailing Surface Winds ............................................................................................................................25-4
Westerly Winds ...........................................................................................................................................25-4
Easterly Winds ............................................................................................................................................25-5
Climatic Zones ............................................................................................................................................25-5
Equatorial Climate (0° to 10° Latitude)........................................................................................................25-5
Tropical Transition Climate (10° to 20° Latitude) ........................................................................................25-5
Arid Sub-Tropical (20° to 35° Latitude) .......................................................................................................25-5
Mediterranean Climate (35° to 40° Latitude)...............................................................................................25-6
Disturbed Temperate (40° to 65° Latitude) .................................................................................................25-6
Polar Climate (65° to 90° Latitude)..............................................................................................................25-7
Modifications to the Idealised Circulation ....................................................................................................25-7
Global Temperature Distribution .................................................................................................................25-7
Mean Sea Level Temperatures – January ..................................................................................................25-7
Mean Sea Level Temperature – July ..........................................................................................................25-8
Seasonal Variations in Temperature...........................................................................................................25-8
Upper Air Temperature Distribution ............................................................................................................25-9
World Pressure Distribution ........................................................................................................................25-9
Mean Sea Level Pressure – January ..........................................................................................................25-9
Mean Sea Level Pressure – July ..............................................................................................................25-10
Upper Winds .............................................................................................................................................25-11
Mean Upper Wind – January ....................................................................................................................25-11
Mean Upper Wind – July...........................................................................................................................25-12
Inter Tropical Convergence Zone (ITCZ) ..................................................................................................25-12
ITCZ – January .........................................................................................................................................25-13
ITCZ – July................................................................................................................................................25-13
Stability and Moisture Content of the ITCZ ...............................................................................................25-14
ITCZ Weather ...........................................................................................................................................25-14
Inter Tropical Front (ITF/FIT).....................................................................................................................25-14
Low Level Winds.......................................................................................................................................25-15
Low Level Winds – January ......................................................................................................................25-15
Low Level Winds – July ............................................................................................................................25-16
Climatic Summary.....................................................................................................................................25-17
Meteorology
xvii
Table of Contents
CHAPTER 26
Climatology – Prevailing Winds and Ocean Currents
Introduction ................................................................................................................................................ 26-1
Europe and the Mediterranean................................................................................................................... 26-1
Africa.......................................................................................................................................................... 26-7
Asia ............................................................................................................................................................ 26-9
The Indian Monsoon ................................................................................................................................ 26-11
The Far East Monsoon............................................................................................................................. 26-13
North America .......................................................................................................................................... 26-15
South America.......................................................................................................................................... 26-15
Australia ................................................................................................................................................... 26-16
Ocean Currents........................................................................................................................................ 26-17
Cold Water Coast..................................................................................................................................... 26-18
Warm Water Coast................................................................................................................................... 26-18
Summary of the Local Winds of the World ............................................................................................... 26-18
CHAPTER 27
Climatology – Tropical Revolving Storms and Tornadoes
Tropical Revolving Storm (TRS)................................................................................................................. 27-1
Characteristics ........................................................................................................................................... 27-2
Visual Indications of the Advance of the TRS ............................................................................................ 27-6
Tornado...................................................................................................................................................... 27-6
Tropical Revolving Storm Areas................................................................................................................. 27-8
CHAPTER 28
Climatology – Regional Climatology
Europe ....................................................................................................................................................... 28-1
Mediterranean ............................................................................................................................................ 28-5
North Atlantic And North America .............................................................................................................. 28-8
Africa........................................................................................................................................................ 28-13
Asia .......................................................................................................................................................... 28-17
Australia and the Pacific........................................................................................................................... 28-19
South America and the Caribbean ........................................................................................................... 28-22
xviii
Meteorology
INTRODUCTION
Meteorology is the study of the Earth’s atmosphere and the physical processes that occur within
it.
The study of Meteorology is important for the pilot because the atmosphere is the medium
through which the aircraft moves. It is essential to know what conditions are present along a
route, and knowledge of the processes in which weather forms is useful for predicting what
conditions may occur during flight.
DEFINITION OF THE ATMOSPHERE
The term atmosphere refers to the gaseous envelope that surrounds the Earth. It is held to the
Earth by the force of gravity. This gaseous envelope moves with the rotation of the Earth and
extends from the surface of the planet up to the boundary of space.
PROPERTIES OF THE ATMOSPHERE
The atmosphere acts as a fluid, is a poor conductor of heat, and only supports life in the lower
levels.
Due to the extent of the volume of air, variations are found both horizontally and vertically in the
following properties:
¾
¾
¾
¾
Pressure
Temperature
Density
Humidity
Later chapters cover each of these properties in detail.
COMPOSITION OF THE ATMOSPHERE
The density of the atmosphere decreases with altitude. This does not affect the composition up to
an altitude of at least 60 km. Ozone and some trace elements are affected by the chemical
reactions in the upper reaches towards 60 km.
Meteorology
1-1
Chapter 1
The Atmosphere
Above 70 km the lower force of gravity causes the atmospheric composition to vary with height.
The following percentages show the composition of dry air in the lower levels:
Nitrogen:
Oxygen:
Argon:
Carbon Dioxide:
78.09%
20.95%
0.93%
0.03%
The graph below represents this composition:
Other trace elements include:
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
Neon
Helium
Krypton
Xenon
Hydrogen
Methane
Iodine
Nitrous Oxide
Ozone
Sulphur Dioxide
Nitrogen Dioxide
Ammonia
Carbon Monoxide
The above list is background information and needn’t be memorised.
The composition of dry clean air shown above does not allow for the effects of water in the
atmosphere (up to 4% in volume), dust and smoke, or carbon dioxide.
1-2
Meteorology
The Atmosphere
Chapter 1
WATER (H2O)
Water can assume all three physical states in the atmosphere, the solid state (ice), the liquid
state (water), and the gaseous state (water vapour).
Water is unique in that it can readily change from one state to another and can co-exist in all
three states.
THE WATER CYCLE
The water cycle starts when solar radiation strikes moist ground or a water surface. The water
then becomes vapour in the air. The concentration of water vapour is greatest in the lower parts
of the atmosphere.
When conditions are correct, water vapour forms clouds and then condenses, becomes droplets,
and falls as precipitation.
These clouds and the precipitation they produce make up part of what is known as weather.
PARTICLES AND DUST
The solid particles in the atmosphere consist mainly of dust and sand from the ground and salt
particles from the oceans. In addition, man has added all sorts of soot and dust.
These solid particles can restrict visibility, for example, with haze or during sand storms.
The amount of solid particles in the air varies, but the existence of these particles is of
fundamental importance to processes such as condensation and the formation of ice.
The condensation process occurs in the lower parts of the atmosphere. Without condensation
nuclei, it would be difficult for water vapour to convert into precipitation and for the formation of
ice.
Meteorology
1-3
Chapter 1
The Atmosphere
CARBON DIOXIDE (CO2)
Carbon dioxide is to be found both naturally in the atmosphere and as a waste product from
burning fossil fuels (carbon compounds). A large part of the carbon dioxide, which is released into
the air, is returned to nature’s own circulation via the oceans.
Carbon dioxide plays a large role in the heating of the atmosphere.
STRUCTURE OF THE ATMOSPHERE
There are five layers in the atmosphere. From the surface upwards these are the troposphere,
stratosphere, mesosphere, ionosphere, and exosphere.
Note: In the diagram above, the ionosphere and the exosphere combine to form the
thermosphere.
TROPOSPHERE
The troposphere extends from the surface up to an average height of 11 km. Within the layer,
temperatures generally decrease as altitude increases.
It is an area of relatively low stability where the over-turning of air is frequent. It holds virtually all
the water vapour in the atmosphere and is the layer where most flying occurs.
The troposphere contains over 75% of the mass of the total atmosphere.
1-4
Meteorology
The Atmosphere
Chapter 1
TROPOPAUSE
The upper boundary of the troposphere is known as the tropopause. It separates the troposphere
from the stratosphere. The temperature ceases to decrease with height at the boundary of the
tropopause.
The height of the tropopause varies with latitude, season of the year, and the weather conditions.
The tropopause is lowest over the poles (approximately 26 000 ft or 8 km) and highest over the
equator (approximately 52 000 ft or 16 km). Its average height is 36 090 ft (11 km) at about 45°
latitude.
Since the thickness of the troposphere is determined by the amount of solar energy and the
vertical mixing, the tropopause is lower over areas where the air is cold than where it is warm.
The left-hand picture below shows that the tropical tropopause height is greater than the polar
tropopause height. The right hand picture shows that, for a given region such as the poles, the
summer tropopause is higher than the winter tropopause.
As the temperature decreases with height, so the temperature at the tropopause over the poles
will be higher than over the equator because the tropopause is closer to the ground here. This is
the opposite situation to the surface temperature.
Typically, the tropopause temperature is -50°C over the poles and -80°C over the equator.
Another feature of the tropopause is that, rather than show a gradual change in height between
the equator and the poles, there are breaks in the tropopause where large temperature
differentials occur.
Meteorology
1-5
Chapter 1
The Atmosphere
The first of these breaks occurs at about 40° latitude, where warm air circulating from the equator
meets colder air from higher latitudes. The second break is at 55° latitude, where tropical air
meets polar air. The third break is between 60° and 70° latitude, where polar air meets arctic air.
This break is more common in the Northern Hemisphere.
The diagram below shows the breaks:
55° lat
60°-70° lat
40° lat
The presence of these breaks can cause strong winds called jet streams. These will be
discussed in detail in a later chapter.
The table below shows the approximate height of the tropopause at various latitudes in winter
and summer:
Latitude
1-6
Winter
Summer
0°
56 000 ft
55 000 ft
10°
55 000 ft
52 000 ft
20°
52 000 ft
51 000 ft
30°
45 000 ft
47 000 ft
40°
38 000 ft
43 000 ft
50°
35 000 ft
38 000 ft
60°
33 000 ft
35 000 ft
70°
29 000 ft
31 000 ft
80°
25 000 ft
29 000 ft
Meteorology
The Atmosphere
Chapter 1
STRATOSPHERE
The stratosphere extends from the tropopause to approximately 50 km above the surface of the
Earth.
Some flying occurs in the lower parts of the stratosphere, so the combination of the troposphere
and lower parts of the stratosphere is therefore often referred to as the aviation atmosphere.
The stratosphere is relatively stable. Initially, the temperature remains constant and then starts to
increase so that it is around 0°C at the top of the layer. This is due to the absorption of ultra-violet
radiation by ozone in the lower layers of the stratosphere and the retransmission of this radiation
as infra-red heat.
The concentration of ozone varies with the latitude, being greater over the poles than the equator.
Therefore, the stratosphere is warmer at higher latitudes.
The region is not an area of still conditions but one of slow vertical movement and strong
horizontal winds.
STRATOPAUSE
This is the boundary that separates the stratosphere from the mesosphere.
MESOSPHERE
In the mesosphere, temperature again decreases with height. The lowest temperature of
approximately -90°C occurs between 80 and 90 km.
MESOPAUSE
This is the upper boundary of the mesosphere.
THERMOSPHERE
This is the outermost layer of the atmosphere that holds the exosphere in its upper regions (at
heights greater than 700 km) and the ionosphere in its lower regions.
The ionosphere is a region where the air becomes ionised by solar radiation. It consists of several
sub-layers. These layers, named the D, E, F1, and F2 layers are important in the transmission of
certain radio waves and will be covered in more depth in Radio Navigation.
The thermosphere is characterised by an increase in temperature with height. At 200 km, the
temperature is generally 600°C. At times of sunspot activity, it can be up to 2000°C.
Meteorology
1-7
Chapter 1
The Atmosphere
INTERNATIONAL STANDARD ATMOSPHERE (ISA)
The conditions of the atmosphere are constantly changing. This causes problems for aviation,
especially with the calibration of pressure instruments. For this reason, the International Standard
Atmosphere (ISA) was devised. It is a purely hypothetical atmosphere that represents an average
picture of the actual atmosphere.
ISA has been in use since 1964 and is the most widely used hypothetical atmosphere. It
possesses the characteristics laid out below:
Mean Sea Level (MSL)
Temperature
Pressure
Density
15°C
1013.25 hPa
1225 g/m3
From MSL to 11 km
(36 090 ft)
Temperature decreases at 1.98°C
per 1000 ft (6.5°C per km)
From 11 km to 20 km
(65 617 ft)
Temperature constant at –56.5°C
From 20 km to 32 km
(104 987 ft)
Temperature rises with height at
0.3°C per 1000 ft (1°C per km)
The chart shows that the ISA temperature is constant above 36 090 ft in the aviation atmosphere.
ISA DEVIATION
In aviation, it is important to know how the atmosphere differs from ISA at any particular time.
Such information is used in performance calculations and in correcting for instrument errors.
ISA deviation is the difference between the ISA temperature and the actual temperature. It can be
a positive or a negative deviation.
Example 1:
You are flying at 30 000 ft. The outside air temperature is -50°C. What is the ISA
deviation?
Answer 1:
The ISA temperature would be 15 – (1.98 × 30) = -44.4°C. The difference
between this and the actual temperature is 5.6°C. The actual temperature is the
lower figure, so the deviation is negative (-5.6°C).
Example 2:
You are flying at 22 000 ft. The ISA deviation is +10°C. What is the outside air
temperature?
Answer 2:
The ISA temperature would be 15 – (1.98 × 22) = -28.56°C. ISA deviation is
+10°C, so the ambient temperature must be higher than this:
-28.56 + 10 = -18.56°C
1-8
Meteorology
The Atmosphere
Chapter 1
The following table is given for you to practice doing ISA calculations. Answers can be found at
the end of this chapter:
Height (ft)
Ambient
temperature (°C)
10 000
-10
ISA
Temperature (°C)
17 000
-12
-34.5
-59.32
+8
-7
38 000
8000
ISA Deviation
+10
-15.84
-48.36
-32.7
+22
-18
Note: For the JAR exams, it is sufficient to round the lapse rate up and use 2°C/1000 ft
for ISA calculations.
JET STANDARD ATMOSPHERE (JSA)
The Jet Standard Atmosphere (JSA) is often used by engine manufacturers. It assumes a mean
sea level temperature of +15°C. The temperature then lapses at 2°C per 1000 ft to infinity. There
is no tropopause in the JSA.
So an aircraft at 40 000 ft with an outside air temperature of –65°C would have:
¾
¾
An ISA temperature deviation of –8.5°C
A JSA temperature deviation of 0°C
Meteorology
1-9
Chapter 1
The Atmosphere
ANSWERS TO ISA DEVIATION QUESTIONS
1-10
Height (ft)
Ambient
temperature (°C)
ISA
Temperature (°C)
ISA Deviation
10 000
-10
-4.8
-5.2
17 000
-30.66
-18.66
-12
25 000
-26.5
-34.5
+8
34 000
-59.32
-52.32
-7
38 000
-46.5
-56.5
+10
8000
-15.84
-0.84
-15
32 000
-26.36
-48.36
+22
15 000
-32.7
-14.7
-18
Meteorology
INTRODUCTION
Chapter 1 introduced the concept of the atmosphere as a fluid. The chapter also discussed the
fact that certain properties of the atmosphere vary both horizontally and vertically.
The fluidity of the air means that it tends to flow from a region of high pressure to a region of low
pressure. It is these pressure differences and the consequent movement of air that are the main
cause of weather.
An understanding of pressure and pressure systems is vital for pilots.
ATMOSPHERIC PRESSURE
Air is made up of particles that, small as they are, are nevertheless under the force of gravity. A
surface must support the weight of the air directly above it.
Atmospheric pressure is the force per unit area exerted by the molecules of air over a specific
surface.
Consider the column of air below:
h2
h1
s2
s1
The height of the column above s2 (h2) is less than that above s1 (h1). There is a larger weight of
air above s1, hence a larger pressure. The cross-sectional area of both surfaces is the same.
Meteorology
2-1
Chapter 2
Pressure and Pressure Systems
MEASURING ATMOSPHERIC PRESSURE
MERCURY BAROMETER
Vacuum
Mercury
Scale
The simplest means of measuring atmospheric pressure is the Mercury Barometer.
A 1 metre tube of mercury is upturned in a reservoir of mercury. Atmospheric pressure is exerted
on the surface of the mercury in the reservoir. The mercury in the tube then sinks to about
760 mm above the reservoir at mean sea level.
The atmospheric pressure is therefore said to be 760 millimetres of mercury (760 mmHg).
As the atmospheric pressure varies, so does the height of the mercury.
2-2
Meteorology
Pressure and Pressure Systems
Chapter 2
ANEROID BAROMETER
Another way of measuring pressure is by using the aneroid barometer. This consists of a partially
evacuated capsule that expands and contracts as the air pressure outside the capsule changes.
A scale indicates these changes by using a system of linkages. The diagram shows the basic
ideas behind the system.
UNITS OF MEASUREMENT
One method of expressing atmospheric pressure was introduced above, that is, mmHg.
The SI unit for force is the Newton. The SI unit of pressure then becomes the N/m2, as pressure
is force per unit area. The N/m2 is also known as the Pascal (Pa).
100 000 N/m2 is known as the Bar. Within one bar is 1000 millibars. This is the unit most widely
used in aviation. The millibar may also be known as the hectoPascal.
To further complicate the issue, some countries use inches of mercury—the United States for
example. Use the following conversion when moving between units:
1000 mb = 1000 hPa = 29.53 inHg = 100 000 N/m2 = 750.1 mmHg
The ISA values at mean sea level are:
1013.25 mb = 1013.25hPa = 29.92 inHg = 101 325 N/m2 = 760 mmHg
Meteorology
2-3
Chapter 2
Pressure and Pressure Systems
PRESSURE VARIATION
Pressure varies horizontally, diurnally, and vertically.
HORIZONTALLY
Pressure varies from place to place and also changes over time. Horizontal pressure differences
lead to movement of air and hence, weather.
DIURNALLY
Pressure also has a twelve-hour oscillation period. In one day there are two peak pressure
values, which occur at around 1000 and 2200 hours. There are two lows, one at around 1600 and
another at 0400 hours. The difference between the high and low values is very small in temperate
latitudes (only about 1 hPa), but is much more significant in tropical and sub-tropical latitudes
(about 3 hPa).
Although the diurnal pressure change in temperate latitudes is often masked by other events,
absence of the expected change in lower latitudes is often a warning of impending severe
weather, such as a tropical revolving storm.
Tropical/subtropical latitudes
– typically 3 hPa
Temperate
latitudes –
typically 1 hPa
VERTICALLY
Pressure always decreases with increase of height. In the ISA we assume that the surface
pressure is 1013.25 hPa. From this we can calculate the pressure for any height.
2-4
Meteorology
Pressure and Pressure Systems
Chapter 2
Pressure (hPa)
Approximate Height (ft)
850
700
500
400
300
200
100
50
5 000 amsl
10 000 amsl
18 000 amsl
24 000 amsl
30 000 amsl
40 000 amsl
53 000 amsl
68 000 amsl
Be sure to learn the figures in the above table.
THE RELATIONSHIP BETWEEN PRESSURE AND
TEMPERATURE
120 ft
120 ft
ISA +
1°C
ISA
ISA 1°C
The diagram above shows three columns of air: one at ISA, one slightly warmer than ISA, and
one slightly colder than ISA. The pressure at the base of all columns is the same.
Cold air is denser than warm air and tends to sink. Therefore, the same pressure is found at a
lower height in the cold column. The pressure decreases more quickly with height than in the ISA
column.
Conversely, warm air is less dense and rises. The same pressure is found at a higher height than
the colder columns. The pressure decreases less quickly with height than in the ISA column.
For a given height interval the decrease in pressure depends on the mean temperature of the
column of air. For the same height interval the pressure change will be greater in a cold column of
air than in a warm column of air.
Meteorology
2-5
Chapter 2
Note:
Pressure and Pressure Systems
This results in a difference in height of 120 ft per degree Celsius. This is
addressed in more detail in later chapters
This phenomenon is important to understand because the altimeter is calibrated to ISA. While
flying in an environment that is colder than ISA, the altimeter detects the same pressure at a
lower height, so you are actually flying at a lower height than you think you are, which is obviously
a potentially dangerous situation.
Thus the phrase:
‘Warm to cold – don’t be bold!’
PRESSURE/HEIGHT CALCULATIONS
It is unlikely that you will have to make pressure/height calculations in the JAR exams, but the
formulae are included here nonetheless.
For calculations involving small intervals of less than 50 hPa, the following formula can be used to
calculate the height change per hectoPascal change in pressure:
H = 96T/P
Where:
H
height in feet
T
mean temperature in K
P
pressure in hectoPascals
Example:
Using the values for ISA MSL. T = 15 + 273 = 288; P = 1013.25
H = (96 × 288) / 1013.25 = 27.3 ft
Therefore, at mean sea level, the height change is 27.3 ft per hPa.
However, as you go higher the rate of pressure fall lessens because the temperature is also
falling. The changes at various heights are laid out below:
Height
Height change per
hPa
MSL
2000 ft amsl
20 000 amsl
40 000 amsl
27 ft
30 ft
50 ft
100 ft
For JAR-FCL examinations, use 1 hPa change as equivalent to 27 ft near the surface.
2-6
Meteorology
Pressure and Pressure Systems
Chapter 2
p2
p1
h2
h1
Use the following formula to calculate an unknown height from knowledge of its pressure:
H2 = H1 + 221.1T(LOG P1 – LOG P2)
Where:
H2
H1
T
P1
P2
height required
known height
the mean temperature of the column of air in K
pressure at h1
pressure at h2
Example:
At MSL the pressure is 1016 hPa, 12°C
At 700 hPa the temperature is 2°C
What height is the 700 hPa level:
The mean temperature of the column is 7°C
h2 = h1 + 221.1T(log P1 – log P2)
h2 = 0 + (221.1 x 280) x (log 1016 – log700)
h2 = 61 908 x 0.1618
h2 = 10 017 feet
This is the height of the 700 hPa level
Meteorology
2-7
Chapter 2
Pressure and Pressure Systems
PRESSURE VALUES
The following are the most likely pressure values that pilots will encounter:
QFE
QFE is the pressure at the datum level of an aerodrome (usually the highest useable point on the
aerodrome). Since it is generally not possible to place a measuring device at this point it is
usually measured elsewhere with corrections applied for the height difference between the
measuring point and the aerodrome datum. These corrections take into account prevailing
temperature.
When you have QFE set, the altimeter reads zero when you are sitting at the datum level of the
aerodrome.
When flying on QFE, the reading on your altimeter is the height above aerodrome level and is
often just referred to as height.
QNH
QNH is the QFE reduced to mean sea level using ISA conditions.
With QNH set, the altimeter reads aerodrome elevation when you are sitting at the datum level of
the aerodrome.
When flying on QNH, the altimeter reading is your height above mean sea level and is generally
referred to as your altitude.
QFF
QFF is the QFE reduced to mean sea level using actual outside air temperature. It is an important
term for meteorology but must never be used in altimetry. Never fly on QFF.
THE STANDARD PRESSURE SETTING
The standard pressure setting of 1013 hPa is often used. The resulting figure is usually divided by
100 and referred to as a Flight Level.
SYNOPTIC CHARTS
A synoptic chart depicts the pressure situation at a particular time. The chart features lines called
isobars. These lines connect places of equal pressure. They are normally drawn for every even
whole millibar. Note that the pressure represented is the QFF.
Another type of line found on some pressure charts is the isallobar, which connects places of the
same pressure tendency and is annotated in millibars per hour. This may be a decrease or an
increase.
Isallobars are useful in predicting the movement of pressure systems.
2-8
Meteorology
Pressure and Pressure Systems
Chapter 2
PRESSURE SYSTEMS
When looking at a synoptic chart, you can see certain patterns. These are called ”pressure
systems” and understanding the properties of these systems can help us forecast the weather.
DEPRESSIONS
A depression is a region of low pressure. It can also be referred to as a low or a cyclone.
The size of depressions can vary quite considerably, for example:
Temperate low
Tropical Revolving Storms
Tornado
up to 1500 km in diameter
approximately 300 km in diameter
tens of metres in diameter
It appears on a synoptic chart as a series of concentric, roughly circular isobars with the lowest
pressure in the centre.
The low pressure in the centre causes air to flow into the low. This is called convergence. This
then causes air in the centre to rise, producing a relatively high pressure at height.
The result is a circulation of air as shown in the diagram below:
H
30 – 35 000 ft
ASCENDING
L
The surface wind blows counter clockwise around a low in the Northern Hemisphere and
clockwise in the Southern Hemisphere. In both cases, wind also blows in toward the centre. The
mechanisms of this are discussed in a later chapter.
Meteorology
2-9
Chapter 2
Pressure and Pressure Systems
The diagram below represents this:
1004
1002
1000
998
996
L
There are many different kinds of depressions. These will be described in later chapters.
DEPRESSION WEATHER
Due to the lifting at the centre of the low, cloud will form and there will be associated precipitation.
The mechanisms of this are described in later chapters. Typical weather is described in the table
below:
Cloud
Full cover from near the surface to the tropopause.
Precipitation
Generally continuous light or moderate. Heavy showers and
thunderstorms possible because of the unstable nature of the air.
Visibility
Good out of precipitation but poor in precipitation.
Temperature
Mild.
Winds
Depends on the pressure gradient of the isobars but normally
strong.
ANTICYCLONES
This is a region of relatively high pressure, appearing as roughly circular, concentric isobars on
the synoptic chart, with the highest pressure in the centre. It is also referred to as a high.
Isobars are generally more widely spaced than in a depression.
Air will flow out of the centre of the high pressure toward areas of lower pressure. This is called
divergence. To replace the diverging air, air descends. This is called subsidence. This results in a
relatively low pressure at height.
Air circulates clockwise around a high in the Northern Hemisphere and counter clockwise around
a high in the Southern Hemisphere, as well as flowing out of the high.
2-10
Meteorology
Pressure and Pressure Systems
1004
Chapter 2
L
1006
1008
1010
30 – 35 000 ft
H
DESCENDING
H
There are two main types of anticyclone: the warm anticyclone and the cold anticyclone.
WARM ANTICYCLONES
Warm anticyclones are a result of an excess of air at height. Air descends and is warmed. The
main example is the sub-tropical highs caused by the circulation of air known as the Hadley cells.
COLD ANTICYCLONES
Cold anticyclones are caused by low surface temperatures and are found in high latitudes. The
low temperatures cause the density of the air to increase and air to subside.
ANTICYCLONIC WEATHER
When anticyclonic weather is present, air is descending, which prevents cloud from forming and
gives generally good weather. There may be some cloud and precipitation at the edge of the
system.
Temperature inversions are possible due to the subsidence.
Meteorology
2-11
Chapter 2
Pressure and Pressure Systems
The table below shows typical weather associated with an anticyclone:
Cloud
None because of the warming effect of subsidence.
Precipitation
None.
Visibility
In summer, hazy conditions can occur; in winter, foggy conditions.
Temperature
Depends on the type. Hot in summer, cold in winter.
Winds
Light.
TROUGHS
A trough is the extension of isobars out from a depression in the shape of a V, with the pressure
getting lower moving out from the centre. Troughs may be frontal or non-frontal.
In frontal troughs, the front forms the centre line of the trough. The weather depends on the type
of front. Frontal weather is discussed in a later chapter.
In non-frontal troughs, the convergence of air at the centre line causes lifting and unstable
weather.
996
1000
998
1002
1004
Centre line
TROUGH WEATHER
2-12
Cloud
For frontal troughs, the cloud types depend on the type of front. With
cold fronts clouds with a large vertical development are expected.
With warm fronts, layer clouds are more likely.
For non-frontal troughs, CB and CU can be expected.
Precipitation
Showers, thunderstorms, hail with either cold frontal or non-frontal
systems. Light to moderate rain and drizzle with warm fronts.
Visibility
Good except in precipitation.
Winds
Moderate with possibility of gusts and squalls.
Meteorology
Pressure and Pressure Systems
Chapter 2
RIDGES
Ridges are an extension from a high pressure system. They are more rounded than troughs;
more like a U shape.
1010
1008
1006
1004
Ridges are often found between two polar front depressions (see later chapters). They provide
periods of good weather.
RIDGE WEATHER
Ridge weather is very similar to anticyclone weather.
COLS
A col is a region of very little pressure variation between two highs and two lows. Winds are
therefore very light and the air remains mostly stationary, so it remains in contact with the ground
for an extended period of time.
1004
1000
1008
996
H
L
COL
1004
1000
996
L
Meteorology
1008
H
2-13
Chapter 2
Pressure and Pressure Systems
COL WEATHER
In summer, extended contact with the hot ground can lead to instability cloud and thunderstorms.
In winter, extended contact with the cold ground can result in the formation of fog or low stratus.
MOVEMENT OF PRESSURE SYSTEMS
Anticyclones tend to be long-lasting (up to 6 months) and move quite slowly. Depressions move
more quickly and generally only last about 2 weeks. Cols generally get quickly absorbed into
other systems, lasting only a few days.
2-14
Meteorology
INTRODUCTION
Altimeters measure altitude, or height, by using the fact that pressure reduces with height. The
altimeter measures the local pressure but presents this as an altitude in feet rather than as a
pressure in hPa.
The altimeter is an aneroid barometer that detects pressure by way of a capsule. Knowledge of
the detailed workings of the altimeter are not required for Meteorology.
The instrument is calibrated to ISA, so altimeters only read accurately in standard conditions.
The altimeter has a baroscale, a knob that allows the pilot to set the particular reference to which
he wishes to relate the aircraft’s height. This is usually one of the following:
QNH
The altimeter reads height above mean sea level, which is generally referred to as altitude. In
non-ISA conditions, mean sea level is not the same place as actual sea level. Hence the altimeter
only reads height above actual sea level in ISA conditions.
QFE
The altimeter reads height above aerodrome level in ISA conditions — generally just referred to
as height.
THE STANDARD PRESSURE SETTING, 1013 hPA
The resulting figure is usually divided by 100 and referred to as a Flight Level.
PRESSURE CALCULATIONS
When making calculations in altimetry, you can assume that 1 hPa corresponds to 27 ft for the
JAR exams, even though in the real atmosphere the pressure lapse rate decreases as altitude
increases, as discussed in the previous chapter.
Meteorology
3-1
Chapter 3
Altimetry
CONVERTING BETWEEN HEIGHT AND ALTITUDE
As discussed above, the vertical distance above aerodrome level is known as height. The
vertical distance above mean sea level is altitude.
QFE is the pressure at aerodrome level. QNH is QFE reduced to sea level using ISA conditions.
Therefore, if the airfield is above sea level, the QFE is of a lower pressure than the QNH, and the
height is lower than the altitude.
If the airfield is below sea level (a rare occurrence, but not impossible) the QFE is higher than the
QNH and the altitude is lower than the height.
Example 1:
An aircraft is flying at an altitude of 3500 ft. The QNH is 1010 hPa. The
QFE is 988 hPa. What is the aircraft’s height?
1010 – 988 = 22 hPa
Using 27 ft per hPa, the elevation of the airfield must be 22 × 27 = 594 ft.
Hence the aircraft height must be 3500 – 594 = 2906 ft.
This situation is depicted graphically below:
3-2
Meteorology
Altimetry
Example 2:
Chapter 3
An aerodrome has an elevation of 1500 ft. The QFE is 965 hPa.
Calculate an approximate QNH.
1500 / 27 = 56 hPa
The airfield is above sea level so the QNH will be higher, hence:
QNH = 965 + 56 = 1021 hPa.
This situation is depicted graphically below:
Meteorology
3-3
Chapter 3
Altimetry
CONVERTING BETWEEN ALTITUDE AND PRESSURE ALTITUDE/FLIGHT
LEVEL
Flight level and pressure altitude is based on a pressure setting of 1013 hPa. For example, if you
have 1013 set on your altimeter and your reading is 35 000 ft, you are at a pressure altitude of
35 000 ft. Flight level is simply pressure altitude divided by 100. In this example, you are at
Flight Level 350.
Altitude is based on the QNH at any particular time. This varies from place to place and with time.
Look at the following example. As you can see, if QNH is lower than 1013 hPa, the altitude is
lower than the pressure altitude. If QNH is higher than 1013 hPa, the altitude is higher than the
pressure altitude.
As before, use 27 ft per hPa.
The diagram below shows the corresponding altitudes for FL 350 with a low QNH and with a high
QNH.
35 000 –
(27 × 10) =
34 730 ft
35 000 +
(27 × 10) =
35 270 ft
FL 350
FL 350
1003 hPa
1013 hPa
1023 hPa
3-4
Meteorology
Altimetry
Chapter 3
PRESSURE CHANGE
If the pressure falls at a place and the altimeter is not reset, the value it shows will be the height
in feet above an incorrect datum.
If the pressure falls, for example, the pressure datum to which the altimeter was originally set will
have lowered. The aircraft height in relation to it will have increased.
1800 hrs – 1020
0600 hrs – 1010
10 hPa = 270 ft
1020 hPa
If an aeroplane flies from one location to another one with a lower pressure, it will be flying with
reference to a particular datum. If the datum lowers, the aeroplane descends. In the example
below, the aircraft is flying at 500 ft on QNH 1020. It flies towards an area with a QNH of 1010.
500
500 – 270 = 230 ft
1020 hPa
1010 hPa
500
10 × 27 = 270 ft
1020 hPa
When flying to an area of lower pressure the altimeter over-reads. Conversely, when flying to an
area of higher pressure, the altimeter under-reads. This gives rise to the memory aid:
‘High to low – beware below!’
Meteorology
3-5
Chapter 3
Altimetry
CORRECTING FOR TEMPERATURE
The altimeter is calibrated so it reads correctly in ISA conditions. However, in the real world it is
rarely ISA conditions. If it is warmer or colder than ISA, the altimeter reads incorrectly. Look at the
following diagram showing three columns of air: one is at ISA, one is ISA - 10°C, and one is
ISA + 10°C.
ISA +10
ISA
ISA -10
If the air is warmer than ISA, it expands upward; the pressure at the top remains the same so the
aeroplane actually is higher than indicated. If the air is colder than ISA, the air sinks. The
pressure at the top remains the same so the aeroplane is lower than indicated.
The altitude at which the aeroplane is actually flying is called the true altitude.
In order to convert between indicated altitude and true altitude, use one of the following formulas.
Both give the same result so it is a matter of personal preference which one you use.
1% per 2.5°C deviation from ISA
4 feet per 1000 feet per 1°C deviation from ISA
Add if warmer than ISA; subtract if colder than ISA.
To summarise, if flying into warmer air, you climb if maintaining the same reading on your
altimeter. If flying into colder air, you descend if maintaining the same reading on your altimeter.
This gives rise to the memory aid:
‘Warm to cold – don’t be bold!’
3-6
Meteorology
Altimetry
Chapter 3
EXAMPLE 1
You are flying at 6000 feet on a QNH of 1008 hPa. The temperature is 8°C. What is your true
altitude?
At 6000 ft the ISA temperature is 15 – (2 × 6) = 3°C. Hence the temperature is ISA + 5. Using
formula 2 you get:
4 × 6 × 5 = 120 ft
It is warmer than ISA so the true altitude is 6000 + 120 = 6120 ft
(using formula 1: 1% of 6000 ft × (5 / 2.5) = 120 ft)
EXAMPLE 2
You are flying at FL 300. The QNH is 976 hPa. The temperature is -58°C. What is your true
altitude?
First you have to convert from FL to altitude. The QNH is lower, therefore altitude will be lower
than FL.
(1013 – 976) × 27 = 999 ft
Altitude is 30 000 – 999 = 29 001 ft
ISA temperature is 15 – (29 × 2) = -43°C.
So it is ISA – 15.
Temperature correction is 4 × 29 × 15 = 1740 ft.
Colder than ISA so true altitude is 29 001 – 1740 = 27 261 ft.
Meteorology
3-7
Chapter 3
Altimetry
CONVERTING BETWEEN QNH AND QFF
You are not expected to calculate precise values of QFF from QNH and vice-versa in the JAR
exams. However, you are required to say whether QFF is higher or lower than QNH in given
conditions.
QFE 998
540 ft
>ISA
QFF <1018
<20 hPa
change
ISA
20 hPa
change
QFF = QNH 1018
<ISA
QFF >1018
>20 hPa
change
Sea level
The diagram shows an aerodrome above sea level. The QFE is 998 hPa and the elevation is 540
ft above sea level. QNH is always calculated using ISA conditions, so over 540 ft there is a 20
hPa change, making the QNH 20 hPa greater than the QFE, that is, 1018 hPa.
QFF is calculated using actual conditions. The left hand column demonstrates what happens
when it is warmer than ISA. The air is less dense so the pressure change is less over the same
height change. So the change is less than 20 hPa, making the QFF <1018.
The right hand column demonstrates what happens when it is colder than ISA. The air is denser
so the pressure change is more over the same height change. So the change is more than 20
hPa, making the QFF >1018.
3-8
Meteorology
Altimetry
Chapter 3
The second diagram shows what happens with an aerodrome below sea level. In this case, when
it is warmer than ISA, the QFF is greater than the QNH, and when it is colder the QFF is less than
the QNH.
SUMMARY
Warmer
than ISA
Colder
than ISA
Meteorology
Aerodrome above
mean sea level
QFF < QNH
Aerodrome below
mean sea level
QFF > QNH
QFF > QNH
QFF < QNH
3-9
Chapter 3
Altimetry
MOUNTAIN FLYING
There is a tendency for air to collect on the windward side of a mountain range. This leads to an
increased pressure on the windward side. Conversely, a lower pressure is experienced on the
leeward side. This means that if you use the QNH from the windward side you may be exposing
yourself to danger as you will be flying into a region of lower pressure.
To ensure adequate terrain clearance you should always use the lowest QNH for the area.
As air tries to flow through a mountain range, the air is obstructed by the range. In order to pass
over the mountain the air must speed up. Bernoulli’s theorem states that if the velocity goes up,
the pressure goes down.
This leads to a lower pressure over the top of the mountain — a potentially dangerous situation.
The greater the wind speed over the mountain, the greater the height loss.
In order to ensure safe terrain clearance you should add an extra margin to your minimum safe
altitude, depending on the wind speed, as shown below:
< 30 kt
31 – 40 kt
41 – 50 kt
51 – 60 kt
> 60 kt
no addition necessary
add 500 ft
add 1000 ft
add 1500 ft
add 2000 ft
In summary, when calculating the actual altitude above high terrain:
Use the correct reference pressure (the lowest QNH).
Correct the reading for temperature deviation as described above.
Allow an extra margin of safety for strong winds.
ALTIMETER SETTINGS
For take-off and landing, the QNH is normally used — never the standard pressure setting.
When climbing, the standard pressure setting is set when passing the Transition Altitude.
When descending, the QNH is set on passing the transition level.
Transition Altitude
The altitude at or below which the vertical position of an aircraft is
referenced to altitude
Transition Layer
The airspace between the transition altitude and the transition level
Transition Level
The lowest flight level available for use above the transition altitude
Note: Pilots may, at their discretion, use QFE for take-off and landing in which case the aircraft
should have two altimeters and the second one should be set to QNH. If the barometric
pressure is very low (e.g. below the altimeter sub-scale minimum setting) then the
standard pressure setting is used and the aircraft is landed with a false altitude indicated
on the altimeter (the QNE).
3-10
Meteorology
Altimetry
Chapter 3
CALCULATION OF MINIMUM USABLE FLIGHT LEVEL
Minimum usable flight level is important to know in the event of a decompression over high
terrain. Sometimes it is defined by the authorities; if not, you can calculate it as follows:
1.
2.
3.
4.
5.
Note the highest elevation within 5 nm of track, then:
a) In the case of the terrain being higher than 6000 ft (1800 m), add 2000 ft to this
value.
b) In the case of the terrain being lower than or equal to 6000 ft (1800 m), add 1000
ft to this value.
Correct for temperature deviation as described earlier in the chapter.
Correct for wind as described earlier.
Convert from an altitude to a pressure altitude.
Round up to the nearest higher flight level.
Meteorology
3-11
Chapter 3
3-12
Altimetry
Meteorology
INTRODUCTION
Temperature is one of the most important variables that affect the atmosphere. The temperature
changes that occur on the Earth’s surface initiate both vertical air movement (leading to cloud
development) and horizontal air movement (wind).
Temperature normally decreases with height. If there is an increase with height, this is called an
inversion.
If temperature stays the same with change in height, this is called an isothermal layer.
TEMPERATURE SCALES
There are three scales of measurement for temperature. These are:
FAHRENHEIT
In the Fahrenheit scale, the freezing point of water is 32°F and the boiling point of water is 212°F.
This scale is not used in meteorology.
CELSIUS
The Celsius scale is widely used. The freezing point of water is 0°C and the boiling point is
100°C.
KELVIN
The Kelvin scale does not have units, but intervals of the scale are equal to 1°C. The scale
relates to absolute zero (−273°C) which is defined as 0K. The freezing point of water is 273K and
the boiling point is 373K.
0K is called absolute zero and is the temperature at which all molecules stop moving completely.
CONVERSION FACTORS
To convert from Celsius to Fahrenheit:
°F = (°C x
9
) + 32
5
Meteorology
4-1
Chapter 4
Temperature
To convert from Fahrenheit to Celsius:
°C = (°F – 32) X
5
9
To convert from Celsius to Kelvin:
K = °C + 273
To convert from Kelvin to Celsius:
°C = K – 273
MEASUREMENT OF TEMPERATURE
Surface temperatures are measured using mercury thermometers housed in a Stevenson screen.
This is a louvred wooden box that allows air to circulate around the thermometers but protects
them from draughts and direct sunlight. It is held 4 ft above the ground so the temperature won’t
be adversely affected by the ground temperature.
High level temperatures are measured using a Radio Sonde, a radio transmitter that is carried
high into the atmosphere (up to 150 000 ft) by a hydrogen balloon and sends back continuous
readings of pressure, temperature, and humidity to stations on the ground.
Temperature is measured to the nearest 0.1°C and reported to the nearest whole number. If the
temperature ends in 0.5, it is rounded to the nearest odd whole number.
4-2
Meteorology
Temperature
Chapter 4
HEATING OF THE ATMOSPHERE
The atmosphere is heated by five different processes:
1. Solar radiation
2. Terrestrial radiation
3. Conduction
4. Convection
5. Latent heat of condensation
A sixth process, advection, is responsible for the horizontal transfer of heat.
We will look at each of these processes in turn.
SOLAR RADIATION
Radiation from the sun is of the short-wave type. Most of the radiation that reaches the Earth’s
surface is of wavelengths less than 2 microns.
Nearly all the radiation passes through the Earth’s atmosphere without heating it. Ultra-violet
radiation is absorbed by ozone in the stratosphere. Still more is reflected by cloud cover. But on a
clear day, about 85% of the sun’s radiation will reach the Earth’s surface.
The radiation does not heat the atmosphere directly but does heat the surface of the Earth. This
process is called insolation. The atmosphere then becomes heated by the other processes
described below.
The amount of insolation (heating of the surface) depends on the angular elevation of the sun.
This in turn depends on latitude, season, and time of day.
Latitude
As can be seen from the diagram below, as you move further from the equator, the
curvature of the Earth means that the same amount of solar radiation is spread over a
larger area of the Earth’s surface. So insolation is less at higher latitudes
Meteorology
4-3
Chapter 4
Temperature
Season
For the same reasons mentioned above, the sun heats the Earth more efficiently if it is
directly overhead. Where this occurs depends on the time of year.
At the equinoxes, the sun is overhead the equator; at Summer Solstice (21st June) it is
overhead the Tropic of Cancer (23.5°N); at Winter Solstice (21st December) it is
overhead the Tropic of Capricorn (23.5°S).
Time of day
The amount of insolation is greatest at noon when the sun is highest in the sky.
TERRESTRIAL RADIATION
The Earth’s surface absorbs large amounts of solar radiation at short wavelengths and retransmits it as smaller amounts of long-wave radiation, between 4 and 80 microns.
This is the main method by which the atmosphere is heated. Since the atmosphere is heated
from below, it gets colder as you move away from the surface of the Earth. This is the reason for
the temperature lapse rate.
CONDUCTION
Conduction occurs when two bodies are touching one another. Heat passes from the warmer
body to the colder body. For example, heat passes from a warm ground surface to the air.
At night, the ground cools quickly due to lack of insolation from the sun. The air in contact with the
ground loses heat by conduction. As air is not a very good conductor, air at a higher level remains
warm, which results in a temperature inversion.
4-4
Meteorology
Temperature
Chapter 4
CONVECTION
As air is heated by conduction or radiation, it becomes less dense and tends to rise. Likewise,
cold air is more dense and subsides. This vertical movement of air is called convection. This
process helps heat the upper levels of the atmosphere.
LATENT HEAT OF CONDENSATION
When heat is used to alter temperature it is called sensible heat. Heat used to alter the state of a
substance is referred to as latent heat (latent meaning hidden), as no temperature change
occurs.
For example, when water turns from vapour to droplets in the atmosphere, it is turning from the
gaseous state to the liquid state. Heat is released when this occurs.
Likewise, when it turns from liquid to gas, it absorbs heat to effect the change, but the actual
temperature remains constant within the substance.
As air is lifted it cools and is no longer able to hold as much water vapour. This condenses out as
water droplets and latent heat is released, warming the atmosphere.
ADVECTION
Advection is the process by which air moves horizontally. The movement is caused by variations
in pressure, but the air takes with it its characteristics, including its temperature.
DIURNAL VARIATION OF TEMPERATURE
The maximum amount of insolation occurs at noon when the sun is high in the sky. As the earth
takes time to heat up, it does not immediately transfer the heat out to the atmosphere — there is
a slight lag. This means that the highest air temperature occurs at about 1500 local time. The
lowest temperature occurs about a half an hour after sunrise, again due to lag.
Meteorology
4-5
Chapter 4
Temperature
THE EFFECT OF CLOUD COVER ON DIURNAL VARIATION
During the day, clouds prevent some solar radiation from reaching the Earth, hence reducing the
maximum temperature that the air near the surface reaches during the day.
At night, clouds trap some of the heat between them and the ground, hence raising the minimum
temperature that the air drops to at night.
The overall effect is to reduce the diurnal variation.
THE EFFECT OF WIND ON DIURNAL VARIATION
During the day, wind causes surface air to be mixed with cooler air above. The amount of time
that any air is in contact with the warm ground is short, so the maximum temperature the air near
the surface reaches is lower compared to calm conditions.
During the night, terrestrial radiation leads to a reduction in air temperature close to the ground.
Any wind causes mixing of the cold surface air with warmer air above. Therefore, the minimum
temperature of the air above the surface at night is not as low as it would be in calm conditions.
The overall effect is to reduce diurnal variation.
THE EFFECT OF SURFACE ON DIURNAL VARIATION
How much a surface heats up when exposed to insolation depends on its specific heat. The
specific heat is the amount of heat required to raise the temperature of the surface by 1°C.
Some examples of surfaces listed in the order of increasing specific heat follows:
1. Bare rock/stone
2. Concrete
3. Dry soil
4. Wet soil
5. Oceans
6. Snow surfaces
Those surfaces that take a long time to heat up also lose their heat very slowly, so the diurnal
variation over the sea is minimal but is much greater over the land.
Not only does water have a much higher specific heat than land, but due to the movement of the
sea surface, the energy is spread to a depth of several metres, whereas solar radiation only heats
the top few inches of the land surface.
Topics found later in the course detail why the different properties of land and sea are important.
4-6
Meteorology
Temperature
Chapter 4
SUMMARY
In summary, greatest diurnal variation can be found over the land, with clear skies and no wind.
Least diurnal variation can be found over the sea and over the ice caps, when skies are cloudy
and it is windy.
THE GREENHOUSE EFFECT
Water vapour and carbon dioxide are transparent to short wavelength radiation, but they are less
permeable to long wavelengths. This means they allow solar radiation to reach the surface, but
do not allow all of the terrestrial radiation to leave the atmosphere and go back into space.
This leads to an increase of temperature at ground level, a process called the greenhouse effect,
since the glass in a greenhouse works in a similar way.
Meteorology
4-7
Chapter 4
Temperature
4-8
Meteorology
INTRODUCTION
Most water in the atmosphere is in the form of water vapour, which is water in its gaseous state.
This water cannot be seen. In order for water to become visible in the form of clouds, mist, or fog
it must turn into water droplets or ice crystals.
WATER STATES AND LATENT HEAT
Water can exist in three basic states: solid (ice), liquid (water), and gas (water vapour). When
changing from one state to another, latent heat is either released or absorbed.
EVAPORATION
This is the change of state from a liquid to a gas. Gas is a higher energy state than liquid so latent
heat is “absorbed” during this process.
Evaporation can take place at any temperature above absolute zero, but the rate of evaporation
is greater at higher temperatures.
MELTING
This is the change of state from a solid to a liquid. Liquid is a higher energy state than solid so
latent heat is “absorbed” during this process.
Meteorology
5-1
Chapter 5
Water in the Atmosphere
SUBLIMATION
Sometimes a substance can turn directly from a solid to a gas or from a gas to a solid without
passing through the intermediate liquid state. The term sublimation can be used to describe this
process in both directions. The change from gas to solid, however, can also be referred to as
deposition.
Latent heat is “absorbed” when a solid turns to a gas.
Latent heat is “released” when a gas turns to a solid. This process is important in the formation of
frost, hail, and some airframe icing.
CONDENSATION
This is the change of state from a gas to a liquid. Liquid is a lower energy state so latent heat is
“released”.
Condensation nuclei must be present in order for condensation to occur in the atmosphere.
Condensation nuclei are tiny particles of hygroscopic (water attracting) material, such as dust and
pollution.
FREEZING
This is the change of state from a liquid to a solid. Solid is a lower energy state so latent heat is
“released”.
For this to occur, freezing nuclei are required, similar to those for condensation. Without them,
the water droplets in the atmosphere become supercooled, which means they remain as a liquid
state despite being lower than freezing temperature.
Supercooled droplets are a major cause of airframe icing. They are discussed again later in the
course.
SATURATION
As water evaporates into the air, there comes a point in which the air can no longer accept any
more water vapour. The amount of vapour that air can hold is dependent on its temperature and
pressure.
The higher the temperature, the more water vapour the air can hold.
When the air contains the maximum amount of water vapour it can hold, it is described as being
saturated.
The air can become saturated in two ways: extra water vapour can be added, or the air can be
cooled, since cooler air holds less water vapour.
HUMIDITY
Humidity refers to the amount of water vapour in the air. It is often expressed as a percentage
and is known as relative humidity.
5-2
Meteorology
Water in the Atmosphere
Chapter 5
ABSOLUTE HUMIDITY
Absolute humidity is the actual mass of water in a given volume of air and is generally expressed
in g/m3.
SATURATION CONTENT
Saturation content is the mass of water a given volume of air can hold, not that which it is actually
holding, again expressed as g/m3.
RELATIVE HUMIDITY
Relative humidity is an expression of how much water vapour is in the air, expressed as a
percentage of the maximum amount the air could hold at that temperature and pressure. Hence:
RELATIVE HUMIDITY (RH)
=
AMOUNT OF WATER VAPOUR IN THE AIR %
AMOUNT OF WATER VAPOUR THE AIR CAN HOLD
=
ABSOLUTE HUMIDITY %
SATURATION CONTENT
Example:
If the absolute humidity is 12 g/m3 and the saturation content is 26 g/m3,
what is the relative humidity?
Relative Humidity = Absolute Humidity / Saturation Content
= (12 ÷ 26) = 0.462 = 46.2%
Please attempt the following simple RH calculations. The answers can be found at the end of the
chapter:
Exercise 1:
Absolute Humidity
(g/m3)
Saturation Content
(g/m3)
6
20
Relative Humidity
(%)
34
14
45
30
HUMIDITY MIXING RATIO
Humidity mixing ratio (HMR) is similar to absolute humidity but is the mass of water in a certain
mass of air. The unit for this is therefore g/kg rather than g/m3.
Typically, the HMR is between 5 and 50 g/kg in temperate latitudes.
Meteorology
5-3
Chapter 5
Water in the Atmosphere
HMR FOR SATURATION CONTENT / SATURATION MIXING RATIO
The saturation mixing ratio (SMR) is the HMR when the parcel of air is saturated. Hence relative
humidity can also be expressed as:
RELATIVE HUMIDITY (RH)
=
HMR %
HMR FOR SATURATION CONTENT
SUPER-SATURATION
As mentioned earlier, condensation only occurs if there are condensation nuclei present. If no
nuclei are present, then the water remains as vapour and the air is described as super-saturated.
This means there can conceivably be a relative humidity greater than 100%.
SATURATION AND DEWPOINT
The graph below shows the HMR for saturation plotted against the temperature in °C. The higher
the temperature, the larger the amount of water the air can hold. However, the relationship is not
linear, it is logarithmic.
30
HMR for Saturation in g/kg
25
20
15
10
5
0
-30
-20
-10
0
10
20
30
Temperature in degrees C
5-4
Meteorology
Water in the Atmosphere
Chapter 5
It follows that if a parcel of air contains a certain amount of water vapour and is cooled, it will be
able to hold less water vapour. If it continues to cool, it eventually reaches a point where the
amount of vapour it can hold is equal to the amount it is actually holding. The air is said to be
saturated.
The temperature at which this occurs is called the dewpoint. A parcel of air at 20°C with a HMR of
7 g/kg (as seen on the graph) is not saturated. Air at 20°C can hold up to 14 g/kg.
What happens if air is cooled to 10°C? Based on the graph, the HMR for saturation is 7 g/kg.
Therefore, the air is saturated — the relative humidity is 100%. So the dewpoint for air containing
7 g/kg is 10°C.
Cooling the air beyond this point results in water vapour condensing to become droplets, which
causes clouds, fog/mist, or dew.
Relative humidity also has an effect on the rate of evaporation. Evaporation does not occur if the
air is saturated. Warmer air can take more vapour so is less likely to be saturated. However,
evaporation can still occur if the air above the liquid is cold, especially if there is a breeze to take
away the saturated air and replace it with dry air.
Note:
The term dry air is used to describe any air that is not saturated. So, even air with
a RH of 99% is still dry. Completely dry air, that is air with an RH of 0%, does not
occur in the atmosphere.
Using the graph above answer the following questions:
Exercise 2:
The HMR is 4 g/kg. The temperature is 20°C. What is the RH?
Exercise 3:
The HMR is 15 g/kg. What is the dewpoint?
Exercise 4:
The dewpoint is 18°C. The RH is 40%. What is the HMR?
CONDENSATION LEVEL
When unsaturated air is cooled, it eventually reaches its dewpoint and water vapour condenses
out as water droplets.
One way in which a pocket of air may cool is if it is lifted. As the air rises it cools. Once it reaches
a level where the RH becomes 100%, any further lifting leads to condensation. This level is
referred to as the condensation level.
As air rises it is said to cool adiabatically. Likewise, as air descends it is said to warm
adiabatically. This process of adiabatics and how it relates to dewpoint and cloud formation is
discussed more fully in the chapter on Stability.
Meteorology
5-5
Chapter 5
Water in the Atmosphere
DIURNAL VARIATION OF HUMIDITY
Assuming the absolute humidity of the air remains constant, the relative humidity varies as the
temperature varies. Cold air can hold less water, so just after dawn, when temperature is at its
lowest, RH is at its highest. This is why mist and fog are most likely to form around dawn.
Throughout the day as the temperature increases with increased insolation, the relative humidity
decreases, dropping to its lowest value at about 1500 LMT when the air temperature is at its
greatest.
After this, the temperature starts to drop again, so the RH starts to rise.
WATER VAPOUR PRESSURE
This is the part of the atmospheric pressure that is exerted by the water vapour present. When
the air is saturated, the water vapour pressure is called Saturation Vapour Pressure. The
dewpoint depends on the vapour pressure. The lower the vapour pressure, the lower the
dewpoint.
As air rises, it expands and cools. Its overall pressure goes down so the pressure exerted by the
water vapour also goes down. This leads to the dewpoint decreasing as well. The dewpoint
decreases by about .5°C per 1000 ft gain in height.
Yet another formula for dewpoint arises from the relationship between water vapour pressure and
saturation vapour pressure:
RELATIVE HUMIDITY (RH)
=
VAPOUR PRESSURE (hPa)
%
CORRESPONDING VAPOUR PRESSURE FOR SATURATION
5-6
Meteorology
Water in the Atmosphere
Chapter 5
SATURATION VAPOUR PRESSURE CURVE
12
Vapour Pressure in hPa
10
8
Ice
6
Water
4
2
0
-25
-20
-15
-10
-5
0
5
10
Temperature in degrees C
Saturation vapour pressure depends on a number of factors. The graph above shows that the
saturation vapour pressure is higher over ice than over water.
Other factors affecting saturation vapour pressure are:
1. Higher above a curved surface than a flat surface
2. Higher over clean water than a salt solution
3. Higher around a supercooled droplet than an ice crystal
Meteorology
5-7
Chapter 5
Water in the Atmosphere
MEASUREMENT OF HUMIDITY
PSYCHROMETER
DRY BULB
WET BULB
MUSLIN
CLOTH
DISTILLED
WATER
To calculate humidity and dewpoint, a Psychrometer or Wet and Dry Bulb Hygrometer is used.
This apparatus consists of two mercury thermometers. One, the dry bulb thermometer, is an
ordinary thermometer that measures the air temperature. The other, the wet bulb thermometer,
has a piece of muslin cloth wrapped around the bulb. The other end of this cloth is dipped in a
container of distilled water.
As the water evaporates from the cloth, latent heat is drawn from the immediate surroundings.
This causes the wet bulb temperature to be lower than the dry bulb temperature. The wet bulb
temperature is the lowest temperature to which the air can cool by evaporation.
Note that if the air is already saturated, no evaporation occurs and the two readings are the
same. In this case the temperature displayed will also be the dewpoint.
The two figures obtained can be used to look up the dewpoint, RH, and HMR from tables.
An approximation of the dewpoint can be made using the following method:
1. Subtract the wet bulb temperature from the dry bulb temperature.
2. Subtract this figure (the wet bulb depression) from the wet bulb temperature.
5-8
Meteorology
Water in the Atmosphere
Example:
Chapter 5
Dry Bulb Temperature
Wet Bulb Temperature
20°C
15°C
Wet Bulb Depression = Dry Bulb Temperature – Wet Bulb Temperature = 5°C
Dewpoint Temperature = Wet Bulb Temperature – Depression = 15° - 5° = 10°C
Please complete the following dewpoint calculations:
Exercise 5:
Dry Bulb Temperature (°C)
Wet Bulb Temperature (°C)
22
15
Dewpoint (°C)
18
10
12
3
HUMIDITY METHOD
Another method of approximating the dewpoint is from the RH and air temperature. The formula
is:
DIFFERENCE BETWEEN TEMPERATURE AND DEWPOINT =
(100 – RH)
5
Example:
The temperature is 23°C and the relative humidity is 80%. What is the
dewpoint?
Difference = (100 – 80) ÷ 5 = 4°C
Dewpoint = 23°C - 4°C = 19°C
Test your understanding of the formula by completing the following table:
Exercise 6:
Air Temperature (°C)
Relative Humidity (%)
18
70
12
4
85
Meteorology
Dewpoint (°C)
2
5-9
Chapter 5
Water in the Atmosphere
ANSWERS TO EXERCISES
Exercise 1:
Absolute Humidity
(g/m3)
Saturation Content
(g/m3)
Relative Humidity
(%)
6
20
30
15.3
34
45
14
46.7
30
Exercise 2:
28.6%
Exercise 3:
21°C
Exercise 4:
4.8 g/kg
Exercise 5:
Dry Bulb Temperature (°C)
Wet Bulb Temperature (°C)
Dewpoint (°C)
22
15
8
18
14
10
21
12
3
Exercise 6:
Air Temperature (°C)
Relative Humidity (%)
Dewpoint (°C)
18
70
12
12
60
4
5
85
2
5-10
Meteorology
INTRODUCTION
The density of a substance is its mass per unit volume. Density in the atmosphere is usually
expressed as grams per cubic metre (g/m3). It may also be expressed as a percentage of the
standard surface density. This is called relative density.
Example 1:
As chapter one detailed, the standard surface density is 1225 g/m3.
Hence if the actual density is 900 g/m3, the relative density would be:
900 × 100 = 73.47%
1225
Example 2:
If the actual density is 1500 g/m3 what is the relative density?
1500 × 100 = 122.45%
1225
A third way in which density may be expressed is as density altitude. This is described later in
the chapter.
THE IDEAL GAS LAWS
An ideal gas is one that is incompressible and without viscosity. The atmosphere is assumed to
be an ideal gas. There are several gas laws that apply.
In the next few formulae, the following key applies:
P = Pressure
V = Volume
T = Temperature
ρ = Density
Meteorology
6-1
Chapter 6
Density
BOYLE’S LAW
At constant temperature, as the pressure of gas increases, its volume must decrease. Therefore
the pressure is inversely proportional to volume:
Pα
1
V
To remove the proportional sign, use:
P=
Constant
V
so:
PV = Constant
or:
P1V1 = P2V2
CHARLES’S LAW
At constant pressure, if the temperature of a gas increases, the gas expands. In other words, its
volume increases. The temperature is proportional to volume:
TαV
To remove the proportional sign, use:
V = Constant x T
so:
V
T
= Constant
or:
V1
T1
=
V2
T2
THE GAS EQUATION
Combining Boyle’s and Charles’s laws, the gas equation becomes (where R is the gas constant):
PV = RT
6-2
Meteorology
Density
Chapter 6
Density can also be a part of the equation. In an ideal gas, as volume increases, density
decreases. This is due to the same mass of air being contained in a larger volume.
So:
ρα
1
V
Substituting this into the ideal gas equation:
P
ρ
= RT
Re-arranging to make density the subject of the equation:
ρ=
P
RT
So, maintaining a constant temperature: if pressure goes up, density goes up. Maintaining a
constant pressure: if temperature goes up, density goes down.
EFFECT OF WATER VAPOUR ON AIR DENSITY
Water vapour is less dense than air: approximately 5/8 of the density of dry air. Therefore, all other
things being equal, the density is lower in more humid atmospheres. This difference is usually
insignificant and can be ignored for aviation purposes. In the tropics, however, where it can be
very humid, it can make a large difference.
VARIATION OF SURFACE AIR DENSITY WITH LATITUDE
Air density is lowest with low pressure and high temperature. So in the equatorial regions, density
at the surface is low.
High pressure and low temperature equates to high density. Examples of this can be found at the
poles or at the centre of a large land mass in winter, (e.g. Siberia).
So, in general, density increases with increasing latitude.
The lowest density can be found at an aerodrome that is not only hot and high, but humid. An
example is Nairobi, which is very close to the equator, so experiences high temperatures and
humid conditions. It is also at an elevation of about 5500 ft, so has all the attributes that contribute
to low density.
VARIATION OF AIR DENSITY WITH HEIGHT
As height increases, both the temperature and pressure decrease. Based on the gas laws, a
decrease in temperature leads to an increase in density and a decrease in pressure leads to a
decrease in density.
Meteorology
6-3
Chapter 6
Density
So, with one law trying to increase the density and one trying to decrease it, will it therefore stay
constant?
The answer is no. Since pressure near the surface decreases by about 10 hPa per 300 ft, this
would produce a reduction in density of about 1%.
A similar height increase would cause a drop in temperature of less than 1°C. This would lead to
an increase in density of about 0.3%.
The change in pressure has more of an effect, therefore, density decreases with height.
This leads to the following observations:
20 000 ft
Density is 50% of the surface value.
40 000 ft
Density is 25% of the surface value.
60 000 ft
Density is 10% of the surface value.
VARIATION OF AIR DENSITY WITH LATITUDE AND HEIGHT
As already mentioned, the air density at the surface tends to increase with increased latitude and
density decreases with increased height. Now it’s time to bring those two factors together.
Consider two columns of air of equal heights. Both columns have the same pressure at the base,
but one column of air is cold and the other warm.
LOW
PRESSURE
HIGH
PRESSURE
1013 hPa
The cold air has a higher density, so as height increases there is a greater reduction in mass and
the change in pressure is greater. Conversely, the warm air is less dense, so there is a small
reduction of mass above as height increases. The change in pressure is less, so pressure at the
top of the cold column is lower than at the top of the warm column.
6-4
Meteorology
Density
Chapter 6
This is important when considering global patterns in density. At the equator, the air temperature
is high, so density at the surface is relatively low, as is pressure. At the Poles, the air temperature
is low, so density at the surface is relatively high, as is pressure.
However, as height increases over the equator, pressure, and therefore density, decreases
relatively slowly, like in the warm column of air described above.
As height increases over the Poles, pressure, and therefore density, decreases relatively quickly,
like in the cold column of air described above.
At the equator there is a relatively low density at the surface, compared to the Poles, but a
relatively high density at height, as the density decreases only slowly.
At the Poles there is a relatively high density at the surface, but a relatively low density at height,
as the density decreases quickly.
At approximately 26 000 ft the density is constant at all latitudes.
DIURNAL VARIATION OF DENSITY
Density is highest when temperatures are lowest, that is, just after dawn. It is at its lowest at
about 1500 LMT when temperatures are highest.
DENSITY ALTITUDE
The density altitude at which you are flying is the pressure altitude in the International Standard
Atmosphere at which that density would occur.
Meteorology
6-5
Chapter 6
Density
Logically, if it is warmer than ISA, your density altitude is higher than your pressure altitude and
vice versa for colder than ISA conditions.
The diagram below shows two columns of air: one is at ISA and the other is warmer than ISA.
ISA
WARMER
THAN ISA
1000 g/m
1000 g/m
3
3
1225 g/m
10 000 ft
3
0 ft
At surface level in ISA the density is 1225 g/m3. At 10 000 ft, the density is 1000 g/m3. The
warmer column of air has been heated such that the air density at the surface has decreased to
1000 g/m3, the same as that at 10 000 ft in ISA conditions.
Hence the density altitude at the surface is 10 000 ft.
CALCULATING DENSITY ALTITUDE
120 ft
120 ft
ISA +
1°C
ISA
ISA 1°C
Density altitude differs from pressure altitude by 118.8 ft per 1°C deviation from ISA. In the JAR
exams it is sufficient to use 120 ft per 1°C deviation from ISA. Add the difference to the pressure
altitude if warmer than ISA, subtract if colder.
6-6
Meteorology
Density
Chapter 6
Example:
The pressure altitude is 20 000 ft. The ISA deviation is 4°C. What is the
density altitude?
It is warmer than ISA so:
Density altitude = Pressure altitude + (120 × ISA deviation)
= 20 000 + 480 = 20 480 ft
Exercise 1:
The pressure altitude is 15 000 ft. The ISA deviation is -5°C. What is the density altitude?
Exercise 2:
The pressure altitude is 8000 ft. The ambient temperature is 9°C. What is the density altitude (use
a lapse rate of 2°C/1000 ft)?
Exercise 3:
The density altitude is 26 000 ft. The ISA deviation is +8°C. What is the pressure altitude?
EFFECT OF DENSITY ON AIRCRAFT PERFORMANCE
Low density reduces the performance of engines and aerofoils.
Engines work by accelerating air backward in order to produce thrust. Less dense air has lower
mass. The lower the mass, the less thrust the engine produces.
The production of lift by aerofoils such as the wings also depends on the density. The formula for
lift is shown below:
2
LIFT = CL ½ ρV S
Where:
CL = COEFFICIENT OF LIFT
ρ = DENSITY
V = TRUE AIRSPEED
S = SURFACE AREA OF AEROFOIL
The amount of lift produced is directly proportional to the density. So if density is low the aircraft
will not produce as much lift, all other factors being equal.
This is very important on take-off and landing. In order to generate enough lift, the aircraft either
has to fly at a lower weight or a higher TAS. If a higher speed is chosen, then the aircraft requires
a longer take-off and landing run.
At an airport such as Nairobi, aircraft often have to operate with reduced weight at the hottest
time of the day.
Meteorology
6-7
Chapter 6
Density
ANSWERS TO EXERCISES
Exercise 1:
Density altitude = 15 000 - (120 × 5) = 14 400 ft
Exercise 2:
ISA temp for 8000 ft = 15 – (2 × 8) = -1°C. ISA deviation is therefore +10°C.
Density altitude = 8000 + (120 × 10) = 9200 ft
Exercise 3:
Density altitude = Pressure altitude + (ISA deviation × 120)
Hence:
Pressure altitude = Density altitude – (ISA deviation × 120)
= 26 000 – (8 × 120) = 25 040 ft
6-8
Meteorology
INTRODUCTION
The processes leading to cloud formation and precipitation depend greatly on the stability of the
atmosphere. In order to understand the concepts of stability and instability, you must understand
the concept of adiabatics.
ADIABATIC PROCESSES
As a bubble of air rises, the pressure in the surrounding atmosphere goes down and the bubble
expands. This leads to the temperature within the bubble decreasing. This is called adiabatic
cooling.
Conversely, if a bubble of air descends, it compresses and the temperature increases. This is
called adiabatic warming.
Air is not a very good conductor, so there is very little exchange of heat with the surrounding
environment.
Hence, an adiabatic process is one in which the temperature changes within the system but there
is no exchange of energy with the surroundings.
THE DRY ADIABATIC LAPSE RATE
When dry air (unsaturated air) is forced to rise, it cools at what is called the Dry Adiabatic Lapse
Rate (DALR). This has been found to be 3°C/1000 ft. This is the same regardless of how close to
saturation the air is. It is also independent of pressure and temperature.
THE SATURATED ADIABATIC LAPSE RATE
Once the air reaches saturation, water vapour starts to condense if the air is cooled any further.
This process of condensation releases latent heat, as discussed in earlier chapters.
This means that the temperature does not decrease as much as if it were dry, due to this extra
heat being added into the system. The rate is referred to as the Saturated Adiabatic Lapse Rate
(SALR).
The actual amount of heat released as latent heat depends on the amount of condensation that
occurs. In cold temperatures, even when the air is saturated, the actual amount of water vapour
present is low, so very little latent heat is released. In this situation, the SALR is nearly as high as
the DALR.
Meteorology
7-1
Chapter 7
Stability
In hot temperatures, saturated air contains a large amount of water vapour and condensation
releases large amounts of latent heat. The SALR, therefore, is considerably lower than the DALR.
The average SALR is taken to be 1.5°C/1000 ft.
THE ENVIRONMENTAL LAPSE RATE
This is the lapse rate of the air in the environment, that is, the air surrounding the adiabatic
system, not within the system itself. This air is not in vertical motion.
The ELR is variable. As discussed in Chapter 1, the average ELR is 1.98°C/1000 ft.
SUMMARY OF ADIABATICS
The following diagram shows the DALR and the SALR. The DALR is constant at 3°C/1000 ft, but
the SALR is not constant. As the height increases, the SALR approaches the DALR.
SALR
Height
DALR
Temperature
Where the ELR falls in this picture is discussed later in this chapter.
STABILITY OF THE AIR
Air that is warmer than its surrounding environment is less dense and rises. This is called
instability.
Air that is colder than its surrounding environment is more dense and sinks. This is called
stability.
Air that is the same temperature as its surrounding environment neither rises nor sinks. It is
neutral.
The stability of the atmosphere depends on the relationship between the ELR and the DALR and
SALR.
7-2
Meteorology
Stability
Chapter 7
ABSOLUTE STABILITY
Consider the following example. The ELR is 1°C/1000 ft. The diagram demonstrates what
happens when air is forced to rise. One bubble of air is dry, one is saturated.
DALR
SALR
4000 ft
3°C
9°C
3000 ft
6°C
10.5°C
2000 ft
9°C
12°C
1000 ft
12°C
ELR 1°C/1000 ft
11°C
12°C
13°C
14°C
13.5°C
15°C
15°C
Dry air
Saturated air
The surface temperature is 15°C. The dry air cools at 3°C, faster than the surrounding
environment is lapsing. This means that at each level the dry bubble of air is colder than the
surrounding environment, and therefore more dense, so it wants to sink.
The saturated air cools at 1.5°C, again faster than the lapse rate of the environment. So at each
level, the saturated bubble is colder and it too wants to sink.
This situation is known as absolute stability since, regardless of whether the air is saturated or
not, the air is stable.
ABSOLUTE INSTABILITY
Now, consider the diagram below. The ELR is 5°C/1000 ft, greater than both the DALR and the
ELR.
DALR
SALR
ELR 5°C/1000 ft
4000 ft
8°C
14°C
3000 ft
11°C
15.5°C
2000 ft
14°C
17°C
1000 ft
17°C
18.5°C
Meteorology
5°C
10°C
15°C
20°C
20°C
Dry air
0°C
Saturated air
7-3
Chapter 7
Stability
The unsaturated air cools at 3°C/1000 ft and at each level is warmer than the surrounding
environment. Thus, it’s less dense and, therefore, tends to keep rising.
The saturated air cools at 1.5°C/1000 ft. At each level it is warmer than the surrounding
environment, hence less dense. It too tends to keep rising.
We call this situation absolute instability.
CONDITIONAL INSTABILITY
Now consider a situation in which the ELR is between the SALR and the DALR, as in the diagram
below.
DALR
SALR
ELR 2°C/1000 ft
4000 ft
8°C
14°C
3000 ft
11°C
15.5°C
2000 ft
14°C
17°C
1000 ft
17°C
18.5°C
20°C
Dry air
12°C
14°C
16°C
18°C
20°C
Saturated air
The environmental temperature is lapsing at 2°C/1000 ft. The unsaturated air is cooling at
3°C/1000 ft and at each level it is cooler than the surrounding environment, so it wants to sink.
The saturated air, however, is cooling at 1.5°C/1000 ft, so at each level it is warmer than the
surrounding environment and it tends to rise.
This situation is called conditional instability. The air is stable when unsaturated, but unstable
when saturated.
7-4
Meteorology
Stability
Chapter 7
SUMMARY OF STABILITY
SALR
DALR
Conditional
instability
Height
Absolute
stability
Absolute
instability
Temperature
The diagram shows the stability of the air when the ELR falls in different areas of the graph.
Absolute Stability
ELR < SALR < DALR
Absolute Instability
ELR > DALR > SALR
Conditional Instability
DALR > ELR > SALR
Note that in all the above cases, an initial trigger action is required to start the air rising. There are
several forms that this trigger can take, which are discussed thoroughly in the chapter on Cloud
Formation.
NEUTRAL STABILITY
There is one more type of stability not yet mentioned. If the air is unsaturated and the ELR is
exactly 3°C/1000 ft, then the rising air is cooling at the same rate that the environment is lapsing.
So the air is neutrally stable.
If the air was saturated, the ELR would have to be identical to the SALR for the air to be neutral.
Meteorology
7-5
Chapter 7
Stability
CONVECTIVE OR POTENTIAL INSTABILITY
Potentially unstable air occurs when horizontal air motion is present at the same time air is being
lifted, such as in a low pressure centre or along a frontal surface.
The air in the lower layers must be saturated and the air in the upper layers must be dry, as
demonstrated in the following diagram.
25 – 24
= 1°C
ELR 3.4°C/1000 ft
– unstable
Cools
at the
DALR
5000 ft
30 – 12
=18°C
8000 ft
Unsaturated air
Cools
at the
SALR
25°C
5000 ft
ELR 1°C/1000 ft
– initially stable
30°C
Saturated air
The diagram above shows that before lifting occurs, the ELR is lower than the SALR, therefore,
the layer is stable. The lower air cools at the SALR as it is lifted because it is saturated. Since the
air above is dry, it cools at the DALR. When the air reaches the top of the obstruction, the
temperature difference between the bottom of the 5000 ft layer and the top has increased, hence
the ELR has increased. It is now greater than the DALR, so the layer is unstable.
In the next diagram, the lower air is dry and the upper air is saturated, so the opposite occurs.
Initially the ELR is high, but as the air cools, the temperature difference decreases, lowering the
ELR to below the SALR and making the layer stable.
7-6
Meteorology
Stability
Chapter 7
14 – 12=
2°C
ELR 0.8°C/1000 ft
– stable
Cools
at the
SALR
5000 ft
30 – 24
=6°C
8000 ft
Saturated air
Cools
at the
DALR
14°C
5000 ft
ELR 3.2°C/1000 ft
– initially unstable
30°C
Unsaturated air
In summary, the following processes increase stability:
1.
2.
3.
4.
Advection of cold air or other cooling at low level
Advection of warm air or other heating of upper air
Decreased humidity at low levels or infusion of dry air at high levels
Descending air motions such as subsidence created behind mountains in high
pressure centres or through divergence at low level
Factors that lead to increased instability are
1. Advection of warm air or heating of the air at low level
2. Advection of cold air or other cooling of the upper air such as night time radiation
from the top of clouds
3. Increased humidity at low level
4. Enforced lifting which may lead to conditional instability (over mountains, on shore
winds at coasts etc)
5. General lifting, as in low pressure centres and in the case of convergence
INVERSIONS
Inversions are extremely stable, as the ELR is in fact negative.
The most common inversion forms at low level during clear nights, when radiation and cooling at
ground level is at its maximum. This is known as a ground inversion.
When the surface is snow covered, the cooling can be intense, and surface temperature is often
10°C lower than at the level of the Stevenson Screen (1 – 2 m). From the screen upward, the air
temperature rises 10 – 20°C in extreme cases.
Meteorology
7-7
Chapter 7
Stability
In broken terrain, the cooled air at the surface drains into the lowest area of the ground, creating
what is called a katabatic wind. This can lead to fog formation. This is discussed in more detail in
later chapters.
When winds are light and the ground is covered with snow, the inversion may be at 4000 ft to
7000 ft and dominates the weather situation.
Inversions can form in the troposphere, when warm air moves over a colder layer of air, for
example, with a warm front. In many cases, clouds form in the inversion but these do not have
strong vertical air currents.
CLOUD FORMATION
As discussed at the beginning of the chapter, you must understand the concepts of stability and
adiabatics in order to understand the processes of cloud formation.
THE DRY THERMAL
Consider the following hypothetical situation. The surface temperature is 10°C and the
environmental temperature lapses normally until height ‘X’, then there is an inversion. The
surface is heated at a particular location, which causes the temperature to rise to 20°C. The air in
this region becomes less dense and starts to rise.
ELR
Height
X
DALR
0
5
10
15
20
Temperature in °C
This is what is known as a “thermal.” In this case, the air is unsaturated so it is called a dry
thermal.
Because the air is unsaturated, it cools at the DALR — faster than the environment and hence
eventually the two lines will intersect. In the hypothetical example, the two lines intersect at X, the
height at which the inversion starts.
If the thermal were to continue to rise it would follow the dotted line, so it would be cooler than its
environment. Therefore, it will be more dense and no longer has the tendency to rise.
If you were to fly below height X, you would experience turbulence due to the updrafts in the
thermal. Above height X, the conditions would be smooth.
7-8
Meteorology
Stability
Chapter 7
FORMATION OF A CLOUD
Dewpoint was not taken into account in the previous example, which assumed the air never
reaches saturation. What would happen if, at some point in the rise of the air, it became
saturated? The following diagram represents this situation.
ELR
Height
X
SALR
LCL
DALR
DP
0
5
10
15
20
Temperature in °C
As before, the trigger action is surface insolation, which leads to the formation of a thermal that
starts to rise. However, now there is a line representing the dewpoint, which has a lapse rate of
0.5°C/1000 ft.
In the diagram, the DALR line intersects the dewpoint line before it intersects the ELR line. Hence
the thermal has reached saturation before it has stopped rising. At this point, water vapour starts
to condense to form cloud.
The thermal is still warmer than the environment so it continues to rise. However, its temperature
now falls at the SALR. It eventually intersects the ELR and stops rising.
So, the base of the cloud is the point in which the DALR intersects the dewpoint line, known as
the lifting condensation level. The top of the cloud is where the SALR intersects the ELR.
Once the thermal reached saturation, the lapse decreased to the SALR. The lower the SALR the
longer it will take for this line to intersect the ELR. Warm air has a higher moisture content when
saturated so it has a lower SALR due to the large amounts of latent heat released.
If the air is cold, the SALR is close to the DALR and the line intersects the ELR quickly. Hence,
warmer air leads to a thicker cloud forming than those formed in colder air. The diagram on the
next page demonstrates this scenario.
Meteorology
7-9
Chapter 7
Stability
ELR
Cloud top for warm air
Height
X
Cloud top for cold air
LCL
DALR
Warm air SALR
Cold air SALR
DP
0
5
10
15
20
Temperature in °C
CALCULATING CLOUD BASE
If the dewpoint was constant, we could quite easily calculate the height that the cloud base would
form. It would simply be:
(T – Td) ÷ 3 × 1000
Where:
T = surface temperature
Td = dewpoint
This would give an answer in feet.
However, since the dewpoint is also lapsing, it is not quite as simple as this. The temperature to
which the thermal must fall must be the same as the temperature to which the dewpoint must fall.
This is referred to ‘t’:
t = T – (3H ÷ 1000)
but also:
t = Td – (0.5H ÷ 1000)
Where ‘H’ is the height of the cloud base in feet.
Hence:
T – 3H ÷ 1000 = Td – 0.5H ÷ 1000
7-10
Meteorology
Stability
Chapter 7
Rearranging the formula to make H the subject:
H = (T – Td)400
Using the same process, the following formula is derived:
h = (T – Td)125
Where ‘h’ is the cloud base in metres.
You must memorise both formulae. The derivation, however, is for your information only. Note
that the above formulae are only valid for convective clouds, that is, those formed by thermals.
FORECASTING CLOUD FORMATION
When forecasters determine whether or not convective clouds are likely to form, they must initially
select a representative environmental lapse rate curve for the air mass in question. The dewpoint
at the ground is checked and then assumptions are made of the development of the air
temperature near the ground (amount of cloud, insolation, estimated maximum temperature, etc.).
The condensation level can be calculated based on the forecast temperature and current
dewpoint.
To forecast convection a comparison is made between the lifting (path) curve with the actual
lapse rate curve. When such a comparison is made, four main types can be distinguished.
The following key applies:
DALR
SALR
ELR
Dewpoint
Meteorology
7-11
Chapter 7
1.
Stability
The condensation level is on the cold side of the lapse rate.
No clouds form, dry thermals only.
Rate of ascent of the thermals 0.5 – 2 m/s.
Over hot (dry) surfaces, the dry thermals may be much stronger.
Height
Temperature
2.
The condensation level is on the warm side of the lapse rate.
The moist adiabatic lapse rate intersects the environmental lapse rate curve rather early.
Small convective clouds form.
Rate of ascent 1 – 4 m/s below clouds, 5 – 10 m/s inside the clouds.
Height
Temperature
7-12
Meteorology
Stability
3.
Chapter 7
The condensation level is on the warm side of the lapse rate curve.
The moist adiabatic air does not intersect the lapse rate curve until high level. Large
convective clouds form.
Hail and electrical discharges may occur.
Rate of ascent at tens of metres/sec in the cloud subjects the aircraft to heavy
turbulence.
Height
Temperature
4.
The condensation level is on the cold side of the lapse rate and no clouds form.
If the air is forced to rise, e.g. over an obstruction, temperature is forced to cool to the
condensation temperature and the thermals begin to rise by themselves. This condition is
called Latent Instability.
Height
Temperature
Meteorology
7-13
Chapter 7
7-14
Stability
Meteorology
ACKNOWLEDGEMENTS
Thank you to Ashley Gibbs for the use of his photographs.
INTRODUCTION
Clouds are collections of water droplets, ice crystals, or a mixture of both. They provide
indications of:
1. possible turbulence
2. poor visibility
3. precipitation
4. icing
The average lifetime of a cloud is 15 – 20 minutes, but cumulonimbus clouds can last 2 – 3 hours.
There are several different types of cloud, all with different characteristics regarding the weather
factors above.
Cloud formation is discussed in detail in the next chapter. This chapter focuses on defining the
different cloud types and their features, with a basic mention of formation processes.
CLOUD TERMS
Cirrus
Cumulus
Stratus
Alto
Nimbus
Lenticularis
Castellanus
Mamma
Fractus
Meteorology
High clouds with a feathery appearance
Clouds with a flat base and a top like a cauliflower
Widespread clouds of great horizontal but little vertical extension
Medium level clouds
Clouds with moderate precipitation
Clouds with a lens like appearance
Clouds with a turret like appearance
Clouds with a base that has a pendulous or pouch like appearance
Clouds with a broken or ragged appearance
8-1
Chapter 8
Clouds
CLOUD CLASSIFICATION
The initial subdivision of clouds is into two main types: layer clouds and clouds of great vertical
extension (or heap clouds).
LAYER CLOUDS
These form in stable air and can be further subdivided into categories according to the height
bands in which they are found. Hence there are three further subcategories as follows:
High level clouds (16 500 ft to 45 000 ft)
Cirrus
CI
Cirrocumulus
CC
Cirrostratus
CS
Medium level clouds (6500 ft to 23 000 ft)
Altostratus
AS
Altocumulus
AC
Low level clouds (Surface to 6500 ft)
Nimbostratus
NS
Stratocumulus
SC
Stratus
ST
Each cloud type has a two letter abbreviation.
Notice that the medium level and the high level bands overlap. This happens because in the
summer the medium level clouds can extend up to 23 000 ft, and in winter the high level clouds
can come as low as 16 500 ft.
CLOUDS OF GREAT VERTICAL EXTENSION
These form in unstable air and air not restricted to a particular height band like the layer clouds.
Cumulus
CU
Surface to 25 000 ft
Cumulonimbus
CB
Surface to tropopause
Nimbostratus
NS
Surface to 15 000 ft
A nimbostratus cloud can be a low cloud or a cloud with vertical extension because when there is
strong lifting, nimbostratus can behave like a heap cloud and extend through several height
bands.
The next few sections look at each of the cloud types in turn and describe the characteristics of
each.
8-2
Meteorology
Clouds
Chapter 8
LOW CLOUDS
STRATUS, ST
Stratus (ST) is a layer cloud with large horizontal extent but little vertical development. It generally
has a very low cloud base (below 1000 ft) and covers the whole sky. The typical depth is
1000 - 1500 ft. The base can be quite diffuse with veils hanging down beneath the cloud.
It is a turbulence cloud, often found in the warm sector of polar front depressions. It can also be
formed when low fog lifts.
ST consists of water droplets that are sub-zero in winter but are not very dense, so light to
moderate icing can be expected. Precipitation may occur as drizzle, freezing drizzle, or snow
grains.
Meteorology
8-3
Chapter 8
Clouds
STRATOCUMULUS, SC
A stratocumulus (SC) cloud is a stratiform cloud caused by turbulence. It can be found between
heights 1000 ft and 6500 ft. Because it is formed by turbulence, you might expect light to
moderate turbulence when flying in or below the cloud. Conditions are calm above the cloud.
Like stratus, this cloud consists of water droplets, so light to moderate icing, drizzle, freezing
drizzle, or snow grains can be expected. In addition, you can expect ice pellets and, from the
thicker stratocumulus, intermittent rain or snow. Heavy snowfall can be experienced in winter.
MEDIUM CLOUDS
ALTOSTRATUS, AS
8-4
Meteorology
Clouds
Chapter 8
Altostratus is similar to nimbostratus but is less deep and less dense. This type of cloud can
cover the whole or a major part of the sky and is an indication of the approach of a warm front.
Altostratus contains water droplets and ice crystals, therefore, it can cause light to moderate
icing. Light to moderate turbulence can also be expected. Precipitation can take the form of
continuous or intermittent rain or snow.
ALTOCUMULUS CASTELLANUS, ACC
Altocumulus castellanus gets its name for the cloud’s appearance, which is similar to castle
turrets extending from the top. It develops from altocumulus when there is mid-level instability. It
can therefore indicate the possibility of CBs forming. It tends to be denser than altocumulus so
icing and turbulence can be moderate to severe.
ALTOCUMULUS LENTICULARIS, ACL
Altocumulus lenticularis is a lenticular cloud, which means it is lens-like in appearance. It is
formed orographically in association with mountain waves.
Icing in this cloud can be severe due to the constant replenishment of moisture by updraughts in
the wave.
HIGH CLOUDS
All high clouds fall within the 16 500 − 23 000 ft band. They use the prefix ‘cirr(o)’.
CIRRUS, CI
Cirrus is a thin wispy cloud. It is associated with the approach of a warm front. It can also indicate
the line of a jet stream.
It consists of ice crystals and does not produce icing or precipitation. Likewise, there is no
turbulence.
Meteorology
8-5
Chapter 8
Clouds
CIRRO-STRATUS, CS
Cirro-stratus is a sheet-like cloud, sometimes with a wispy veil underneath. It causes a bright ring
around the sun and the moon, known as the halo phenomenon. It is associated with warm
fronts.
Like cirrus, it consists of ice crystals and does not produce icing, precipitation, or turbulence.
CIRRO-CUMULUS, CC
8-6
Meteorology
Clouds
Chapter 8
Cirro-cumulus is divided into smaller cloud elements that look like the scales of a mackerel. It is
formed when there is turbulence within cirrus or cirrostratus.
Cirro-cumulus consists of ice crystals and occasionally freezing water droplets. There is no icing
or precipitation. There may be light turbulence.
CLOUDS WITH GREAT VERTICAL DEVELOPMENT
CUMULUS
This photo features heap clouds, which are clouds that generally have greater vertical than
horizontal extent. They are formed convectively and the base can be found between 3000 and
7000 ft in the summer and 700 and 4000 ft in the winter. The tops can extend to 25 000 ft.
Cumulus clouds consist of water droplets, which are supercooled above the freezing level.
Precipitation can be present when the cloud has a vertical extent greater than 10 000 ft. It can
take the form of rain or snow showers.
When the cloud becomes towering without being ‘iced’ (cirrus forming) at the top, it is called
towering cumulus, TCU.
Strong vertical currents can be present and larger CU should be avoided. Moderate to severe
icing conditions can be encountered, but because the time taken to traverse the cloud is usually
short, any ice build up tends to be small.
Meteorology
8-7
Chapter 8
Clouds
CUMULONIMBUS
Cumulonimbus is a towering cumulus cloud with a top that has turned into cirrus. This is called
the anvil and extends in the direction of the wind. The anvil is fibrous and diffuse in appearance.
This cloud is very hazardous to aircraft. It is very dense and consists of water droplets of varying
sizes, so moderate to severe icing may be expected. Moderate to severe turbulence is also likely.
CB can give precipitation in the form of rain or snow showers and hail.
Due to the severe weather conditions associated with this cloud, it is discussed in detail in a
separate chapter on thunderstorms.
CLOUD AMOUNTS
Not only is the type of cloud important, but also the amount of cloud. If half or less than half of the
sky is covered with clouds, there should be little if any problem in avoiding them. If more than half
the sky is covered, avoidance becomes difficult.
In aviation meteorology, the sky is divided into eight equal parts called oktas. You can describe
the amount of cloud as a number of oktas, for example 4 oktas. This would mean that 4/8ths, one
half, of the sky is covered.
8-8
Meteorology
Clouds
Chapter 8
In meteorological messages, use three letter abbreviations. These correspond to a number of
oktas as specified below:
SKC
Sky clear
0 oktas
FEW
Few
1 – 2 oktas
SCT
Scattered
3 – 4 oktas
BKN
Broken
5 – 7 oktas
OVC
Overcast
8 oktas
Aerodrome reports use an observation area with a radius of 5 km around the airport plus the area
in the direction of approach. The exception is CBs, which are reported if they are within a 15 km
radius of the airport.
For airfields equipped with instrument landing systems, cloud base reports are referenced to the
site of the middle marker beacon.
You may also see or hear the term CAVOK in meteorological messages. This means ceiling and
visibility OK. In the following conditions you can replace the visibility, weather, and cloud group in
a meteorological report with the word CAVOK.
1. Visibility > 10 km.
2. No clouds occur below 5000 ft or the highest Minimum Sector Altitude, whichever is the
greater.
3. No CB in the vicinity (> 15 km).
4. No precipitation (except ice crystals), thunderstorms, low snowdrift, shallow fog, low
drifting dust or sand, or sand or dust storms.
CLOUD BASE
In addition to the amount and type of cloud, the cloud base is also reported based on the distance
from the ground to the cloud.
The cloud base is the lowest zone in which the type of obscuration perceptibly changes from that
corresponding to clear haze to that corresponding to water droplets or ice crystals.
A METAR or MET REPORT uses 100 ft intervals for clouds up to 10 000 ft, and 1000 ft intervals
for those above 10 000 ft.
For example, you may receive the following in a report:
FEW003 SCT010 BKN040
The numbers after the descriptive abbreviations give the cloud base in hundreds of feet, so
FEW003 means 1 – 2 oktas with a cloud base of 300 ft.
Meteorology
8-9
Chapter 8
Clouds
CLOUD CEILING
The cloud ceiling is the height above aerodrome level of the lowest layer of cloud of more than
4 oktas.
MEASURING CLOUD BASE
AIREPS
There are several ways to measure the cloud base. The cheapest and easiest way is to use
AIREPs (reports from the pilots of aircraft). This may not always be possible on approach and
departure routes that aren’t used frequently.
HUMAN OBSERVATION
The general weather service uses an imperfect method in which the observer estimates the cloud
base. Estimated cloud bases can have large errors and have to be supplemented by the direct
measurements to be used in aviation meteorology.
BALLOONS
If the cloud base is low, as is the case of ST/SC clouds, balloons that rise at a known rate can be
used to determine the cloud base. The time taken for the balloon to disappear into the cloud is
measured, and the measurement is converted into a distance.
CEILOMETER
Most ceilometers use a light-pulse that is reflected by the cloud. The laser reaches the higher
levels without any significant scattering. The reflected light-pulse is received by a light-sensitive
cell and half the time of transport gives the measurement of the cloud base. One problem of this
type of ceilometer is that precipitation can also give reflecting light-pulses, which leads to the
cloud base measurement being too low.
ALIDADE
The Alidade is used at night. The alidade is positioned a known distance from a searchlight. The
searchlight is shone on the cloud and the alidade measures the angle above the horizontal of the
searchlight glow on the base of the cloud. The cloud base is calculated by trigonometry.
VERTICAL VISIBILITY
If fog is thick or snowfall is heavy, the cloud base loses its importance and vertical visibility is
reported. Vertical visibility indicates at what height above the ground the pilot of an aircraft should
have visual contact with the ground vertically down below.
An important difference between cloud base and vertical visibility is that the cloud base mostly
indicates a height in which the pilot can see forward, while vertical visibility only indicates at what
height the pilot can see vertically down.
8-10
Meteorology
Clouds
Chapter 8
SUMMARY OF CLOUD TYPE AND CHARACTERISTICS
Cloud Type
Height
Composition Turbulence
Icing
Visibility
Significance
Cirrus
CI
16 500 ft to
45 000 ft
Ice crystals
Nil
Nil
1000 m +
Found 400 to 600
nm ahead of a warm
front
Cirrostratus
CS
16 500 ft to
45 000 ft
Ice crystals
Nil
Nil
1000 m +
Found 400 to 600
nm ahead of a warm
front
Cirrocumulus
CC
16 500 ft to
45 000 ft
Ice crystals
Light
Nil
1000 m +
Found 400 to 600
nm ahead of a warm
front when
turbulence exists
Altocumulus
AC
6500 ft to
23 000 ft
Water
droplets and
ice crystals
Light to
moderate
Light to
moderate
20 to
1000 m
Turbulence cloud
Altostratus
AS
6500 ft to
23 000 ft
Water
droplets and
ice crystals
Light to
moderate
Light to
moderate
20 to
1000 m
Warm front 200 nm
ahead. Merges with
NS as the front is
approached
Nimbostratus
NS
Ground level to
6500 ft. Can be
10 000 ft to
15 000 ft
merging into AS
at higher levels
Water
droplets but
can be ice
crystals at
medium
levels
Moderate
to severe
Moderate
to severe
10 to 20 m Warm front very
close
Stratocumulus
SC
1000 ft to 6500
ft
Water
droplets
Light to
moderate
Light to
moderate
10 to 30 m Turbulence cloud
Stratus
ST
Ground level to
6500 ft
Water
droplets
Nil to light
Occasion
ally light
to
moderate
10 to 30 m Turbulence cloud
Cumulus
CU
1000 ft to
25 000 ft
Water
droplets and
ice crystals
Moderate
to severe
Moderate
to severe
Less than
20 m
Cumulonimbus
CB
1000 ft to
45 000 ft
Water
droplets and
ice crystals
Moderate
to severe
Moderate
to severe
10 to 20 m Instability cloud
Altocumulus
castellanus
AC
C
6500 ft to
23 000 ft
Water
droplets and
ice crystals
Moderate
to severe
Moderate
to severe
-
An indication of
unstable air at mid
levels; can indicate
approaching CB
Altocumulus
Lenticularis
AC
L
6500 ft to
23 000 ft
Water
droplets and
ice crystals
Moderate
to severe
Moderate
to severe
-
Associated with
mountain waves
Meteorology
Instability cloud.
Large CU may
develop into CB
8-11
Chapter 8
8-12
Clouds
Meteorology
INTRODUCTION
This chapter covers the formation of clouds in more depth than previous chapters.
Clouds form when air rises and cools adiabatically. If rising air cools to its dewpoint, the water
vapour will condense out as water droplets.
The height at which this occurs is the condensation level. This is also the level the cloud base
occurs.
There are several different lifting processes that can lead to cloud formation. They are as follows:
1. Turbulence
2. Convection
3. Orographic uplift
4. Frontal uplift
5. Convergence
TURBULENCE
CONDITIONS
Turbulence clouds can form whenever there is a stable layer. Such a stable layer may occur if
there is an inversion or isothermal layer above it, preventing lifting.
If the wind speed is greater than about 10 kt, turbulence within the layer can lead to a steepening
of the lapse rate.
Note:
Although a wind speed of greater than 10 kt is necessary for turbulence clouds to
form, once formed it can persist at lower speeds.
If this steepening is such that the saturation layer occurs within the turbulent layer, then
turbulence clouds form.
Meteorology
9-1
Chapter 9
Cloud Formation
MECHANISM
The diagrams below show what happens when there is a stable layer of 3000 ft thickness and
turbulent mixing occurs within the layer.
Isothermal layer
3000 ft - 12°C
2000 ft - 13°C
Stable layer –
ELR of
1°C/1000 ft
1000 ft - 14°C
0 ft - 15°C
The above diagram shows the layer before turbulence commences. The layer is stable, the ELR
being only 1°C/1000 ft. Surface temperature is 15°C, making the top of the layer 12°C. Above
3000 ft is an isothermal layer, where the temperature remains 12°C (although this could equally
be an inversion layer).
Isothermal layer
3000 ft
6°C
12°C
2000 ft
9°C
15°C
1000 ft
12°C
18°C
0 ft
15°C
21°C
The above diagram shows the situation during turbulence. Pockets of air are circulated within the
layer. Due to the nature of air as a bad conductor, the pockets cool or warm adiabatically.
9-2
Meteorology
Cloud Formation
Chapter 9
As you can see from the diagram, this means bubbles of air ascending to the top of the layer is
6°C, colder than the environmental temperature. Descending bubbles of air are 21°C when they
reach the bottom of the layer, warmer than the environment.
Isothermal layer
3000 ft
2000 ft
1000 ft
0 ft
9°C
12°C
15°C
18°C
The final diagram shows the situation after turbulence. The temperature at any one level
becomes the average of the temperatures of the bubbles that have ascended and those that have
descended.
The surface temperature has increased and the temperature at the top of the layer has
decreased. Overall the ELR has increased. It is now 3°C/1000 ft. This may result in the dewpoint
being reached below the top of the layer.
For example, assume that the surface dewpoint is 12°C. The dewpoint lapses at 0.5°C so it
would not be reached before the top of the layer in the pre-turbulence case. However, after
turbulence the dewpoint would fall within the layer, hence saturation would occur and clouds
would form, as shown in the next diagram.
Meteorology
9-3
Chapter 9
Cloud Formation
Isothermal layer
3000 ft
2000 ft
9°C
12°C
1000 ft
DEWPOINT
10.5°C
11°C
11.5°C
15°C
0 ft
18°C
12°C
By comparing the new environmental temperature with the dewpoint at various levels, you find
that the cloud base is at 2400 ft.
CLOUD TYPES
The following cloud types are formed by turbulence:
1. Stratus
2. Stratocumulus
3. Altocumulus
4. Cirrocumulus
CONVECTION
Convective processes were introduced in the chapter on Stability, but the processes are
recapped below.
CONDITIONS
Convective clouds form when the surface is heated. This heat energy passes to the air above the
surface by conduction. This air is now warmer than the surrounding environment so it starts to
rise, that is, convection occurs. If the rising air reaches its dewpoint before it reaches the same
temperature as the environment, condensation occurs.
9-4
Meteorology
Cloud Formation
Chapter 9
MECHANISM
The following key applies to the next few diagrams:
DALR
SALR
ELR
Dewpoint
Height
Temperature
In the diagram above, the surface is heated, which starts a vertical motion of air. Initially, the air
cools at the DALR until it reaches the dewpoint. Water vapour then starts to condense out as
droplets and a cloud starts to form. The level at which this occurs is the condensation level and
is coincident with the cloud base.
The air now cools at the SALR. Lifting, and hence cloud formation, ceases when the rising air
reaches the same temperature as the surrounding environment.
The temperature to which the surface must be heated in order for air to be lifted to its
condensation level is the critical temperature.
In the diagram, the DALR intersects the dewpoint curve when the dewpoint temperature is quite
close to the environmental temperature at a low height. Only a small amount of lifting occurs after
this point, so the cloud form is quite small.
Such small clouds are not large enough vertically to produce precipitation. They are usually
isolated (forming over hot spots on the surface) and the sky is otherwise clear. They are,
therefore, referred to as fair weather cumulus/cumuli.
This is common on warm summer days. As temperatures fall in the evening, they tend to
disappear.
If fair weather cumulus form in the morning it may mean there will be large Cu or Cb later on in
the day when insolation increases, for example in the next diagram.
Meteorology
9-5
Chapter 9
Cloud Formation
Height
Temperature
The relative positions of the ELR and the dewpoint curves are the same. The only difference is
that the surface is heated to a much higher temperature. The DALR intersects the dewpoint curve
at a greater height. After this point, there is much more lifting before the SALR intersects the ELR.
So, with greater surface heating there is a much bigger cloud, but one with a higher cloud base. If
this cloud exceeds 10 000 ft in height, it may produce precipitation.
Another factor is the stability of the atmosphere. The steeper the environmental lapse rate, the
longer it takes for the temperature of the rising air to reach the same temperature as the
environment, so the larger the cloud that forms.
ADVECTION
Another way for convective clouds to form is with advection. Advection is the horizontal
movement of air. If cold air passes over a warm surface it becomes heated from below, starting
the process of convection. Typical convective clouds such as cumulus and cumulonimbus can
form.
An example of this is cold air passing over a warmer sea surface such as polar air moving south
over the North Atlantic.
CLOUD TYPES
The following types of clouds are formed convectively:
1. Cumulus
2. Towering cumulus
3. Cumulonimbus
OROGRAPHIC UPLIFT
CONDITIONS
Orographic clouds form when air is forced to rise over an obstruction, such as high ground. This
may occur in a stable or an unstable environment. The type of cloud that forms depends on the
stability and moisture content of the atmosphere.
9-6
Meteorology
Cloud Formation
Chapter 9
MECHANISM
In stable conditions, air is forced to rise over the obstruction. Initially, it cools at the DALR. Once it
reaches its dewpoint, cloud starts to form. This formation is a stratiform cloud. The air cools at the
SALR.
As it passes over the crest of the ridge, the lifting force no longer is present so the air flows down
the other side. It initially warms at the SALR. Since much of its moisture has condensed out as
cloud, it becomes unsaturated again at a lower temperature than the original dewpoint.
Hence the base of the cloud is higher on the leeward side than the windward side.
The air then warms at the DALR. The diagram below shows the temperature at ground level on
the lee side is higher than that on the windward side. This warming wind is known as the Foehn
Wind.
6°C
4000 ft
6°C
7.5°C
3000 ft
7.5°C
9°C
2000 ft
10.5°C
1000 ft
13.5°C
0 ft
16.5°C
12°C
15°C
In drier conditions, the cloud base may be above the top of the ridge. If this happens, the clouds
that form are altocumulus lenticularis.
Meteorology
9-7
Chapter 9
Cloud Formation
Altocumulus Lenticularis (Lenticular Cloud)
These clouds get their name from their lens shape and, generally, indicate the presence of
mountain waves, which are discussed in detail in the chapter on Windshear and Turbulence.
These types of clouds can cause severe turbulence.
The cloud is being continuously replenished with moist air. It, therefore, contains a high
concentration of supercooled droplets. Icing, therefore, can also be severe.
If the conditions are unstable, the obstruction provides the initial lifting force. After the crest is
reached, the air continues lifting due to the unstable nature of the air. The cloud that forms is a
cumuliform rather than a stratiform.
The bulk of the cloud forms on the windward side of the obstruction. Most of the precipitation falls
here as well. The lee side is said to be in rain shadow.
9-8
Meteorology
Cloud Formation
Chapter 9
Cap Cloud
Another situation that causes orographic uplift is when the atmosphere is initially stable then
becomes unstable. Initially, stratiform clouds form. If this is at a medium level, it becomes
altocumulus. If the atmosphere then becomes unstable, this can develop into altocumulus
castellanus. Stratocumulus can develop into stratocumulus castellanus but this is rare.
Meteorology
9-9
Chapter 9
Cloud Formation
CLOUD TYPES
The following clouds can be formed orographically:
IN UNSTABLE CONDITIONS
Cumulus
Cumulonimbus
IN STABLE CONDITIONS
Stratus
Stratocumulus
Altocumulus
Altocumulus lenticularis
WHEN ATMOSPHERE IS INITIALLY STABLE AND LATER BECOMES UNSTABLE
Altocumulus castellanus
FRONTAL UPLIFT
CONDITIONS
A front is the boundary between two air masses, generally in motion, with different properties.
Usually the comparison is made between the relative temperatures of the air masses. There are
two main types of front: the warm front and the cold front.
A warm front is found when warm air is replacing cold air. A cold front is found when cold air is
replacing warm air. In both cases the warm air, being less dense, rises up over the cold air.
Looking at it from the point of the warm front, the warm air slides up over the cold air it is
replacing. From the point of view of the cold front, the cold air undercuts the warm air it is
replacing.
The fronts have different properties and hence the cloud types that form along them differ.
MECHANISM
THE WARM FRONT
The warm air rises over the cold air, forming a sloping front with a gradient of only about 1 to 150,
so the lifting is very gentle and a stratiform cloud forms. From the ground up, the types of cloud
that forms will be stratus, nimbostratus, altostratus, cirrostratus, and cirrus.
Note that when flying toward a warm front from the cold side, you will encounter a progressively
lowering cloudbase.
The gradient is such that the first cloud, the high cloud cirrus, can be encountered up to 600 nm
ahead of the surface position of the front.
9-10
Meteorology
Cloud Formation
Chapter 9
WARM FRONT
CI
WARM AIR
CS
AS
NS
COLD AIR
ST
THE COLD FRONT
Cold air pushes underneath the warm air it is replacing. The slope of the cold front is very
different from that of the warm front. It averages a slope of 1 to 50, and close to the ground it can
be almost vertical, sometimes forming a protruding area that looks like a nose, as shown in the
next diagram.
The air may be unstable, but if it is not, it can be made so by the large amount of enforced lifting.
Hence the type of cloud which forms on this kind of front is generally cumuliform in type, although
there can be shallow bands of stability where NS and CI can form.
Since the front is steeper, the associated cloud ceases no more than about 200 nm after the
passage of the surface front.
Meteorology
9-11
Chapter 9
Cloud Formation
COLD FRONT
CI
CU/CB
WARM AIR
COLD AIR
NS
CLOUD TYPES
COLD FRONTS ONLY
Cumulus
Cumulonimbus
WARM FRONTS ONLY
Stratus
Altostratus
Cirrostratus
MAINLY WARM FRONTS, OCCASIONALLY COLD FRONTS
Nimbostratus
Cirrus
9-12
Meteorology
Cloud Formation
Chapter 9
CONVERGENCE
CONDITIONS
Wherever there is convergence, air is forced to rise. Such convergence occurs in depressions
and non-frontal troughs.
MECHANISM
As air converges into the low pressure area, the air at the centre of the low, or the centre line of
the trough, is forced to rise. This leads to instability and saturation, hence the formation of clouds.
CLOUD TYPES
The cloud types that form are those that are associated with instability. These are cumulus,
cumulonimbus, and towering cumulus.
Meteorology
9-13
Chapter 9
9-14
Cloud Formation
Meteorology
INTRODUCTION
Clouds can consist of a combination of water droplets, supercooled water droplets, and ice
crystals. Individual water droplets and ice crystals are very small and light, and due to upcurrents
in the clouds, they do not fall as precipitation on their own.
If they combine with other water droplets or ice crystals they become progressively heavier. If the
upcurrents in the cloud are not strong enough to support their weight they fall as precipitation.
It follows that the stronger the upcurrents are, the heavier the droplet or crystal has to be in order
for precipitation to occur. So the largest droplets fall from convective clouds such as cumulus and
cumulonimbus.
PRECIPITATION PROCESSES
There are two theories concerning the formation of precipitation. These processes are not
mutually exclusive and, given the right conditions, may both occur within the same cloud.
BERGERON THEORY (THE ICE CRYSTAL EFFECT)
Where sub-zero conditions occur, both ice crystals and water droplets may be present. Water
vapour may sublimate onto the ice crystals. Collision with supercooled droplets allows the crystal
to grow in size.
Once the crystal reaches a sufficient size, it falls as precipitation. The type of precipitation
depends on the temperature of the air through which it falls. If sufficiently warm, the crystal melts
and falls as a rain droplet. If not, it might fall as snow.
The difference in saturation vapour pressure between ice and water is greatest at approximately
-12°C, so clouds reaching this temperature produce precipitation. Snow has a relatively low rate
of fall, so a cloud thickness of 1500 to 3000 ft is sufficient if the temperature at the cloud top is
approximately -8°C to -12°C.
If supercooled water droplets fall through colder air they might freeze and form freezing rain. This
is common with nimbostratus clouds on a warm front. The droplets fall through the front into
colder air.
In dense clouds such as cumulonimbus, there may be a sufficient concentration of supercooled
water droplets for them to freeze onto ice crystals to form a snow pellet.
Meteorology
10-1
Chapter 10
Precipitation
COALESCENCE THEORY (CAPTURE EFFECT)
The Bergeron Theory requires part of the cloud to be below 0°C, so ice crystals are present. In
many clouds in lower latitudes, no part of the cloud is below 0°C yet precipitation still falls. The
Coalescence Theory covers this scenario.
In the cloud there are water droplets of varying sizes. The larger, heavier droplets fall faster and
collide with smaller droplets on their way down. When the droplets become sufficiently heavy,
they fall as precipitation.
INTENSITY OF PRECIPITATION
Precipitation is described by the following terms:
Rainfall Rate (mm per hour)
Rain
Rain/Hail Showers
Snow Accumulation
(cm per hour)
Slight
< 0.5
<2
< 0.5
Moderate
0.5 to 4
2 to 10
0.5 to 4
Heavy
>4
10 to 50
>4
Violent
> 50
CONTINUITY OF PRECIPITATION
Continuity of precipitation is described using the three terms described below.
Showers
Showers are of short duration and are associated only with convective clouds, that is,
cumulus and cumulonimbus.
Intermittent
Intermittent is associated with layer clouds. Precipitation falls from time to time with short
breaks.
Continuous
Continuous precipitation is that which falls for periods of an hour or longer without breaks.
Continuous precipitation is also associated with layer clouds.
10-2
Meteorology
Precipitation
Chapter 10
PRECIPITATION TYPES
The following table describes the different types of precipitation and the clouds they fall from.
Precipitation Type
Drizzle
Freezing Drizzle
Snow Grains
Cloud Type
ST or SC
Comments
Diameter:
Visibility:
0.2 to 0.5 mm
500 to 3000 m
Imperceptible impact.
Drizzle does not make a splash on the ground.
Rain (continuous)
Thick AS and NS
Diameter:
Visibility:
0.5 to 5.5 mm
3000 to 5.5 km
1000 m in heavy rain
Perceptible impact:
Drops have to be large to overcome the upcurrents in the cloud in order to fall.
Larger drops break up into smaller drops as the
rain falls.
Snow (continuous)
Thick AS and NS
Grains/Needles:
Pellets:
< 1 mm diameter
2 to 5 mm diameter
Flakes:
A collection of crystals greater than 4 mm in
diameter. The lower the temperature the smaller
the flake size.
Surface temperature must be < 4°C for snow to
reach the ground before melting.
Hail
CB
Diameter:
Weight:
Height:
Rain (intermittent)
Snow (intermittent)
Thick AS and SC
Rain Showers
Snow Showers
Heavy
CB
CU
5 to 50 mm
up to 1 kg
up to 48 000 ft
and
A mixture of rain and snow or snow that has
partially melted in the descent.
Sleet
Sleet falls when the temperature is between + 5°C
to + 6°C
Soft Hail, or
Graupel
CB
Ice Pellets
SC
Small rounded pellets of less than 5 mm diameter
Can be the early stage of hail growth
Diameter:
< 5 mm
Transparent pellets either spherical or rounded.
Meteorology
10-3
Chapter 10
Precipitation
HAIL
Hail forms by the ice crystal effect when there are updraughts stronger than 10 m/s.
Hail can cause serious damage to an airframe, especially with larger hailstones. The table below
summarises the strength of updraughts required to produce stones of various sizes and masses.
Vertical Speed
Type of Hail
Diameter
Weight
10 m/s
Small Hail (Graupel)
< 5 mm
1g
20 m/s
Hail (Grêle)
2 cm
9g
30 m/s
6 cm
80 g
40 m/s
10 cm
370 g
70 m/s
14 cm
1 kg
In the UK and Northern Europe, the updraughts in thunderstorms are rarely strong enough to
allow the hailstones to grow to any appreciable size. Large hailstones are more likely to be
encountered in heat air mass thunderstorms in tropical locations.
10-4
Meteorology
INTRODUCTION
It is estimated that every day there are about 44 000 thunderstorms across the planet.
Thunderstorms develop from well-developed cumulonimbus clouds. Not all cumulonimbus clouds
develop into thunderstorms, however. The features described in this chapter apply to very active
CBs as well as actual thunderstorms.
CONDITIONS
Thunderstorms are most likely to occur with the following combination of conditions:
1. An environmental lapse rate greater than the SALR through a depth of at least 10 000 ft
and extending to above the freezing level.
2. Sufficient water vapour to provide early saturation and to form and maintain the cloud.
3. A trigger action to start the lifting process. This can take several forms.
TRIGGER ACTIONS
There are five different possible trigger actions:
1. Convection
2. Orographic uplift
3. Advection
4. Convergence
5. Frontal lifting (generally in association with cold fronts and occlusions)
THUNDERSTORM CLASSIFICATION
Thunderstorms are generally classified as one of two types:
1. Heat or airmass — in this case the trigger action is one of the first four above.
2. Frontal — the trigger action is the fifth in the list.
Meteorology
11-1
Chapter 11
Thunderstorms
HEAT/AIRMASS THUNDERSTORMS
CONVECTION
Although heat/airmass thunderstorms can form with one of four triggers, convection is the most
likely one. Since surface heating is greater in the summer, statistically these thunderstorms are
more likely in the summer. They are also more likely during the day and over land and tend to be
isolated, especially if they have formed in a cold air mass. The cold air mass thunderstorms tend
to dissipate in the evening.
Thunderstorms that form in a warm air mass may form a multicell structure.
A multicell thunderstorm is a cluster of CBs where various cells at differing stages interact. The
downdraughts from dissipating and mature cells spread out as a pool of cold air along the ground
surface. This forces the updraught in the front of the system to ascend providing the uplift for the
formation of more CB clouds.
These can persist until late into the evening.
OROGRAPHIC UPLIFT
With orographic uplift, thunderstorms can occur at any time of the day or night, in summer and in
winter. If the uplift is over a range of hills they may occur in a line formation. Thunderstorms are
formed when the conditions are unstable or conditionally unstable.
Orographic processes may enhance an existing thunderstorm that moves over the obstruction.
ADVECTION
With advection, storms can occur in the day or at night, in summer or in winter. In summer, they
can be caused by maritime air from a cold sea passing over the warm land and being heated
from below. However, the more common case is in winter, when cold, moist air moves over a
progressively warmer sea. A prime example of this would be polar maritime air moving south. The
process then becomes similar to the convective case above.
CONVERGENCE
The fourth type of trigger is convergence. This can be in association with low pressures or nonfrontal troughs. Time of day and year depends on the type of low. The different types of lows are
discussed in a later chapter.
When associated with a trough, thunderstorms can form in a line along the centre line of the
trough and can cause difficulties for a pilot trying to avoid them.
FRONTAL THUNDERSTORMS
Frontal thunderstorms are more frequent in winter due to the increased frequency in the passage
of fronts. They can form over land or sea, by day or night, and are associated with both cold
fronts and occluded fronts.
Because they are associated with a front, these thunderstorms tend not to be isolated but to form
in a line. They can be embedded in other clouds and are difficult to identify, especially when
formed on an occlusion in which there are significant layer clouds present.
They are often accompanied by line squalls, which is a line of thunderstorms formed just ahead
of the front.
11-2
Meteorology
Thunderstorms
Chapter 11
IDENTIFICATION OF THUNDERSTORMS
A thunderstorm cloud, whether of the air mass or frontal type, usually consists of several selfcontained cells, each in a different state of development. New and growing cells can be
recognised by their cumuliform shape with clear-cut outline and cauliflower top. The tops of more
mature cells appear less clear-cut and are frequently surrounded by fibrous cloud.
Development of cells is not always seen since other clouds may obscure the view. In frontal or
orographic conditions, extensive layer cloud structures may obscure a view of the development of
cumulonimbus thunderstorm cells, or ACC.
STAGES OF DEVELOPMENT
There are three stages in the development of a thunderstorm, summarised in the diagram below.
40 000
Altitude (feet)
30 000
Updraught
20 000
10 000
Updraught
5000
Updraught
0
GROWTH STAGE
Downdraught
MATURE STAGE
Downdraught
DISSIPATING STAGE
GROWTH STAGE
In this stage, several small cumulus clouds combine together to form a large cumulus of about
5 nm across. Strong updraughts are present, typically on the order of 1000 fpm, but can be as
great as 4000 fpm.
Air is drawn in from the sides and underneath the cloud, replacing the lifting air within the cloud.
This stage lasts approximately 15 to 20 minutes.
MATURE STAGE
The mature stage is characterised by the onset of precipitation. This precipitation is produced by
the combination of ice crystals and water droplets. The precipitation causes downdraughts of
approximately 2000 − 3000 fpm.
Meteorology
11-3
Chapter 11
Thunderstorms
The updraughts are still present, increasing to as much as 10 000 fpm, though 5000 fpm is a
more typical figure. Tops can reach the tropopause, which can be in excess of 50 000 ft in low
latitudes.
Cloud tops can rise by as much as 5000 fpm. The tops of the clouds are affected by a stronger
upper wind which causes it to tilt in the direction of the wind.
This mixture of updraughts and downdraughts causes strong turbulence within and below the
cloud.
The downdraughts are colder than the surrounding air when they reach the base of the cloud,
due to some water droplets evaporating and latent heat being absorbed. Once clear of the base
of the cloud, they warm at the saturated adiabatic lapse rate and remain colder than the
surrounding air.
This combined with the absorption of latent heat intensifies the temperature difference between
the downdraughts and the environment and causes the downdraught to descend even more
rapidly.
This strong downdraught of cold air reacts with the ground and causes a gust front extending up
to 17 nm ahead of the storm. Also at this stage, there may be roll (rotor) clouds, which are
stratocumulus caused by turbulence.
Other hazards associated with this stage, such as microbursts and lightning, are discussed later
in this chapter.
The mature stage lasts approximately 20 – 30 minutes.
DISSIPATING STAGE
This stage commences when the local supply of moisture is no longer sufficient to support the
storm.
The stage is characterised by the appearance of an anvil. This occurs when the cloud top
reaches the tropopause and is spread out by the strong upper winds to form a flat-topped anvil
shape. This anvil is part of a cirrus cloud.
The cloud at this stage can be referred to as Cumulonimbus capillatus.
Updraughts cease and the cloud starts to dissipate as the downdraughts remove the moisture
from the cloud. The precipitation diminishes and the downdraughts are too strong to support roll
clouds. Lightning might still occur.
The dissipating stage lasts about 30 minutes but the cloud can persist for 2 to 3 hours.
11-4
Meteorology
Thunderstorms
Chapter 11
SUPERCELL THUNDERSTORMS
Supercell thunderstorms are severe local storms that form when there is:
¾
Great depth of instability
¾
Strong vertical windshear
¾
A stable layer between the warm lower air and cold upper air
In the mature stage of these storms there are severe updraughts and downdraughts, which can
give rise to very violent weather such as torrential rain, large hail, strong winds, and even
tornadoes. The mature stage can last for several hours.
MOVEMENT OF THUNDERSTORMS
Thunderstorms formed in a col or slack pressure gradient tend to move erratically, but generally
thunderstorms move with the wind at the 700 hPa level, which is equivalent to approximately
10 000 ft.
Supercell thunderstorms in the Northern Hemisphere tend to move 20° to the right of the 500 hPa
(18 000 ft) wind.
SQUALL LINES
Squall lines are usually formed in the warm air mass ahead of a cold front. Squall phenomena are
most frequent during the evening and early night. They are not very common in Western Europe.
Squall lines are more common over large continental areas such as Eastern Europe or, more
frequently, North America. A squall line with thunderstorms also contains hail, and tornadoes can
occur.
Although the CB along the squall can seem very small and insignificant compared to the frontal
clouds behind, in reality the most intense weather phenomena are caused by squalls.
HAZARDS
TURBULENCE AND WINDSHEAR
Turbulence is moderate to severe in thunderstorms, caused by updraughts and downdraughts
within the cloud. Gusts associated with thunderstorms can cause vertical displacements of up to
5000 ft. The effects can be felt up to 40 miles away.
Severe turbulence can be encountered several thousand feet above the cloud tops, as well as
within and below the cloud. Flying within a few thousand feet of the tops of CBs should be
avoided.
Windshear is a more sustained change in windspeed or direction. It is, therefore, likely to be more
dangerous, especially on the approach where the effect on an aircraft’s airspeed can have
serious consequences. In the most extreme cases changes of as much as 80 kt in speed and 90°
in direction can be experienced within a layer of only a few hundred feet.
Meteorology
11-5
Chapter 11
Thunderstorms
GUST FRONT
Some thunderstorms may have a well defined area of cold air flowing out from a downdraught in
all directions, but tending to lead the storm along its line of movement. A gust front might extend
out 24 to 32 km from the storm centre and can be felt from the surface to about 6000 ft. The cold
air undercuts warm air and windshear may be associated with it.
This gust front can be quite distant from the cloud and without precipitation it does not show up
on weather radar and can therefore be quite unexpected. Occasionally there may be roll cloud
associated with it.
Storm Movement
Possible Roll
Cloud Formation
Warm Air Inflow
Turbulence
Outflow
Downdraughts
Gust Front
MICROBURSTS
Microbursts are strong downdraughts of air that descend from the centre of CB clouds with
speeds up to 60 kt down to levels as low as 300 ft. They are typically less than 5 km across and
last from 1 to 5 minutes.
As the downdraughts approach the ground, the air splays out in all directions. The following
diagram shows an aircraft approaching the CB. It initially experiences a strong headwind (A), then
a downdraught (B), followed by a tailwind (C).
11-6
Meteorology
Thunderstorms
Chapter 11
Microbursts are the most extreme example of windshear and can result in large airspeed changes
that can result in the loss of large aircraft.
There are two types of microburst: wet and dry. The wet type has large amounts of precipitation
associated with it so shows up well on weather radar. In the dry type any precipitation has
evaporated before reaching the ground, so is less easy to identify. Some virga may show up on
radar.
Dry microbursts are generally the more severe type and tend to be associated with heat airmass
thunderstorms over dry near-desert regions. The evaporation of the precipitation absorbs latent
heat and enhances the downdraughts.
HAIL
Hail can be encountered in the cloud, below the cloud, and beneath the anvil. Since it is not
possible to tell whether or not a given storm produces hail, for avoidance purposes it is safer to
assume that it will. The stronger the lifting and the greater the moisture content, the greater the
chance of hail.
Hail can be up to 14 cm in diameter and can be encountered up to 45 000 ft, producing severe
skin damage with even a short exposure.
ICING
Any flight in cloud or precipitation can result in icing when the temperatures are below zero. Icing
can occur down to temperatures as low as -40°C. Icing is more severe near the base of the cloud
where the droplets are larger. This is discussed more thoroughly in the chapter on Icing.
Carburettor icing is also a risk and can occur in the temperature range -10°C to +30°C.
Meteorology
11-7
Chapter 11
Thunderstorms
LIGHTNING
Various processes can lead to different charges separating within a CB cloud.
In a CB cloud, hail can collide with water droplets and ice crystals in the cloud. This results in a
net transfer of positive ions from the warmer hail to the colder supercooled water droplet or ice
crystal. This results in the positively charged ice crystal/water droplet moving upward in
updraughts and the negatively charged hail falling downward with gravity.
As a water droplet falls within a cloud it gathers speed. Once it reaches about 9 m/s it starts to
split. Larger parts of the split droplet become positive and smaller parts become negative. The
small negative parts are lifted higher up the cloud than the larger positive parts.
Supercooled water droplets might also freeze onto hail. Tiny splinters of ice break off, become
negatively charged and ascend within the cloud.
These processes result in a net charge difference within the cloud. Once this reaches a potential
difference of about 3 million volts per metre over a distance of about 50 metres, a discharge of
current, lightning, takes place.
Most lightning occurs within 10°C (approximately 5000 ft) of the freezing level.
Hazards associated with lightning are temporary blindness caused by the flash, interference with
compasses and other instruments, and possible airframe damage.
STATIC
Static causes interference on LF, MF, HF, and VHF radio equipment. In severe cases a visible
discharge may occur, called St. Elmo’s Fire, which is a purple light around windscreen edges,
wing tips, propellers, and engine nacelles. Although not dangerous in itself it is an indication that
the air is highly charged and lightning is likely.
WATER INGESTION
Turbine engines have a limit to the amount of water they can ingest. If the updraught velocity in
the thunderstorm approaches or exceeds the terminal velocity of the falling raindrops, very high
concentrations of water may occur. It is possible that these concentrations can be in excess of
the quantity of water turbine engines are designed to ingest, which could result in flame-out
and/or structural failure of one or more engines.
To eliminate the risk of engine damage or flame-out, it is essential to avoid severe storms. During
an unavoidable encounter with extreme precipitation, the recommendation is to follow the severe
turbulence penetration procedure contained in the approved aircraft flight manual, with special
emphasis on avoiding thrust changes unless excessive airspeed variations occur. Water can exist
in large quantities at high altitudes even where the ambient temperature is as low as -30° C.
11-8
Meteorology
Thunderstorms
Chapter 11
TORNADOES
Tornadoes are associated with severe thunderstorms. They form with massive convergence in a
trough with sharply inclined isobars. Differing wind directions give a rotating twist and the lifted air
becomes a spiral.
They are very localised — less than 300 metres across — and the lifting can be so strong that it
can pick up water from a sea surface or dust from the land. Wind speeds in the vortex can reach
200 kt.
If the funnel does not touch the ground it is called a funnel cloud; if it does touch, it is called a
tornado.
Tornadoes are common in the United States but rare in the UK and Europe.
PRESSURE VARIATIONS
Pressure variations can cause the given QNH/QFE to be in error, sometimes by as much as 1000
ft. Local gusts exacerbate the problem and VSIs are also subject to errors. Aircraft should be
flown for attitude rather than altitude.
WEATHER RADAR
Weather radar is provided to enable pilots to avoid thunderstorms and is designed to detect areas
of heavy precipitation.
The strength of the echo is not necessarily an indication of the strength of the associated
turbulence. Radar return intensities may be misleading because of attenuation resulting from
intervening heavy rain. This may lead to serious underestimation of the severity of the rainfall in a
large storm, and an incorrect assumption of where the heaviest rainfall is likely to be
encountered.
The echo from that part of an area of rain furthest from the radar is relatively weaker and the
actual position of the maximum rainfall at the far edge of the storm area is further away than
indicated on the radar display, sometimes by distances up to several miles. Additionally, a storm
cell beyond may be completely masked.
The high rate of growth of thunderstorms and the danger of flying over or near to the tops both of
the main storm and the small convective cells close to it must be considered when using weather
radar for storm avoidance.
Meteorology
11-9
Chapter 11
Thunderstorms
AVOIDANCE CRITERIA
When using weather radar the following avoidance criteria should be used:
Echo Characteristics
Flight Altitude
Shape
Intensity
Gradient of
Intensity
Rate of Change
0 to 20 000 ft
Avoid by 10 nm
echoes with
hooks, fingers,
scalloped edges
or other
protrusions
Avoid by 5 nm
echoes with
sharp edges or
strong intensity
Avoid by 5 nm
echoes with
strong gradients
of intensity
Avoid by 10 nm
echoes showing
rapid change of
shape, height or
intensity
20 to 25 000 ft
Avoid all echoes by 10 nm
25 to 30 000 ft
Avoid all echoes by 15 nm
Above 30 000 ft
Avoid all echoes by 20 nm
General rules:
¾
If a storm cloud has to be overflown, maintain at least 5000 ft vertical separation from the
cloud tops.
¾
If the aircraft has no weather radar, avoid any storm cloud by 10 nm that is tall, growing
rapidly, or has an anvil top.
¾
Avoid flying under a CB overhang.
11-10
Meteorology
INTRODUCTION
Visibility is a measurement of atmospheric clarity. Reduction in visibility can be caused by:
¾
Water droplets, such as cloud, fog, or rain.
¾
Solid particles, such as sand, dust, or smoke.
¾
Ice, such as crystals, hail, or snow.
Poor visibility is more common in stable conditions, for example, beneath an inversion. Visibility is
generally better upwind of towns and industrial areas, away from the atmospheric pollutants.
TYPES OF VISIBILITY REDUCTION
There are several types of visibility reduction. These are:
Mist
Caused by very small water droplets in a RH of more than 95%. The visibility is between
1000 and 5000 metres.
Fog
Also water droplets. Visibility is less than 1000 metres and RH is very close to 100%.
Haze
Caused by solid particles such as sand, dust, or smoke. There is no lower or upper limit
to visibility but haze is not reported above 5000 m visibility.
TYPES OF VISIBILITY
METEOROLOGICAL VISIBILITY
Meteorological visibility is also known as Meteorological Optical Range (MOR) and is the
furthest horizontal distance on the ground that an observer with normal eyesight can recognise a
dark-coloured object. At night, lights of known power are used. Readings are taken at a person’s
eye level.
RUNWAY VISUAL RANGE
Runway Visual Range (RVR) is the maximum distance in the direction of take-off or landing at
which a pilot in the threshold area at 15 ft above ground can see marker boards by day, or
runway lights by night. It is only used when the meteorological visibility is less than 1500 metres
or when fog is reported or forecast.
Meteorology
12-1
Chapter 12
Visibility
OBLIQUE VISIBILITY
When flying at altitude, slant visibility is the maximum distance a pilot can see to a point on the
ground. The oblique visibility is the distance measured along the ground from the point directly
beneath the aircraft to the furthest point the pilot can see.
The distinction is made in the diagram below.
SLANT
VISIBILITY
DOWNWARD
VISIBILITY
OBLIQUE
VISIBILITY
MEASUREMENT OF VISIBILITY
BY DAY
Measurement by day is made by reference to suitable landmarks at known distances from the
observing position.
BY NIGHT
Measurement by night is done by using a suitable arrangement of lights of known power as a
substitute for landmarks.
If this is not possible, a Gold’s Visibility Meter can be used. A variable filter in the viewing
mechanism adjusts until light is no longer seen and the reading off the meter gives an equivalent
daylight visibility.
12-2
Meteorology
Visibility
Chapter 12
MEASUREMENT OF RUNWAY VISUAL RANGE
HUMAN OBSERVER
When an observation of runway visual range is taken by a human observer, the observer is
positioned 76 metres from the centreline of the runway in the touchdown area. The observer
sights the number of marker boards or lights in the appropriate direction. Then, the number of
observed boards or lights is converted into a distance and reported. Human reporting is
inaccurate at the maximum and minimum reporting ranges and visibilities < 100 m and > 1200 m
are unlikely to be reported.
INSTRUMENT REPORTING
Instrument reporting is done with an instrument called a transmissometer, which consists of a
projector and a receiver.
The receiver contains photoelectric cells which measure the opacity of the air and give an
equivalent daytime visibility.
RVR REPORTING
Three transmissometers are positioned alongside the runway giving three readings, one for
touchdown, one from the mid-point, and one for the stop-end of the runway.
RVR is reported in increments of 25 m up to 200 m, 50 m up to 800 m, and 100 m over 800 m.
Sometimes not all three readings are transmitted. The touchdown reading is always reported but
the mid-point and stop-end values may be omitted if certain conditions are met. If one reading is
omitted, the second figure in the group must be specified as the mid-point or stop-end value.
The conditions for the omission of midpoint and stop-end RVR values are that:
a. They have equal to or greater values than the touchdown value, and.
b. They are above 400 metres.
E.g.
300/500/600 would be reported as R 300.
300/350/500 would be reported as R 300 mid-point 350.
OR
c.
E.g.
Meteorology
They are 800 metres or greater.
900/850/950 would be reported as R 900.
900/850/750 would be reported as R 900 stop-end 750.
12-3
Chapter 12
Visibility
VISIBILITY WHILE FLYING
EFFECT OF SUN AND MOON
Visibility is reduced looking into the sun due to the harsh glare of the strong rays. Conversely,
looking into the moon may improve visibility at night as it casts a gentle light on water surfaces
and other ground based features.
WITH A DEEP HAZE LAYER
When flying within the layer at different heights the slant visibility stays the same. When flying
higher, the vertical component of the slant visibility increases, so the horizontal component, that is
oblique visibility, decreases.
Conversely, while flying above the layer flying higher increases oblique visibility.
12-4
Meteorology
Visibility
Chapter 12
WITH A SHALLOW FOG LAYER
If the fog is shallow the pilot may be able to see the airfield quite clearly from directly above it.
Once the pilot descends and turns onto final, visibility may be much poorer looking through the
horizontal extent of the fog instead of the depth. It is important, therefore, to heed the visibility
readings given by the tower even if your own observations are different.
Meteorology
12-5
Chapter 12
Visibility
TYPES OF FOG
RADIATION FOG
At night, the ground loses its heat by radiation. The ground becomes cold and cools the air in
contact with it. If this lowers the air temperature below the dewpoint, water vapour condenses out
as droplets, resulting in fog if there is a light wind, or dew/frost if there are calm conditions.
Conditions necessary for radiation fog to form are:
¾
Clear sky which increases the rate of terrestrial radiation (fog can still form in light, high
cloud cover such as scattered cirrus).
¾
High relative humidity so that only a little cooling will be required for the air to reach
saturation.
¾
A light wind of 2 to 8 kt which mixes the air bringing warmer air from above to the surface
to be cooled and thickening the fog.
Radiation fog is most common in autumn and winter when there is a long night giving the land
time to cool. It occurs at night and early morning after a prolonged period of cooling. It doesn’t
occur over the sea as the sea has insufficient diurnal variation. It forms first in the valleys due to
katabatic effect and is common in anticyclones, ridges, and cols where the air remains in contact
with the ground for a prolonged period.
Dispersal of the fog can occur by:
¾
The increase of insolation during the course of the morning, raising the temperature
above the dewpoint and evaporating the fog away from the base.
¾
The increase of thermal turbulence during the morning which lifts the fog to form low
stratus.
¾
An increase of cloud cover preventing the loss of radiation from the lower atmosphere
and raising the temperature of the air above the dewpoint.
¾
Replacement of the air mass with a drier air mass by advection.
12-6
Meteorology
Visibility
Chapter 12
ADVECTION FOG
Advection fog forms when warm moist air flows over a cold surface. It can occur over land or sea.
Conditions necessary for it to form are:
¾
A wind of up to 15 kt (20 kt over the sea).
¾
A high relative humidity so little cooling is required to bring the air to saturation.
¾
The cold surface over which the air moves must have a temperature lower than the
dewpoint of the warm moist moving air.
Advection fog is common over land areas in winter and early spring when the land is colder than
the sea and over sea areas in late spring and early summer when the land becomes warmer than
the sea.
This type of fog is much more persistent than radiation fog and can last several weeks. Examples
are the coast of Newfoundland and the Kamchatka peninsula where the temperature difference
between land and sea is extreme.
Dispersal comes when there is a change of airmass or an increase in windspeed beyond that
described in the conditions above.
Meteorology
12-7
Chapter 12
Visibility
Some types of advection fog experienced in and around the UK are listed below:
Thaw Fog
These fogs occur over land surfaces in winter and spring when severe frost or
snowfall gives way to milder Atlantic air from the southwest.
Haar
Frequent in the spring and early summer off the Northeast coast of the UK. The
sea is at its coldest having been cooled gradually through the winter months.
Warm air from the continent passes over the colder sea.
Sea Fog
Common in the approaches to the English Channel during the spring and early
summer when the sea is still cool. If the wind speed is over 25 kt then the fog will
lift into ST.
STEAMING FOG (ARCTIC SEA SMOKE)
Steaming fog occurs at very high latitudes over sea areas such as around Iceland, Greenland,
and Norway. It is similar to advection fog in that the airmass is moving but in this case it is a cold
moist air mass passing over a warmer sea.
Normally this would lead to convection and the formation of cumuliform cloud. However, in this
case the air is too cold and stable for sufficient lifting to occur. Instead, the small amount of lifting
and evaporation from the sea leads to saturation and fog formation.
At such high latitudes the water content is likely to be ice crystals giving the fog a white
appearance which is the reason for its nickname of Arctic Sea Smoke.
FRONTAL FOG
Frontal fog is associated with warm fronts and warm occlusions. Precipitation from NS cloud
above the front falls into the colder air beneath the front, saturating the colder air. Additionally, the
precipitation wets the ground and the moisture then evaporates into the air just ahead of the front
aiding saturation.
12-8
Meteorology
Visibility
Chapter 12
This produces a band of fog up to 200 nm wide that travels just ahead of the front as shown in the
diagram.
HILL FOG
Hill fog is really stratiform cloud that forms when there is orographic lifting in stable conditions.
The cloud stays next to the surface obscuring the tops of the hill or mountain.
A nice example is the tablecloth effect on Table Mountain in Cape Town, South Africa.
Meteorology
12-9
Chapter 12
Visibility
OTHER VISIBILITY REDUCERS
SMOKE FOG (SMOG)
Smoke fog is a combination of ordinary water droplet fog and solid particles. It occurs in industrial
cities when there is an inversion layer preventing air from lifting and removing the pollutants.
In addition to being visibility reducers themselves, the solid particles are hygroscopic nuclei and
enhance the severity of the fog.
DUST AND SAND
Dust is a solid particle less than 0.08 mm in diameter. Sand is between 0.08 mm and 0.3 mm in
diameter. Winds can carry these particles aloft causing dust or sand storms.
In dust storms, the wind is upwards of 15 kt and the dust can rise to up to 15 000 ft agl. In sand
storms, the winds are upwards of 20 kt but these remain within a few feet of the surface due to
the weight of the particles.
Both types tend to be daytime phenomena as wind strengths are usually insufficient at night.
Visibility in dust or sand storms is generally less than 1000 m.
PRECIPITATION
Precipitation also causes reduction in visibility.
Drizzle reduces visibility more than rain, as drizzle consists of large numbers of small water
droplets. Drizzle can lower the visibility to 500 m.
The worst type of precipitation is snow. Heavy snow can lower the visibility to 50 m and possibly
even less if it is blowing or drifting.
For more information, see the chapter on Precipitation.
12-10
Meteorology
Visibility
Chapter 12
VISUAL ILLUSIONS
SHALLOW FOG
If the pilot enters a shallow fog layer on descent it can give the illusion that the aircraft has
pitched up. If the pilot believes this illusion and pitches the nose down, a very dangerous situation
can arise, especially if this happens on the approach to land.
RAIN SHOWERS
A rain storm moving toward the aircraft can give the illusion of the horizon moving lower, causing
the pilot to reduce power or lower the nose unnecessarily.
LAYER CLOUD
In the absence of a well-defined horizon, the pilot may orientate himself with respect to layer
clouds. If the layer clouds are not parallel to the ground, the orientation to a false horizon will
cause banking.
RAIN EFFECTS
Rain can have two opposing effects:
1. Rain falling between the aircraft and visual landmarks such as the runway lights will
diffuse the light and make the objects or runway lights appear further away than they
really are. The pilot might perceive this as being low on approach.
2. Rain on the windscreen can make runway lights bloom, making the runway appear closer
than it really is. The pilot might perceive this as being high on approach and may make
adjustments to the aircraft’s power and/or attitude which will result in undershooting the
runway.
Meteorology
12-11
Chapter 12
12-12
Visibility
Meteorology
INTRODUCTION
Ice accretion can have serious implications for performance and handling of aircraft. Modern
aircraft are equipped with efficient anti-icing and de-icing equipment. However, these systems
may become inoperative or icing conditions may be so severe that these systems become unable
to cope.
Even if these systems operate perfectly there is quite a significant fuel cost in running the
systems. The preferred approach would be to avoid the conditions in which severe icing may
occur. It is necessary for the pilot to understand the conditions and the risks associated with icing.
CONDITIONS
Ice forms on an airframe if the following three conditions are present:
1. Water is present in a liquid state.
2. The ambient air temperature is below 0°C.
3. The airframe temperature is below 0°C.
EFFECTS OF ICING
The detrimental effects of icing can include the following:
AERODYNAMIC
Ice forms mostly on the leading edges of the airframe and aerofoils. This spoils the aerodynamic
shape of the airframe and leads to:
¾
¾
¾
Reduced lift (up to 30%)
Increased drag (up to 40%)
Increased weight
The increased weight coupled with loss of lift leads to an increased stalling speed. The added
weight and increased drag results in greater fuel consumption.
In addition, ice accumulation may lead to control surfaces becoming jammed, especially where
ice has broken off in chunks from other surfaces and become lodged.
WEIGHT
The rate of accumulation of ice is rarely constant across the airframe. This inconsistency may
lead to a shifting centre of gravity which causes instability and difficulty controlling the aircraft.
Uneven ice build-up on propellers can lead to severe engine vibration and possible engine
damage.
Meteorology
13-1
Chapter 13
Icing
INSTRUMENTS
Ice may block the pitot and static inlets leading to gross instrument errors in the altimeter,
airspeed indicator, vertical speed indicator and Machmeter.
The safety implications of this are far-reaching.
OTHER EFFECTS
Other miscellaneous effects include:
¾
¾
¾
¾
¾
Skin damage from chunks of ice breaking off propellers
Obscuration of windscreens
Increased skin friction and associated performance effects
Radio interference due to ice build-up on aerials
Landing gear deployment/retraction problems if ice forms in gear wells or freezes gear
doors closed
ICING DEFINITIONS
Any pilot encountering unforecast icing should report the time, location, level, intensity, icing type,
and aircraft type to the ATS unit they are in contact with. The following definitions are the
reporting definitions for levels of icing:
TRACE
Ice becomes perceptible; rate of accumulation slightly greater than the rate of sublimation. It is
not hazardous. De-icing/anti-icing equipment is not used unless ice is encountered for more than
one hour.
LIGHT
The rate of accumulation might create a problem if flight in this environment exceeds one hour.
Occasional use of de-icing/anti-icing equipment removes/prevents accumulation. It does not
present a problem if anti-icing equipment is used.
Note: The ICAO definition of light icing is: “Change of heading or altitude not considered
necessary.”
MODERATE
The rate of accumulation is such that even short encounters become potentially hazardous and
the use of de-icing/anti-icing equipment, or diversion, is necessary.
Note: The ICAO definition of moderate icing is: “Change of heading or altitude considered
desirable.”
SEVERE
The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the
hazard. Immediate diversion is necessary.
Note: The ICAO definition of severe icing is: “Immediate change of heading and/or altitude
necessary.”
13-2
Meteorology
Icing
Chapter 13
SUPERCOOLED WATER DROPLETS
In order for a droplet of water to freeze, it not only must be below freezing point, but there must
be a freezing nucleus present. This could take the form of salt, dust, pollen, or smoke particles.
There are less freezing nuclei than condensation nuclei. Hence it is a frequent occurrence that a
droplet cools to a temperature below zero but there is no freezing nucleus available. When this
occurs, the droplet stays in liquid form even though it is below zero. It is then referred to as a
supercooled water droplet. These droplets can exist in temperatures as low as -40°C.
Most icing is caused by aircraft colliding with these droplets while in cloud or fog. As the droplet
touches the airframe its surface tension breaks down and it starts to freeze.
SIZE OF SUPERCOOLED WATER DROPLETS
There are two factors dictating the size of the supercooled water droplets in a cloud.
First, consider the type of cloud. Layer clouds only have small water droplets, so when these
become supercooled they remain small. Cumuliform clouds can have small and large water
droplets, so the size of the droplets when supercooled varies.
The second factor is temperature. Once the temperature drops below -20°C, large supercooled
droplets freeze, regardless of the lack of a freezing nucleus. So even in cumuliform cloud, if the
temperature drops below -20°C, only small supercooled droplets will be present.
LARGE SUPERCOOLED WATER DROPLETS
In summary, large supercooled water droplets occur:
1. In CU and CB from 0°C to -20°C.
2. In NS at temperatures from 0°C to -10°C.
3. If the NS has been enhanced by orographic uplift, between 0°C and -20°C.
SMALL SUPERCOOLED WATER DROPLETS
In summary, small supercooled water droplets occur:
1.
2.
3.
4.
In CU and CB from -20°C to -40°C.
In NS at temperatures from -10°C to -40°C.
If the NS has been enhanced by orographic uplift, between -20°C and -40°C.
In ST, SC, AS, AC from 0 to -40°C.
Note: Supercooled water droplets do not occur in the cirriform clouds. These consist of ice
crystals.
Meteorology
13-3
Chapter 13
Icing
FREEZING PROCESS
When a supercooled water droplet impacts an airframe, not all of it freezes instantly. The fraction
that freezes instantly depends on the temperature of the droplet.
For every degree below zero, 1/80 of the droplet will freeze on impact. So if the temperature is
-20°C, 1/4 will freeze on impact; if the droplet is -40°C, 1/2 will freeze on impact.
So with a warmer droplet, the freezing process is slower. As a fraction of the droplet freezes,
latent heat is released which delays the freezing of the remainder of the droplet. This allows the
liquid part to flow over the airframe (called flowback) and freeze more gradually.
Also, the size of the droplet is important. Large droplets tend to retain latent heat better, so
freezing is delayed even more, allowing a greater spread of the droplet.
The importance of these differences is discussed below.
TYPES OF ICING
CLEAR ICE (GLAZE ICE)
Clear ice, or glaze ice, forms when large supercooled droplets impact with an airframe. When the
droplet impacts the airframe it does not freeze instantly. It starts to freeze and as a result some
latent heat is released. This raises the temperature slightly, allowing the water to flow over the
airframe before subsequently freezing. This results in a clear coating of ice which adheres
strongly to the surface of the aircraft.
Clear ice is a very serious form of icing which is heavy and difficult to remove. Uneven formation
on propellers can lead to vibration and chunks breaking off and causing skin damage.
The weight addition, which can be uneven, leads to stability and control problems and the aerofoil
shape is spoiled. Because of this, clear ice is usually described as moderate to severe.
Since large droplets only occur in CU, CB, and NS, this type of ice is only found in those clouds,
and only in the temperature range 0°C to -20°C.
RIME ICE
This forms from impact with small supercooled droplets. When the droplet impacts, most of the
droplet freezes instantly with little or no flowback.
Air becomes trapped between the droplets causing the ice to be opaque or cloudy. It is a granular
coating which is generally easy to remove. It can cause some loss of the aerofoil shape and an
increase in surface friction. It can also cause blockage of air intakes.
Usually rime icing is classed as light to moderate as build up is generally light enough for antiicing measures to cope.
This type of icing can occur in any cloud where there are small supercooled droplets. Hence it will
occur in layer clouds at any temperature below zero (except cirriform clouds which consist of ice
crystals). It will also occur in cumuliform clouds where temperatures are below -20°C.
It may also occur in freezing fog.
13-4
Meteorology
Icing
Chapter 13
MIXED ICE
This is a combination of clear ice and rime ice and occurs where both types of water droplets are
present. This applies to clouds where the temperature is close to the transition between small and
large supercooled droplets. This will be within a few degrees of:
1. -20°C for CU and CB.
2. -10°C for NS.
3. -20°C for NS enhanced by orographic uplift.
RAIN ICE
This type of icing is very severe and very similar to clear ice. It is common beneath a warm front
or an occlusion, when precipitation falls from NS cloud above the front. The warm rain falls into
colder air and becomes supercooled. If the aircraft is above the freezing level, the airframe is
below zero and the droplets strike the airframe and form ice in the same way as described above
in the section on clear ice.
The colder the air is below the front, the more common this type of icing becomes. Hence, it is a
common occurrence over large land masses such as North America and Central Europe, but is
much rarer over the UK where the temperatures are milder.
HOAR FROST
This type of icing occurs when air is cooled to the temperature at which saturation occurs and the
airframe is below 0°C. The frost forms by sublimation, that is, water vapour turns directly to ice
without passing through the liquid state.
Note that the temperature to which the air must be cooled for saturation to occur is called the
frost point in this situation, rather than the dewpoint.
It is a white crystalline deposit of the kind you find on your car on a cold morning.
It can occur on the ground when the aircraft is parked, or during flight.
Meteorology
13-5
Chapter 13
Icing
The correct conditions for hoar frost formation occur when an aircraft takes off from an aerodrome
at a sub-zero temperature and climbs through an inversion into warm moist air. Likewise, if an
aircraft descends from a very cold region into a warm moist layer, the same conditions will be
present.
This causes similar problems to those caused by rime ice.
FACTORS AFFECTING THE SEVERITY OF ICING
There are several factors which affect icing severity. These are detailed below.
SIZE OF SUPERCOOLED WATER DROPLETS
As discussed above, larger supercooled droplets cause more severe icing of the clear type, and
small supercooled droplets cause rime ice, which is less serious. The size of the droplets
depends on the type of cloud and the ambient temperature. This was discussed above and is
summarised below:
Type
Severity
Conditions
Clear / Glaze Ice
Moderate to severe
Caused by large supercooled droplets,
hence only found in cumuliform clouds such
as CU and CB, and also in NS and ACC
which have heap-type characteristics.
Rime Ice
Light to moderate
Caused by small supercooled droplets. In
layer clouds from 0°C to -10°C. In
cumuliform clouds from -20°C to -40°C.
Light
Caused by small supercooled droplets. In
layer clouds from -10°C to -40°C.
N/A
In CI, CS and, CC (only ice crystals are
present).
Nil
CONCENTRATION OF SUPERCOOLED WATER DROPLETS
The higher the concentration of supercooled droplets, the more serious the icing risk. Upcurrents
are stronger in the convective clouds, hence able to support a higher concentration of droplets.
This increases the risk in these clouds.
There is a higher concentration of droplets at the base of the cloud. This is for two reasons. First,
gravity tends to increase the density lower down. Second, the base is where condensation
commences, where the temperature is higher so the water content of the moist air is greater.
OROGRAPHIC UPLIFT
Where clouds have formed orographically, or existing clouds have been enhanced by lifting
against a hill or mountain, uplift is stronger, so the cloud can support a higher concentration of
water droplets, and also a greater size of droplet. For both these reasons, icing tends to be more
severe.
CLOUD BASE TEMPERATURE
The higher the temperature, the greater the amount of water vapour the air can hold. If the cloud
starts to form at a high temperature, the moisture content will be greater making the concentration
of droplets greater. Upcurrents result in the concentration of water droplets at all levels of the
cloud being greater so the icing will be more severe.
13-6
Meteorology
Icing
Chapter 13
AEROFOIL SHAPE
Air flowing around thin, low-drag aerofoils tends to follow the shape quite closely, whereas air
flowing around thick, high-drag aerofoils tends to be deflected away from the surface more.
Hence, supercooled water droplets are more likely to adhere to the thin aerofoil shape.
Aircraft with low-drag aerofoils tend to fly at a higher speed, and so they impact with more
droplets in a given amount of time.
This may be offset by kinetic heating effect, more details of which are given below. If the skin
temperature is raised to above zero, no icing will occur.
KINETIC HEATING
As an aircraft travels through the air it experiences kinetic heating of its surface which is related to
its true airspeed. The formula is as follows:
Temperature Rise (°C) =
(
TAS
100
)
2
So if the true airspeed is 300 kt, the temperature rise will be 9°C.
If this raises the temperature to above zero, no ice will form. However, it also has the potential to
worsen the effect of icing. If the temperature were a low sub-zero temperature and was heated to
a temperature which was still below zero, this may lead to increased flowback and a greater
likelihood of clear ice.
Hence it is important not to assume that kinetic heating will always improve the situation.
ENGINE ICING
Icing can occur in both piston and turbine engines. The types of icing and conditions for formation
differ between the engine types. Icing can occur to a much higher temperature in piston engines
than in turbine engines. The processes involved are described below.
PISTON ENGINE ICING
Several different types of icing can occur.
IMPACT ICING
Impact icing occurs in the intake area of the engine. It forms by direct impact of supercooled
water droplets with the surface, in much the same way as airframe icing. Temperatures need to
be sub-zero for this to occur.
FUEL ICING
Fuel icing is caused by water in the fuel freezing in the pipes and reducing or preventing fuel flow
to the engine. Again, the temperature needs to be below zero.
Meteorology
13-7
Chapter 13
Icing
CARBURETTOR ICING
ICE
FUEL
INTAKE AIR
This is the only form of icing where the ambient temperature can be above zero. It is caused by
two things:
1. Latent heat being absorbed from the surroundings as fuel evaporates.
2. As air passes through the venturi its speed increases, but its pressure, and therefore its
temperature, go down.
The temperature reduction can be in excess of 30°C. So even at quite high temperatures the air
may be cooled to a temperature below zero. If the air has sufficient moisture, content icing
occurs.
The effects can be more severe if a low throttle setting is used with the carburettor butterfly only
partially open. A total blockage may occur.
Carburettor icing is common on warm, humid days as the moisture content of the induction air is
high.
Indications that the conditions for carburettor ice formation may be present include wet ground or
dew, reduced visibility from mist or fog, proximity to clouds, or precipitation.
JET ENGINE ICING
As for piston engines, the problem of fuel icing in the supply pipes exists.
Impact icing may accumulate in the intakes of a jet engine. If this breaks off, it can cause blade
damage.
In the early intake stages, there is a pressure reduction which can lead to adiabatic cooling on the
order of 5°C. This is a particular problem if the aircraft is at high revs, such as on approach or
climb-out.
13-8
Meteorology
Icing
Chapter 13
In potential icing conditions, use engine igniters to help prevent failures. If there is precipitation or
the outside air temperature is less than 10°C, engine anti-icing systems should be switched on.
ICE PROTECTION
ANTI-ICING
Anti-icing measures are designed to prevent the formation of ice. They include:
¾
¾
¾
¾
¾
Kill-frost paste applied to the leading edges.
Heated windscreen and pressure head.
Hot air system on leading edges and tailplane.
Hot air system on engine cowling lips and spinner.
Anti-icing fluids.
DE-ICING
De-icing measures are designed only to remove icing after it has formed, not to prevent its
formation. Examples are:
¾
¾
¾
¾
Meteorology
De-icing fluids.
Pulsating rubber boots.
Hot air systems.
Electrical heating systems.
13-9
Chapter 13
13-10
Icing
Meteorology
INTRODUCTION
Wind is the horizontal movement of air over the surface of the Earth due to forces acting upon it.
It is expressed as a wind velocity, which is a combination of direction and speed. The direction
given is always that from which the wind is blowing.
Calm
20 kt, further additions up to 45 kt
1 to 2 kt
50 kt
5 kt
60 kt
10 kt
65 kt, further additions as
necessary
15 kt
The wind is depicted as a straight line coming from the periphery of a circle. The examples above
show a wind direction of 090°.
The wind speed is normally given in knots. Other units used are kilometres per hour and metres
per second.
Direction is usually given in °T. Exceptions to this are in an ATIS or verbally from the control
tower, where wind direction is given in °M. This is because runway direction is magnetic, enabling
the pilot to calculate the wind components if the wind speed is also given in magnetic.
Meteorology
14-1
Chapter 14
Wind
TERMS ASSOCIATED WITH WIND
Veer is a change of direction in a clockwise direction.
Back is a change of direction in an anti-clockwise direction.
Gust is a sudden increase in wind speed lasting a few seconds.
Squall is a wind speed increase of at least 16 kt to a uniform speed of at least 22 kt lasting for at
least one minute. Squalls are often associated with CBs.
Lull is a decrease in wind speed lasting from a few seconds to a few minutes.
Gale is a mean surface wind of 34 kt or more, or gusting to 43 kt or more.
Hurricane is a wind with a mean surface value of 63 kt or more.
Wind Gradient is the gradual change in wind velocity between the surface and the top of the
friction layer.
Gust factor is calculated by the following formula:
GUST FACTOR % = (MAXIMUM GUST SPEED – MINIMUM LULL SPEED) * 100%
MEAN WIND SPEED
For example:
A wind averaging 35 kt with gusts to 50 kt and lulls of 20 kt would
have a gust factor of:
(50 – 20) × 100= 85.7%
35
FORCES ACTING UPON THE AIR
There are two main forces acting upon the air. These are:
1. The Pressure Gradient Force.
2. Geostrophic Force.
14-2
Meteorology
Wind
Chapter 14
There is a third force, friction, that acts close to the surface. The thickness of the friction layer
varies.
THE PRESSURE GRADIENT FORCE
The Pressure Gradient Force (PGF) is the force that initiates movement of air. If there is a region
of high pressure adjacent to a region of low pressure, the air flows from the high pressure to the
low pressure. If there were no other forces acting, this would continue until the two pressures
were equal, resulting in no more pressure gradient.
PGF
H
L
1004
1002
1000
998
The diagram shows the pressure in mb or hPa. As seen in the diagram, the PGF acts at right
angles to the isobars.
Calculate it using the following equation:
PGF =
dp
ρdn
where:
dp
dn
ρ
=
=
=
the pressure difference between two points
the horizontal distance between the two points
air density
THE GEOSTROPHIC FORCE
This is also referred to as the Coriolis Force.
Geostrophic force is due to the rotation of the Earth and the law of inertia. The Earth rotates at a
fixed speed. At the Equator, the line of latitude with the largest circumference, objects on the
Earth move faster than those at higher latitudes, because they have to travel a longer distance in
the same amount of time.
In the diagram below, the thick horizontal arrows show how a position on the Earth moves in a
given time at the equator and at two temperate latitudes, one in the Northern Hemisphere, one in
the Southern Hemisphere.
Meteorology
14-3
Chapter 14
Wind
Four different situations are shown. A and B show movement away from the Equator in the
Northern and Southern Hemispheres respectively.
C and D show movement towards the Equator in the Northern and Southern Hemispheres
respectively.
Take, for example, situation A.
A parcel of air leaves the point represented by the start of the thick horizontal arrow at the
Equator and travels due north. As it travels, the point on the ground from which it left and the
point on the ground for which it is aiming move due to the Earth’s rotation.
You would expect the parcel of air to end up at the point of the arrow at the higher latitude, that is,
the initial aiming point after following a path represented by the dashed line.
However, due to inertia the parcel of air moves at the speed of objects at the Equator, so travels
further east than expected, following a path represented by the thick diagonal arrow.
Hence, the parcel of air appears to have turned right in the Northern Hemisphere.
Now look at the Southern Hemisphere, situation B. You can see that the parcel of air appears to
turn left.
Now look at situations C and D. You will find that the same rule applies for movement toward the
Equator.
14-4
A
C
B
D
Meteorology
Wind
Chapter 14
In summary, due to Coriolis effect, objects appear to turn right in the Northern Hemisphere, and
left in the Southern Hemisphere.
Geostrophic Force (GF) can be calculated using the following equation:
GF = 2 Ω ρ V SIN θ
Where:
Ω = THE ANGULAR ROTATION OF THE EARTH
ρ = AIR DENSITY
V = WINDSPEED
θ = LATITUDE
Note that the Pressure Gradient Force must initiate movement of a parcel of air before
Geostrophic Force can come into play. Geostrophic Force has no effect on a stationary parcel of
air.
As the Geostrophic Force is proportional to SIN θ, it is zero at the Equator and a maximum at the
poles. Within 15° of the Equator Geostrophic Force is negligible.
THE GEOSTROPHIC WIND
As already discussed, movement of air is initiated by the Pressure Gradient Force. The air is then
affected by the Geostrophic Force. The Geostrophic Force initially acts at right angles to the
pressure gradient force, so produces a resultant wind that is at an angle between the two.
However Geostrophic Force is now no longer at right angles to the wind, and so acts from the
resultant wind (see diagram below). This process continues until the PGF and the GF are acting
in opposite directions and are in balance, as shown in the diagram.
PGF
GW
1004
GF
1012
Meteorology
14-5
Chapter 14
Wind
The resultant wind is now at right angles to the PGF. In the Northern Hemisphere it will be 90° to
the right of the PGF, in the Southern Hemisphere 90° to the left.
This resultant wind is called the Geostrophic Wind and flows parallel to the straight isobars as
shown in the diagram. It gives rise to Buys Ballot’s law, which states:
“In the Northern Hemisphere with your back to the wind, the low pressure is on your left.”
Note that the opposite is true for the Southern Hemisphere.
This wind does not take the third force, friction, into account and is taken to be the wind just
above the friction layer.
The equation for geostrophic force applies:
GF = 2 Ω ρ V SIN θ
Since PGF and GF are now in equilibrium, the following is also true:
PGF = 2 Ω ρ V SIN θ
The formula can be re-arranged to make V the object as follows:
V=
PGF
2 Ω ρ SIN θ
Hence the windspeed (V) is proportional to the PGF and inversely proportional to the latitude.
Therefore as latitude decreases, the windspeed increases. This continues until about 15° of the
equator, where the equation breaks down due to the negligible geostrophic force.
If the windspeed at a certain latitude is known, the windspeed at another latitude, assuming the
same isobar spacing, can be calculated using the relationship described above. The derived
formula is as follows:
VLAT A SIN LAT A = V LAT B SIN LAT B
For example:
If the geostrophic wind speed is 40 kt at 30°N, calculate the
geostrophic wind speed at 60°N
40 SIN 30 = V SIN 60
V = 23 KT
In summary, the conditions for the geostrophic wind are:
¾
Above the friction layer.
¾
Greater than 15°N/S.
¾
A pressure system that is not changing rapidly.
¾
Straight and parallel isobars.
14-6
Meteorology
Wind
Chapter 14
THE GEOSTROPHIC WIND SCALE
Consider the formula for windspeed again:
V=
PGF
2 Ω ρ SIN θ
Weather charts are usually for a limited latitude range and altitude. The angular rotation of the
Earth is constant so the denominators of the equation can be replaced by a constant:
V=
PGF
K
This simple relationship means that windspeed can be determined from the pressure gradient
force, which in turn comes from the isobar spacing. A scale called the Geostrophic Wind Scale is
printed on the chart. An example is shown:
100 50
30
20
15
10
5 kt
As you can see, the relationship is not linear but logarithmic.
To find the windspeed for a given point, measure the distance between successive isobars
passing through that point, and compare this to the scale. Align your measured distance with the
left end of the scale but read the speed off from the right. In the example above the isobars are
well spaced, giving a speed of about 18 kt. The closer the isobars, the stronger the wind.
THE GRADIENT WIND
The Geostrophic Wind only applies to straight parallel isobars. When dealing with curved isobars
the situation becomes slightly more complicated.
Consider circular pressure systems. In the Northern Hemisphere the pressure gradient force and
geostrophic force act opposite to each other, and the resultant wind is 90° to the right of the PGF.
However, the wind follows the curved isobars so the air starts to rotate around the centre of the
system.
This rotation brings an additional force into play, called centrifugal force. This is a force acting
outwards from the centre of the system.
In the next two diagrams, the following key applies:
CF — Centrifugal Force
PGF — Pressure Gradient Force
GF — Geostrophic Force
GW — Gradient Wind
Meteorology
14-7
Chapter 14
Wind
GW
CF
L
PGF
GF
In the case of a low pressure system, centrifugal force opposes the pressure gradient force,
hence the resultant wind speed is lower than the geostrophic wind for the same isobar spacing.
This is termed sub-geostrophic. If a geostrophic wind scale is used it will over-read.
The resultant wind is called the Gradient Wind, and blows anti-clockwise around a low pressure
system in the Northern Hemisphere.
H
GF
PGF
CF
GW
In the case of a high pressure system, centrifugal force supports the pressure gradient force,
hence the resultant wind speed is higher than the geostrophic wind for the same isobar spacing.
This is termed super-geostrophic. If a geostrophic wind scale is used it will under-read.
The resultant Gradient Wind blows clockwise around a high pressure system in the Northern
Hemisphere.
14-8
Meteorology
Wind
Chapter 14
WINDS NEAR THE EQUATOR
At latitudes less than 15° the formula for geostrophic wind breaks down due to the low value of
the geostrophic force. With straight isobars the wind tends to flow across the isobars from high to
low pressure.
However, with curved isobars the situation is different. In some situations the centrifugal force
becomes so large that it balances the pressure gradient force. When this happens, the wind is
said to be cyclostrophic. Examples are in a tropical revolving storm or a tornado.
THE SURFACE WIND
Both the Geostrophic and the Gradient wind act above the friction layer. The third force, friction,
must be taken into account in this layer.
The strength of the frictional force depends on the following factors:
¾
The roughness of the landscape – the rougher the landscape, the greater the friction;
¾
Stability of the air – an unstable air mass creates thermal turbulence. This causes the
slow surface wind to interact with faster higher winds, resulting in increased wind speed
at the surface;
¾
Season – in summer the turbulence layer is thicker over land due to surface heating. The
same effect will be seen as above;
¾
Type of system – the layer is thicker in low pressure than in high;
¾
Windspeed – the higher the windspeed, the greater the resulting frictional effect.
Friction between the moving air and the surface slows the air down. Therefore V, the windspeed,
decreases. Any decrease in V leads to a decrease in geostrophic force, according to the
geostrophic wind equation discussed above.
PGF
PGF
SW
GW
FF
GF
GF
2000 ft wind
Surface wind
In the above diagram GW is Geostrophic Wind, SW is Surface Wind, and FF is Friction Force.
Meteorology
14-9
Chapter 14
Wind
If the geostrophic force reduces then PGF and GF will no longer be in balance. PGF dominates
so the surface wind deflects toward the PGF, that is, deflected toward the low pressure. As seen
in the diagram, this will be a back in the Northern Hemisphere. In the Southern Hemisphere it will
be a veer. In both cases the surface wind will be slower than the wind above the friction layer.
Note: The above process applies equally to the wind around curved isobars.
The number of degrees of deflection and the reduction in windspeed for different situations are
shown in the table.
Deflection of Surface
Wind from 2000 ft wind
Speed of Surface Wind
as a % of the 2000 ft
wind
15°
30°
45°
75%
50%
25%
Over the Sea
Over the Land by Day
Over the Land by Night
DIURNAL VARIATION OF THE SURFACE WIND
The following paragraphs describe the diurnal variation of winds at different heights. Note that
these are for the Northern Hemisphere. In the Southern Hemisphere the speed changes are the
same but changes of direction are opposite.
SURFACE WIND
During the day, surface heating causes turbulent mixing and an increase in wind speed at the
surface. During night the air cools down, turbulence ceases, and the friction has full effect.
Night to day
Veer and increase
Day to night
Back and decrease
Over land from night to day the surface wind approximately doubles and veers by about 15°.
Windspeeds are highest at around 1500 hours as this is when there is greatest surface heating.
Windspeeds are lowest at around 0600 hours when temperatures are lowest.
1500 FT WIND
By day 1500 ft lies within the friction layer, hence is affected by friction. By night it lies above the
layer so is not affected.
Night to day
Back and decrease
Day to night
Veer and increase
2000 FT WIND
2000 ft is generally above the friction layer by day and by night, hence experiences little diurnal
variation.
Night to day
Day to night
14-10
Little variation
Little variation
Meteorology
Wind
Chapter 14
MEASUREMENT OF SURFACE WIND
At an airport, wind is measured by placing the sensors 10 metres above an even-ground surface.
This prevents false readings caused by surges due to ground obstacles or uneven ground. The
wind vane gives direction as shown in the simple version below.
Wind Vane
270°
360°
180°
90°
The most common wind velocity sensor is the cup anemometer, shown below. Pressure tube
anemometers may also be used. The cup anemometer tends to under-read the value of gusts
and over-read the average wind speed due to its inertia.
3-CUP
ANEMOMETER
Meteorology
14-11
Chapter 14
Wind
ISALLOBARIC EFFECT
If the pressure gradient changes, the three forces of PGF, GF, and centrifugal force are
temporarily out of balance. The wind tends to flow across the isobars from high to low until
balance is restored. An Isallobar is a line joining places that have an equal rate of pressure
change, hence the term Isallobaric Effect.
When air blows toward an area of falling low pressure, this is called convergence. When air
flows outwards from an area of increasing high pressure this is called divergence.
14-12
Meteorology
INTRODUCTION
The previous chapter explored lower winds which come about as a result of pressure differences
on a large scale. In this chapter more localised wind effects will be explored.
These tend to become apparent when the pressure gradient is slack or when the same air mass
remains in contact with the ground for an extended period, such as in a stable high pressure
system.
LAND AND SEA BREEZES
These winds are common when there is an anticyclone with a light pressure gradient on a clear
sunny day.
SEA BREEZE
During the day, the land heats up more quickly than the sea. The air in contact with the land
heats up and rises by the process of convection which leads to a decrease in pressure at the
surface and an increase in pressure at approximately 1000 — 2000 ft agl.
This causes air at that height to move over the sea. Air then descends over the sea causing an
increased pressure at the surface of the sea. Air then flows from the slightly higher pressure over
the sea surface to the lower pressure over the land surface and creates the sea breeze.
The circulation is shown in the diagram below.
Return Flow
Warm
Sea Breeze
L
Meteorology
Cool
H
15-1
Chapter 15
Local Winds
Sea breezes are typically 10 kt in temperate latitudes and extend to about 10 nm either side of
the coastline. In tropical areas they can be 15 kt and extend to 40 or 50 nm inland.
Initially the wind will be at right angles to the coastline but as insolation increases throughout the
day the wind will extend further from the coast and due to this longer fetch Coriolis effect comes
into play. This causes a veer in the Northern Hemisphere and a back in the Southern
Hemisphere.
LAND BREEZE
After sunset the land starts to cool down much more rapidly than the sea. This leads to a reversal
of the above situation. The sea surface experiences a lower pressure and the land a higher
pressure as shown in the diagram. The wind now blows from the land to the sea.
Return Flow
Cool
Warm
H
L
The temperature difference between land and sea is less at night so the land breeze is weaker
than the sea breeze – typically half the speed (5 kt in temperate latitudes) – and only extends to
about 5 nm out to sea.
OPERATIONAL IMPLICATIONS OF THE LAND AND SEA BREEZES
At coastal airfields, the landing and take-off direction is reversed from day to night if the runway is
at right angles to the coast. During the day landing/take-off will be towards the sea and at night
towards the land.
Coastal airfields with runways running parallel to the coast experience crosswinds when the sea
and land breeze are well-established.
Fog off the coast can be blown inland during the day reducing visibility at coastal airfields.
Lifting of air over land by the sea breeze can cause small cumulus clouds to form which assist
pilots in the identification of coastlines.
15-2
Meteorology
Local Winds
Chapter 15
KATABATIC AND ANABATIC WINDS
These winds occur on hillsides and valley sides and tend to form in slack pressure gradients.
KATABATIC WIND
During the night a hillside cools down rapidly. The air in contact with it is cooled by conduction
and becomes more dense than the free air next to it. It therefore flows down the hillside.
The katabatic wind is more apparent if the sky is clear as radiation is greater. If the slope is snow
covered this also assists.
The air remains in contact with the ground at all times and does not warm adiabatically. The
average speed is 10 kt.
If this wind occurs in a valley cold air collects at the bottom increasing the likelihood of fog or
frost.
Meteorology
15-3
Chapter 15
Local Winds
ANABATIC WIND
Anabatic wind is the opposite of the Katabatic wind and occurs during the day on slopes which
are subject to direct sunlight. As insolation increases, the air in contact with the land warms up,
becomes less dense and flows up the slope.
The Anabatic wind is typically weaker than the Katabatic (about 5 kt) since it flows against the
force of gravity.
15-4
Meteorology
Local Winds
Chapter 15
FOEHN WIND/EFFECT
This topic was already mentioned in the chapter on Cloud Formation.
The Foehn Wind was named for a warm dry wind that occurs in the Alps. There are several other
winds in other parts of the world which are caused by the same effect, such as the Chinook,
which flows down the east side of the Rocky Mountains.
The Foehn Wind occurs when air is forced to rise up a mountain side in stable conditions. It cools
initially at the DALR until it reaches saturation. At this point, cloud starts to form and the air
continues to rise, but now cools at the SALR.
Once it reaches the top of the mountain it starts to flow down the other side. Initially it warms at
the SALR but quickly becomes unsaturated as much of its moisture has already been lost. It then
warms at the DALR.
Since the cloud base is higher on the lee side, the air at the base on that side will be warmer than
on the windward side. The difference can be as much as 10°C (20°C with the Chinook).
8000 ft - 0°C
0°C
6000 ft - 3°C
3°C
4000 ft - 6°C
9°C
2000 ft - 9°C
15°C
0 ft - 15°C
21°C
Meteorology
15-5
Chapter 15
Local Winds
VALLEY/RAVINE WIND
When wind blows against a mountain barrier it finds its progress impeded. If there is a gap or
valley it is forced to flow through this. The restriction acts like a venturi and the wind speeds up.
Wind speeds of 70 kt can be experienced.
The combination of high wind speeds and rough terrain can result in turbulence at low level. An
additional hazard results from the fact that small changes in the general direction of the wind can
lead to sudden reversals in direction of the ravine wind.
15-6
Meteorology
Local Winds
Chapter 15
HEADLAND EFFECT
Where the 2000 ft wind blows parallel to the coast around a headland or cape the isobars push
together causing an increase in pressure gradient and hence an increase in wind speed.
LOW-LEVEL JET
A Low Level Jet (LLJ) is defined as a narrow, horizontal band of relatively strong wind (usually
between 20 and 80 kt) located between 500 to 5000 feet AGL. They are often several hundred
miles long and a few hundred miles wide. There are four common types of LLJ.
NOCTURNAL JET
When the ground cools quickly, an inversion may build, and the wind quickly slows along the
surface by friction. However above the inversion, the wind is not affected by friction, and the cold
calm air along the ground serves as a gliding layer.
The result is a strong wind, just above the inversion. Maximum wind speed is usually attained
about 4 − 8 hours after sunset, the time depending on the latitude. The wind abates when
insolation and convection destroys the inversion layer.
VALLEY INVERSION
Often accentuated in mountainous regions where cold air drains into the bottom of a valley, valley
inversions create an elevated stable layer and surface inversion. Wind speeds of more than 50 kt
are sometimes reported above such inversions.
Meteorology
15-7
Chapter 15
Local Winds
COASTAL JET
Water temperature differentials along many coasts around the world create elevated inversions or
shallow frontal zones where low level jet (LLJ) phenomena occur. These LLJ can persist both day
and night for as long as the temperature differentials last.
LOW LEVEL JET IN FRONT OF AN EXTRA-TROPICAL COLD FRONT
A large temperature contrast across a cold front can create a similar wind phenomenon as the
shallow coastal front and a pronounced LLJ forms ahead of a cold front inside the warm air mass.
15-8
Meteorology
INTRODUCTION
Air masses are large volumes of air with properties of humidity and temperature which remain almost
constant in the horizontal.
This phenomenon of more or less constant properties arises from the fact that the air in air masses
remains stationary over its source for an extended period of time. This essentially means that air
masses originate only in high pressure areas, as low pressures tend to be temporary features.
ORIGIN AND CLASSIFICATION
Air masses are initially classified by the latitude from which they originate. This gives us three main
types:
¾
Tropical
¾
Polar
¾
Arctic
They are further subdivided depending on whether they originate over sea or land:
¾
Maritime
¾
Continental
This gives us five main air masses:
1. continental Tropical (cT)
2. maritime Tropical (mT)
3. continental Polar (cP)
4. maritime Polar (mP)
5. Arctic (not subdivided) (A)
Meteorology
16-1
Chapter 16
Air Masses
maritime
Arctic (mA)
COLD
continental
Polar (cP)
maritime
Polar (mP)
continental
Tropical (cT)
maritime
Tropical (mT)
WARM
Tropical air originates in the sub-tropical high pressure zones. An example of continental tropical air
would be the air mass which originates in North Africa.
Maritime tropical air originates in the permanent high pressures over the oceans. In the North Atlantic
this is the Azores high. There is an equivalent high pressure in the North Pacific.
Continental polar air originates in the high pressures over large land masses, hence this air mass is
mainly a winter phenomenon. Examples of sources are Siberia and North America.
Maritime polar air originates in the north of the North Atlantic and North Pacific.
Arctic air originates over the North Polar ice cap. Since the region is ice covered, arctic air is not
subdivided into continental and maritime. In the Southern Hemisphere there is an Antarctic air mass
originating over the South Polar ice cap.
MODIFICATION OF AIR MASSES
As the air masses pass over other regions as they travel away from their sources, their properties
alter. In general, the following rules apply:
¾
16-2
An air mass passing over a warmer area:
•
Becomes warmer.
•
Becomes more unstable.
•
Experiences a reduction in relative humidity.
Meteorology
Air Masses
¾
Chapter 16
An air mass passing over a colder area:
•
Becomes colder.
•
Becomes more stable.
•
Experiences an increase in relative humidity.
AIR MASSES AFFECTING EUROPE
We have introduced the various types of air masses. The next sections go into more detail about the
kind of weather conditions that these air masses bring to Europe.
ARCTIC
Originating over the North Polar ice cap, the arctic air mass is very cold and stable at the source. It
has a low absolute humidity and low relative humidity.
It is more common in the winter and moves south if there is a high pressure to the west of the UK and
a low pressure to the east.
H
L
As an arctic air mass moves south toward Scotland, it becomes warmer and more unstable. It also
picks up moisture from the sea to the north of Scotland. Over land, large cumulus will form bringing
very cold weather, snow showers, and possible blizzards.
If it occurs in summer, there will be rain showers and the region will experience a marked drop in
temperature.
POLAR
MARITIME POLAR
A maritime polar air mass is cold and stable at its source, with a low absolute humidity but a high
relative humidity.
The air mass which comes to the UK originates in the far North Atlantic in the Greenland/Iceland
areas. As it moves south over the sea it becomes heated in the lower layers and becomes unstable. It
also picks up moisture.
Meteorology
16-3
Chapter 16
Air Masses
Once it reaches the UK it produces unstable weather with cumulus, cumulonimbus with heavy
showers, and sometimes thunderstorms and hail.
Visibility is generally good outside of cloud and showers.
At night in winter the clouds clear and radiation can lead to an inversion and radiation fog.
RETURNING MARITIME POLAR
This is maritime polar air that has reached the UK via an indirect route. It occurs when the air gets
deflected by a low pressure system in the North Atlantic.
This results in the air first travelling to the south of the North Atlantic before changing direction and
approaching the UK from the south-west.
The result is that the air becomes unstable as it travels south. Once it has turned north the lower
layers become stable, but the upper layers remain unstable.
In summer, convection can break through the lower stable layer resulting in Cu, Cb, and
thunderstorm activity, with hail and heavy showers.
CONTINENTAL POLAR
A continental polar air mass is mainly a winter phenomenon which originates in Siberia. It is very cold,
stable, and dry. It brings a cold easterly wind to the UK, with mainly good visibility except for some
occasional industrial smoke from Northern Europe.
If the air mass originates from further north it may pass over the North Sea on its way to the UK. In
this case it will become unstable and increasingly moist, resulting in cumulus clouds and heavy
showers on the east coast of England and Scotland.
The conditions are not as severe as those associated with maritime polar as the air mass has a much
shorter sea passage.
In the summer, the high pressure over Siberia replaces low pressure as the land mass heats up. Air
originating in this area is then generally referred to as continental tropical.
Occasionally there may be a high pressure over Scandinavia. This results in an air mass passing
over the North Sea. This sea will now be colder than the surrounding land areas, so the air mass will
become cooled and more stable. It will absorb moisture as it passes over the sea.
This results in what is referred to as Haar conditions on the coast of east Scotland and north-east
England. These conditions are very low stratus with drizzle, advection fog, and bad visibility. In the
northeast of England, these conditions are colloquially termed Sea Fret.
16-4
Meteorology
Air Masses
Chapter 16
TROPICAL
MARITIME TROPICAL
A maritime tropical air mass originates in the Azores high in the south of the North Atlantic. It is warm
and stable with a high absolute humidity and a moderate relative humidity.
As it moves northeast, it cools and becomes more stable with increased relative humidity.
On reaching the south-west coast of the UK it produces low stratus and stratocumulus with drizzle
and poor visibility. Advection fog occurs over the land areas in winter and early spring and sea areas
in late spring and early summer.
In summer the increased insolation and convection clears the low cloud resulting in clear skies and
good visibility, with occasional fair weather cumulus.
CONTINENTAL TROPICAL
A continental tropical air mass originates in North Africa and south-east Europe, plus Siberia in the
summer. It is a warm dry air mass which brings clear dry weather with generally good visibility.
Occasionally, some dust haze comes north from the Sahara region.
Occasionally the air mass picks up some moisture over the Mediterranean and becomes unstable but
this moisture is lost as showers over France.
AIR MASS SUMMARY
ARCTIC
Normal Winter Only
Source Region
North Polar Ice
Cap
Conditions at
Source
Temperature
Cold
Relative Humidity
Low
Absolute Humidity
Low
Modifications
Weather
Moves south and is
heated from below,
becoming unstable
Arrives over Europe as
extremely cold, moist and
unstable
Evaporation from sea
causes increased
dewpoint and RH
CU or CB give heavy snow
showers, possibly TS on
north and north-east facing
coasts
Inland clear and cold
Meteorology
16-5
Chapter 16
Air Masses
MARITIME POLAR
Summer
Source Region
Sea areas
around Iceland
and Greenland
Conditions at
Source
Modifications
Cold
The air mass is heated as
it moves south-east
Relative Humidity
High
It becomes unstable over
a great depth
Absolute
Humidity
Low
Moisture evaporates from
the ocean so RH
increases
Temperature
Weather
Widespread CU and CB
activity overland with
moderate to heavy
showers of rain or hail
Moderate to severe icing
and turbulence in cloud
Visibility good outside
cloud
Winter
Source Region
Sea areas
around Iceland
and Greenland
Conditions at
Source
Temperature
Cold
Relative Humidity
High
Absolute
Humidity
Low
16-6
Modifications
Weather
The air mass is heated as
it moves south-east to a
greater extent than in
summer
Day – as above but more
severe. Strong gusts and
squalls common
Becomes unstable over a
great depth
Night – skies clear with
possible radiation fog
Moisture evaporates from
the ocean so RH
increases
Meteorology
Air Masses
Chapter 16
RETURNING MARITIME POLAR
Summer
Source Region
Sea areas
around Iceland
and Greenland,
with a low
pressure to the
west of Ireland
Conditions at
Source
Temperature
Cold
Relative Humidity
High
Absolute Humidity
Low
Modifications
As the air moves south it
becomes unstable over a
great depth
Continuous evaporation
raises the dew point and
the RH remains high
The depression west of
Ireland drags the air mass
in an anti-clockwise
direction towards Europe
The movement into colder
regions stabilises the air
mass in the lower layers
and leaves the upper
layers unstable
Near the surface the air
mass has similar
characteristics to the mT
Weather
Day
Warmer than average
temperatures with a
relatively high RH
Insolation heats up the land
surfaces
As air moves over the
heated surfaces the lower
layer becomes unstable
leading to the development
of CU and CB
CB produce widespread TS
and showers which are
most marked in the
afternoon
Visibility moderate to good
Night
Convective activity dies out
as surface temperatures
fall
CU may spread into SC
Visibility moderate
Meteorology
16-7
Chapter 16
Air Masses
RETURNING MARITIME POLAR
Winter
Source Region
Sea areas
around Iceland
and Greenland,
with a low
pressure to the
west of Ireland
Conditions at
Source
Temperature
Cold
Relative Humidity
High
Absolute
Humidity
Low
Modifications
Weather
As the air moves south it
becomes unstable over a
great depth
As for the mT although
medium level instability
may be encountered
Continuous evaporation
raises the dew point and
the RH remains high
CB formation over
mountains
The depression west of
Ireland drags the air mass
in an anti-clockwise
direction toward Europe
Medium level ACC may be
apparent
The movement into colder
regions stabilises the air
mass in the lower layers
and leaves the upper
layers unstable
Near the surface the air
mass has similar
characteristics to the mT
16-8
Meteorology
Air Masses
Chapter 16
CONTINENTAL POLAR
Normal Winter Only
Source Region
Siberia,
Northern Europe
and Scandinavia
Conditions at
Source
Temperature
Cold
Relative Humidity
Low
Absolute
Humidity
Low
Modifications
Weather
Moves over the cold
winter land of Europe and
remains cold, dry, and
stable
If the airflow is from the
east via continental
Europe, the weather is very
cold and very dry with no
precipitation
If the air passes over the
relatively warm North Sea
the air is heated from
below and the absolute
humidity increases
If the airflow is over the
North Sea, Then CU and
CB can give showers on
the east coast of UK
(In summer the air mass is
rare. With a high pressure
over Scandinavia in early
summer a North Easterly
flow occurs. The air is dry,
warm, and stable, leading
to Haar conditions)
Meteorology
16-9
Chapter 16
Air Masses
MARITIME TROPICAL
Summer
Source Region
The Azores high
Conditions at
Source
Temperature
Warm
Relative Humidity
Mod
Absolute Humidity
High
Modifications
As it moves north-east
toward Europe, the air
is cooled from below,
increasing stability
Continual evaporation
gives a high dew point
and high RH
Weather
Advection fog likely over
the sea
Warm moist conditions with
some ST or SC, visibility
moderate or poor
Winter
Source Region
The Azores high
Conditions at
Source
Temperature
Warm
Relative Humidity
Mod
Absolute Humidity
High
16-10
Modifications
As it moves north-east
toward Europe the air is
cooled from below,
increasing stability
Continual evaporation
gives a high dew point
and high RH
Weather
Extensive low SC giving
continuous drizzle or light
rain
Temperatures above the
seasonal average, with
moderate to poor visibility
Advection fog forms if the
air flows over a snow
covered surface. This flow
can also cause a general
thaw
Meteorology
Air Masses
Chapter 16
CONTINENTAL TROPICAL
All seasons, but more common in summer
Source Region
North Africa and
South East
Europe
Conditions at
Source
Temperature
Warm
Relative Humidity
Low
Absolute Humidity
Low
Meteorology
Modifications
Moves north and is
cooled from below
becoming more stable
Movement overland
keeps the humidity low
Weather
Hot, dry conditions
Sometimes hazy with dust
from the Sahara
Some cloud and
precipitation over France if
the air mass picks up
moisture and becomes
unstable over the
Mediterranean
16-11
Chapter 16
16-12
Air Masses
Meteorology
INTRODUCTION
The previous chapter discussed air masses, where the properties of temperature and humidity
are relatively constant in the horizontal throughout the air mass.
Also discussed was how the properties of air masses differ from those of other air masses. The
boundary between two air masses with different properties is called a front.
Fronts can produce quite active weather. This chapter discusses the characteristics of various
types of front.
TYPES OF FRONT
Where two air masses meet, the warmer air is less dense and rises up over the colder air. This
gives a sloping frontal surface.
Initially this chapter explores the three main types of front.
WARM FRONT
Where warm air replaces cold air, as shown below, it is called a warm front. Also shown below is
the symbol used on synoptic charts to represent the warm front.
Meteorology
17-1
Chapter 17
Fronts and Occlusions
COLD FRONT
Where cold air replaces warm air, as shown below, it is called a cold front. Also shown below is
the symbol used on synoptic charts to represent the cold front.
QUASI-STATIONARY FRONT
Where there is little frontal movement, and neither air mass can be said to be replacing the other,
it is termed a quasi-stationary front. A diagram representing this situation is shown below along
with the chart symbol for the quasi-stationary front.
PRESSURE SITUATION AT A FRONT
As an aircraft flies from a warm air mass into a cold air mass across a front, if it maintains the
same true altitude then the colder air means higher density and hence higher pressure.
Shown below is the view from above as an aircraft flies along an isobar towards the front. Once it
crosses the front, the pressure increase means that the isobars have changed orientation. They
bend towards the low pressure.
The greater the temperature change at the front, the greater the change in direction of the
isobars. As the isobars determine the direction of the wind, one would expect stronger windshear
when the temperature change is greater.
17-2
Meteorology
Fronts and Occlusions
Chapter 17
SEMI-PERMANENT FRONTS OF THE WORLD
In both hemispheres there are several semi-permanent or quasi-stationary fronts.
ARCTIC FRONT
This is the boundary between arctic and polar air and is found at latitudes above 65°.
POLAR FRONT
A polar front is the boundary between polar and tropical air. It is found between latitudes 35° and
65° in the Northern Hemisphere and at around 50° in the Southern Hemisphere.
In winter the polar front stretches from Florida to south-west UK. In the summer it retreats north,
stretching from Newfoundland to the north of Scotland.
In this region, a phenomenon called the Polar Front Depression arises. This is the major factor
in the weather patterns found in the UK and Europe and will be discussed later in this chapter.
MEDITERRANEAN FRONT
This front only exists in winter when there is low pressure in the Mediterranean. It is the boundary
between polar continental or maritime air and tropical continental air.
Meteorology
17-3
Chapter 17
Fronts and Occlusions
INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
Originally called the inter-tropical front, the inter-tropical convergence zone was renamed since it
is not really a front. It is a boundary zone around 300 nm wide between tropical air masses on
either side of the heat equator. Since both masses are tropical, the word ‘front’ is misleading
hence the name change.
It is also sometimes referred to as the Equatorial Trough or just the Heat Equator.
The ITCZ is discussed in considerably more detail in the chapters on Climatology.
CHARACTERISTICS OF FRONTS
This section explores characteristics of the warm front and the cold front, including the likely kinds
of weather to expect.
WARM FRONT
A warm front occurs when warm air replaces cold air. It rides up over the cold air forming a
sloping frontal surface with an average gradient of about 1:150.
Since warm air is less dense, its progress is retarded by the cold dense air ahead of it. The front
therefore travels at about 2/3 of the geostrophic wind speed that would otherwise be expected
from the isobar interval along the front.
The gentle slope of the front means that lifting will not be strong enough to form cumuliform cloud.
Instead, layer cloud will form. Approaching the front from the cold air side layer clouds appear in
the following order: Ci, Cs, As, Ns.
A progressively lowering cloud base results. The cirrus cloud will be seen up to 600 nm in
advance of the surface position of the front.
17-4
Meteorology
Fronts and Occlusions
Chapter 17
No precipitation will be experienced prior to reaching the altostratus. where you will see virga –
precipitation that doesn’t reach the ground. As you approach the nimbostratus the rain will
become continuous moderate or heavy.
As the front approaches, the pressure drops, but once it passes the fall will be arrested. However,
since the air behind the front is warmer, it settles to a lower value than that preceding the front.
The wind veers, but since the passage of the system is quite slow, this change tends to be
gradual and doesn’t usually result in problematic windshear.
COLD FRONT
A cold front occurs when cold air replaces warm air. The cold air undercuts the warm air because
it is more dense and its progress is not impeded by the warm air it replaces. It therefore moves at
the geostrophic wind speed.
The cold front is much steeper, averaging about 1:50. Sometimes it becomes vertical and even
bulges out into the warm air forming a nose-like protrudence.
Cold front lifting is much greater hence this front produces cumuliform cloud such as Cu and Cb
and possible thunderstorm activity. There may be shelves of nimbostratus or cirrus cloud
extending into the cold air when there is a stable layer.
Since the slope is much steeper than that of the warm front, the band of associated cloud only
spans up to about 200 nm.
Meteorology
17-5
Chapter 17
Fronts and Occlusions
COLD FRONT
CI
CU/CB
WARM AIR
COLD AIR
NS
As the front approaches, the pressure drops due to the rising air, but after its passage it rises
again and settles at a greater value than that preceding the front since the air is now colder.
Wind direction changes over a much shorter passage of time than that of the warm front. Hence
strong windshear tends to be associated with active cold fronts.
POLAR FRONT DEPRESSIONS
These form on the polar front – the boundary between polar and tropical air. At the front the
pressure is lower as the warm air rises up over the cold air. Moving away from the front on either
side the pressure increases.
Obeying Buys Ballots Law the wind flows along the isobars with the low pressure to the left. As
the diagram below illustrates the wind on either side of the front flows in opposite directions.
17-6
Meteorology
Fronts and Occlusions
Chapter 17
The above situation causes friction which leads to the formation of waves or ripples along the
front. As the size of the ripples increases with increasing wind speed, the warm air bulges into the
cold air as shown below.
More warm air flows into the depression, causing the depression to deepen.
The result is a system shaped like a shark fin, with a warm front followed by a cold front. The tip
of the shark fin is a low pressure centre.
Meteorology
17-7
Chapter 17
Fronts and Occlusions
Growth of a polar front depression takes about four days. The depression dies away as it fills
which typically takes ten days.
The system moves in an easterly direction under the influence of the westerly upper winds,
forming an overall picture like that shown on the synoptic chart below. This is known as a
westerly wave.
WEATHER ASSOCIATED WITH THE POLAR FRONT
DEPRESSION
INTRODUCTION
As a polar front depression passes over a point, the first weather experienced will be that
associated with a warm front before the cold front arrives. The weather in this sector will depend
on the stability of the air in this sector, as described below.
After the warm front comes the cold front, bringing with it the expected cold front weather. After
the cold front passes there will be a period of cold clear weather before the arrival of the next
polar front depression.
A typical picture is shown in the next figure:
17-8
Meteorology
Fronts and Occlusions
Chapter 17
WARM FRONT
The weather associated with the passage of the warm front is summarised in the table below:
Warm Front
In Advance
At the Passage
In the Rear
Pressure
Steady fall
Fall arrested
Little change or slow
fall
Wind
Backing slightly and
increasing
Veer and decrease
Steady direction
Temperature
Steady or slow rise
Rise
Little change
Dewpoint
Rise in precipitation
Rise
Steady
Relative Humidity
Rise in precipitation
May rise further if not
already saturated
Little change, may be
saturated
Cloud
CI, CS, AS, NS in
succession,
increasing to 8 oktas
Low ST
ST, SC may persist
perhaps some CI
Weather
Light continuous from
AS becoming
moderate continuous
from NS
Precipitation eases
or stops
Dry or intermittent
rain or snow
Visibility
Good except in
precipitation
Poor, often mist or
fog
Moderate or poor,
mist or fog may
persist
WARM SECTOR
The weather in the warm sector depends on the stability of the air. If the air is stable it is called a
kata front. The clouds will be mainly stratiform.
Meteorology
17-9
Chapter 17
Fronts and Occlusions
Unstable air produces cumuliform cloud, with the possibility of embedded CBs.
17-10
Meteorology
Fronts and Occlusions
Chapter 17
COLD FRONT
The weather associated with the passage of the cold front is summarised in the table below:
Cold Front
In Advance
At the Passage
In the Rear
Pressure
Fall
Sudden rise
Rise continues more
slowly
Wind
Backing and
increasingly
becoming squally
Sudden veer,
perhaps squall
Further squalls before
settling
Temperature
Steady, but falling in
pre-frontal rain
Sudden fall
Little change, variable
in showers
Dewpoint
Little change
Sudden fall
Little change
Relative Humidity
Rise in pre-frontal
precipitation
Remains high in
precipitation
Rapid fall as
precipitation ceases,
variable in showers
Cloud
ST or SC, AC, AS
then CB
CB, CU sometimes
NS and CI
Lifting rapidly
Weather
Some rain, perhaps
thunder
Heavy rain or
snow, perhaps hail
and thunder
Heavy rain or snow for
usually a short period,
sometimes more
persistent, then fine
Visibility
Moderate or poor,
perhaps fog
Good except in
showers
Becomes excellent
well behind the front
OCCLUSIONS
Consider the polar front depression. The warm front is followed by a cold front. As previously
mentioned, the cold front moves at a speed equivalent to the geostrophic wind speed expected
by measuring the isobar spacing at the front.
Meteorology
17-11
Chapter 17
Fronts and Occlusions
However, the warm front is moving at only 2/3 this speed. Hence as the polar front depression
travels east across the North Atlantic, the cold front gains on the warm front, progressively
narrowing the warm sector between the two fronts. Eventually it catches up with the warm front,
as shown in the diagram below.
A
B
This occurrence is called an occlusion. The two types of occlusions are warm and cold.
Which type of occlusion occurs depends on the relative temperatures of the air masses ahead of
the warm front (A) and behind the cold front (B). If the air at A is colder, it is termed a warm
occlusion; if the air at B is colder, it is a cold occlusion.
Both air masses are in fact part of the same air mass, the polar air. However, as an air mass
travels, its characteristics change according to the surface over which it passes.
In the UK in summer, the most common type of occlusion is the cold occlusion. This is because
the air ahead of the warm front has spent a greater length of time over the warmer land, but the
air behind the cold front has much more recently been over the cold sea.
Conversely, in winter, the sea is warmer than the land; hence the common occlusion type is the
warm occlusion.
17-12
Meteorology
Fronts and Occlusions
Chapter 17
Occlusions are shown as below on the synoptic chart. As you can see, for a warm occlusion, it is
the warm front which continues along the same line. For a cold occlusion, it is the cold front that
continues.
WARM OCCLUSION
As shown above, in a warm occlusion the air behind the cold front is less cold than the air ahead
of the warm front. Hence it rides up over the air in front.
The warm front extends down to the surface, but the cold front doesn’t. The warm sector is never
in contact with the ground.
The expected cloud types are the same as with a warm front initially, with cumuliform cloud
coming at around the same time as the later warm front clouds.
Most of the weather will be experienced before the passage of the surface front.
Meteorology
17-13
Chapter 17
Fronts and Occlusions
COLD OCCLUSION
In the case of a cold occlusion, the air behind the cold front is colder, so it undercuts the air in
front of the warm front. The cold front extends to the surface but the warm front does not. Again,
the warm sector is no longer in contact with the ground.
Expect the same pattern of clouds as for the warm occlusion, but more of the cloud occurs after
the passage of the surface front.
BACK BENT OCCLUSION
As the polar front depression travels, the occluded section can lag behind, in which case it may
bend back on itself. This can give a region of intense weather at the two occluded sections and
the low pressure between them.
17-14
Meteorology
INTRODUCTION
The chapter on Wind introduced the concepts of the geostrophic wind. The formula for
geostrophic wind speed is given again here:
V=
PGF
2Ωρ
SIN θ
Just like lower winds, the upper winds are caused by the same forces: pressure gradient force,
geostrophic force, and cyclostrophic force.
This means that the geostrophic wind formula above also applies to upper winds. Since the wind
speed is inversely proportional to air density, wind speed would be expected to increase as height
increases and density decreases.
For example, the density at 20 000 ft is approximately half that at the surface, thus doubling the
wind speed.
THERMAL WIND COMPONENT
INTRODUCTION
The following diagram shows two columns of air, one cold, and one warm. The surface pressure
is the same in both cases – in the example we have used 1020 hPa.
Pressure falls more quickly over cold air and less quickly over warm air, so the air pressure over
the cold air would be expected to be lower than that at the same height over the warm air.
Meteorology
1010 hPa
1009 hPa
1020 hPa
1020 hPa
18-1
Chapter 18
Upper Winds
The wind must obey Buys Ballot’s Law:
“In the Northern Hemisphere with your back to the wind, the low pressure is on
your left.”
Hence, in this example, the wind must be blowing off the page. This gives a new law similar to
Buys Ballot’s Law:
“In the Northern Hemisphere with your back to the upper wind, the cold air is on
your left.”
In the Southern Hemisphere the cold air is on your right.
CALCULATING THE THERMAL WIND COMPONENT
To calculate the thermal wind component use the following formula:
THERMAL WIND SPEED = TEMP GRADIENT PER 100 NM X
ALTITUDE DIFF (FT)
1000
8000 ft
24°C
18°C
200 nm
For example, to calculate the thermal wind component for the above picture case, determine that
the temperature gradient is 3°C/100 nm and the thickness of the layer is 8000 ft. Hence the
thermal wind component is:
3X
8000
1000
= 24 KT
The direction of the wind depends on the relative positions of the cold and warm air masses.
Note:
18-2
This formula is only valid for the 50° latitude. For other latitudes, multiply the answer by
sin 50 ÷ sin Latitude.
Meteorology
Upper Winds
Chapter 18
UPPER WIND
If the geostrophic wind was calm, the upper wind at any level would simply be the thermal wind
component over the layer between that level and 2000 ft.
Thermal wind
component
Geostrophic
wind
Upper wind
If there is a geostrophic wind, then the upper wind will be the vector sum of the geostrophic wind
and the thermal wind component. Resolve this graphically or by using the CRP-5.
Continuing on from the previous example, assume a geostrophic wind of 040/20 with cold air to
the north, in the Northern Hemisphere. The following steps show how to calculate the upper wind
for 10 000 ft using a CRP-5.
STEP 1
The cold temperature is to the north. Using Buys Ballot’s Law with the wind
behind, the low temperature is on the left. The wind direction must be from 270°.
STEP 2
Having already calculated the thermal wind speed at 24 kt, the thermal wind
component is 270/24 kt.
STEP 3
Set the 2000 ft wind velocity using the zero line.
Meteorology
18-3
Chapter 18
Upper Winds
STEP 4
Set the TWC. The origin of the TWC is the end of the geostrophic wind
component.
STEP 5
Move the end of the TWC component to the centre line and read off the upper
wind at 10 000 ft — 325/20.
Note:
18-4
If the geostrophic and the thermal wind component are in opposite directions, the
wind first decreases in speed as height increases, becoming calm before
reversing in direction and increasing in speed.
Meteorology
Upper Winds
Chapter 18
GLOBAL UPPER WINDS
The diagram below shows the Earth with warm air over the equator and a decreasing
temperature as we move towards the poles. In the Northern Hemisphere the wind keeps the low
temperature to its left, in the Southern Hemisphere it keeps the low temperature to its right. In
both cases this gives a westerly wind.
Exceptions to the rule occur in the tropics and over the poles, where the upper winds are easterly.
JET STREAMS
INTRODUCTION
A jet stream is a wind greater than 60 kt in speed, which manifests itself as a long corridor of wind
with typical dimensions of 1500 nm in length, 200 nm in width and 12 000 ft in depth.
They are caused by large temperature differences in the horizontal.
Meteorology
18-5
Chapter 18
Upper Winds
The wind speed is fastest at the core and decreases with movement away from the core.
60 kt
80 kt
100 kt
120 kt
Speeds in excess of 100 kt are quite common, but it is rare for jet streams to be faster than
200 kt. However, jets of 300 kt have been reported on occasion. These extreme examples tend to
occur in the east Asia/Japan area.
COMMON JET STREAMS
The table below shows the common global jet streams.
Latitude
Pressure Level
Polar front jet stream
45° to 65° N/S
300 hPa – 30 000 ft
Sub-tropical jet stream
20° to 40° N/S
200 hPa – 45 000 ft
Equatorial jet stream
10° to 15° N/S
100 hPa – 55 000 ft
Polar jet stream
70° to 80° N/S
50 hPa – 75 000 ft
Details of the equatorial and the polar jet stream are not required for the course, but this chapter
goes into more detail about the Sub-tropical and the Polar Front jet streams.
SUB-TROPICAL JET STREAM
These occur above the sub-tropical anti-cyclones and are caused by the circulation of the Hadley
cells. The Hadley cells are a circulation which starts with lifting over the heat equator due to
surface heating. When the air reaches the tropopause it flows away from the equator to higher
latitudes.
At approximately 30° latitude, the air is cooled such that it starts to descend. Where it reaches the
surface it forms the sub-tropical anti-cyclones. It then flows into the low pressure at the heat
equator.
The following diagram shows the circulation of air on the Earth.
18-6
Meteorology
Upper Winds
Chapter 18
Polar
cell
Ferrel
cell
Hadley
cell
The sub-tropical jet stream forms when air from the Hadley cells meet air from higher latitudes.
Due to the large amount of air, not all of it descends; some of it is forced to flow horizontally. In
both the Northern and Southern Hemispheres geostrophic force turns it to the right.
In both cases this results in a westerly jet.
PGF
GF
PLAN VIEW
NH
PGF
GF
Heat equator
Heat
equator
SH
The sub-tropical jet streams exist all year round but move as the heat equator moves. In winter
they exist in the latitude band 25° — 40° and in the summer are found in the latitude band 40° to
45°.
Meteorology
18-7
Chapter 18
Upper Winds
POLAR-FRONT JET STREAM
Like the name suggests, a polar-front jet stream occurs on the Polar Front. The following diagram
shows the position of the jet stream in cross section and in plan view.
The diagram shows that the jet stream forms in the warm (tropical air) just below the warm air
tropopause. In the plan view the jet stream appears to be in cold sector. However, it is the surface
position of the fronts that is shown. The fronts slope so in fact the jet is in the warm air.
Unlike the sub-tropical jet stream, the polar front jet stream is not in a constant westerly direction.
It follows the patterns of the polar front depressions and forms a zig-zag shape which is westerly
on average.
They are less permanent than the sub-tropical jets, tending to die out a bit in summer. Average
speeds in summer are 60 kt; in winter, 80 kt.
Like the sub-tropical jet stream, the polar front jet streams change position with the movement of
the heat equator. Approximate positions are between 40°N and 65°N and at around 50°S.
18-8
Meteorology
Upper Winds
Chapter 18
WINDS AROUND A POLAR FRONT DEPRESSION
This chapter has explored the polar front depression and the pattern of isobars around it. The
2000 ft wind follows the isobars in an anti-clockwise direction around the low pressure centre, as
shown in the diagram.
Super-imposed onto this is the polar front jet stream, which obeys the rule of always keeping the
cold air to its left. As a result, the 2000 ft wind and the upper wind often come from different
directions. This is summarised below:
Position
2000 ft wind
Upper wind
Trend
Ahead of the warm front
South-westerly
North-westerly
Veer and increase
In the warm sector
Westerly
Westerly
Increase
Behind the cold front
North-westerly
South-westerly
Back and increase
CLEAR AIR TURBULENCE
The windshear within and around jet streams leads to friction within the atmosphere. This causes
turbulence known as clear air turbulence (CAT), due to the fact that it is not caused by clouds or
by proximity to the ground.
The most severe CAT is found level with the core of the jet on the cold air side. A secondary area
of severe CAT is found above the core, above the warm air tropopause.
Meteorology
18-9
Chapter 18
Upper Winds
If CAT associated with a polar front jet stream in the Northern Hemisphere is experienced,
descend and turn to the south. This brings the aircraft into the warm air and away from the
strongest turbulence.
IDENTIFICATION OF JET STREAMS
It is usually impossible to identify a jet stream visually. However, if the air is moist, there may be a
trail of cirrus cloud associated with the jet stream, as shown below.
This cirrus is caused by a lowering of pressure and temperature around the jet stream, due to the
high velocity of the air. This cools the air to its dewpoint causing some water vapour to sublimate
out as ice crystals.
Another way to identify a jet stream is by looking at meteorological charts, like those discussed
below. Other important charts are discussed in the chapter on Upper Air Charts.
CONTOUR CHARTS
For lower winds, use synoptic charts. These show isobars (lines of constant pressure) and from
this the direction of the wind is predictable.
For upper winds a different system is used. Rather than using a chart for a given height above
mean sea level and showing the different pressures on the chart, charts with constant pressure
are used and the lines drawn join places of constant height above mean sea level at which that
pressure occurs.
This is useful for high altitude flying as flights are conducted at flight levels/pressure altitudes, that
is, the aircraft flies along a line of constant pressure.
Common charts in use are as follows:
Pressure (hPa)
Equivalent Pressure
Altitude (feet)
700
500
300
250
200
150
10 000
18 000
30 000
34 000
39 000
53 000
The lines joining places of equal height are called contour lines and the heights are expressed in
one of two ways. The number may represent the height in 100s of feet or the height in
decametres (10s of metres).
18-10
Meteorology
Upper Winds
Chapter 18
A line with a low value means that the pressure for which the chart is produced is found at a lower
height, whereas a high value means the pressure is found at a greater height. As can be seen
from the following diagram, this means that areas of low contour heights are areas of low
pressure.
31 000 ft
30 000 ft
< 300 hPa
> 300 hPa
300 hPa
29 000 ft
Since the wind follows Buys Ballot’s law, it flows with the low contour lines to its left in the
Northern Hemisphere. As for a synoptic chart, the closer the contour lines, the stronger wind.
THICKNESS CHARTS
Another chart used to discern wind direction is the thickness chart which shows the thickness of
the layer between two given pressure values.
As shown in the diagram below, a low thickness value is associated with cold air and a high value
is associated with warm air. Lines of constant thickness are called isopleths.
500 hPa
Cold air – low
thickness value
Warm air – high
thickness value
1000 hPa
Meteorology
18-11
Chapter 18
Upper Winds
In the Northern Hemisphere the thermal wind keeps the cold air to its left, hence it travels parallel
to the isopleths keeping the low value isopleths to its left. In the Southern Hemisphere the low
value isopleths are to the right.
If the isopleths are closely spaced, this indicates a steep temperature gradient and hence
stronger winds.
18-12
Meteorology
WINDSHEAR
The following meteorological factors can cause windshear:
1.
2.
3.
4.
5.
6.
7.
8.
Inversions
Mountain waves and rotors
Katabatic winds (fall winds)
Sea breeze fronts
Air mass fronts
CB cloud
Low level jet
Jet streams
DEFINITIONS AND THE METEOROLOGICAL BACKGROUND
In discussing windshear it is not easy to find a definition which satisfies both meteorologist and
pilot. At its simplest, windshear is a change in wind direction and/or speed in space, including
updraughts and downdraughts. Despite the emphasis on the windshear hazard in recent years,
there are still some who argue that aviators have lived with windshear since the dawn of aviation,
seeing it as an extreme form of wind gradient, which would itself fit this definition.
DEFINITION
Variations in vector wind along the aircraft flight path of a pattern, intensity, and duration so as to
displace an aircraft abruptly from its intended path requiring substantial control action.
LOW ALTITUDE WINDSHEAR
Low altitude windshear is windshear along the final approach path or along the runway and along
the takeoff and initial climb out flight path.
Further refinement offers:
¾
¾
¾
Vertical windshear as the change of horizontal wind vector with height, as might be
determined by two or more anemometers at different heights on a mast;
Horizontal windshear as the change of horizontal wind vector with horizontal distance as
might be determined by two or more anemometers mounted at the same height at
different points along a runway;
Updraught/downdraught shear as changes in the vertical component of wind with
horizontal distance.
Meteorology
19-1
Chapter 19
Windshear and Turbulence
Setting aside the basic windshear definition above, the other definitions allow for changes of
vector wind from the relatively minor event upwards. The essence of the windshear with which
this chapter is concerned is spelt out by the basic definition with its emphasis on abrupt
displacement from the flight path and the need for substantial control action to counteract it.
A windshear encounter is a highly dynamic and potentially uncomfortable event; to think of
windshear as an aggravated form of wind gradient is unwise. Windshear can strike suddenly and
with devastating effect, sometimes beyond the recovery powers of experienced pilots flying
modern and powerful aircraft. An encounter may cause alarm, a damaged landing gear, or a total
catastrophe. The first and most vital defence is avoidance.
METEOROLOGICAL FEATURES
The most potent examples of windshear are associated with thunderstorms (cumulonimbus
clouds), but windshear can also be experienced in association with other meteorological features
such as the passage of a front, a marked temperature inversion, a low-level wind maximum, or a
turbulent boundary layer. Topography or buildings can exacerbate the situation; particularly in a
strong wind.
THUNDERSTORMS
The chapter on Meteorological Notes describes thunderstorm formation and how the wind flows
in and around the thunderstorm which causes the most severe windshears. Diagrams do no
justice to the violence of totally dynamic and unpredictable thunderstorms with turbulence, hail,
windshear, and lightning as separate or joint hazards. Shears and draughts may strike from all
angles and are certainly not limited to the horizontal or vertical; an assessment of the aircraft’s
actual angle of attack relative to some thunderstorm wind flows is difficult to make, which in turn
makes the risk of a stall harder to gauge. This is significant if a thunderstorm is encountered on
the approach or following take-off.
FRONTAL PASSAGE
Fronts, whether warm, cold, or occluded, vary in strength. It is only well developed active fronts,
with narrow surface frontal zones and with marked temperature differences between the two air
masses, which are likely to carry a risk of windshear.
Warning signs to look out for include sharp changes in wind direction indicated on the weather
charts by an acute angle of the isobars as they cross the front, a temperature difference of 5º C
or more across the frontal zone, and the speed of movement of the front, especially if 30 kt or
more.
It should be mentioned that windshear is possible in fronts which are slow moving, stationary or
even reversing direction. The passage of a vigorous cold front poses the greater risk though,
relative to a warm front, as the period of windshear probability is likely to be much shorter and
occurs just after the surface passage of the front. With a warm front, the effect precedes the
passage and is more prolonged.
To illustrate the potential severity of frontal windshear, there is the case of a twin jet aircraft
caught by the passage of a cold front while flaring to land. Within about ten seconds, the wind
shifted from 230/10 kt to 340/16 kt, so that a 10 kt crosswind from the left and slight tail wind
changed to an 8 kt crosswind from the right with 14 kt headwind. The pilot, finding directional
control for landing to be difficult, wisely carried out a missed approach from a very low level.
19-2
Meteorology
Windshear and Turbulence
Chapter 19
This is a classic case of horizontal windshear. A sea-breeze front may occasionally present a
hazard; for example if it impinges on a thunderstorm it may significantly alter the outflow from the
storm; a catastrophic accident in the USA in 1975 involved such a feature.
INVERSIONS
Vertical windshear is nearly always present in the boundary layer, but this normally involves a
gradual change in the wind with which pilots are well familiar. A hazard exists, however, when an
unexpectedly strong vertical shear develops.
This can occur broadly in two situations:
¾
¾
A low-level jet (more accurately referred to as a low-level wind maximum) can form just
below the top of, or sometimes within, a strong radiation inversion which may develop at
night under clear skies. Other low-level jets may develop in association with a surface
front, particularly ahead of cold fronts;
On occasions, low-level inversions develop and decouple a relatively strong upper flow
from layers of stagnant or slow moving air near the surface. Windshear may be
pronounced across the interface.
TURBULENT BOUNDARY LAYER
Within the boundary layer, turbulence becomes a windshear hazard in two different situations:
¾
¾
Strong surface winds are generally accompanied by large gusts and lulls (horizontal
windshear). Roughly speaking, the stronger the mean wind, the greater the gust or lull.
Thermal turbulence (updraughts and downdraughts) is caused by intense solar heating of
the ground, which is more common in hot countries, but can occur anywhere on a hot
sunny day.
TOPOGRAPHICAL WINDSHEARS
Either natural or man-made features affect the steady state wind flow and cause windshears of
varying severity. The strength and direction of the wind relative to the obstacle are significant and
a change of direction of relatively few degrees may appreciably alter the residual effect. The flow
of wind across a mountain range is a simple large scale example, with waves and possibly a rotor
forming on the leeside.
Wind blowing between two hills or along a valley, or even between two large buildings may be
funnelled, thus changing direction and increasing in speed, or a strong flow may be heavily
damped. Either way, this creates the possibility for shear, with sudden changes of wind vector
becoming a hazard.
Usually local effects become well known and predictable, with warnings given on aerodrome
approach plates (e.g. Gibraltar). Large airport buildings adjacent to busy runways can create
hazardous local effects and typical windshear problems, such as loss of airspeed and abrupt
crosswind changes, causing upsets to airliner-size aircraft which have been near to major
accidents.
On smaller aerodromes, lines of trees can mask the wind and cause problems at a late stage in
the approach. These incidents usually contribute to a pilot’s experience, but damaged landing
gear can result from wind effects of greater significance than a steep wind gradient or low-level
turbulence alone.
Meteorology
19-3
Chapter 19
Windshear and Turbulence
THE EFFECTS OF WINDSHEAR ON AN AIRCRAFT IN FLIGHT
Windshear affects aircraft in many different ways and during an encounter the situation is
constantly changing, especially during the more dynamic thunderstorm windshears. Particular
types of aircraft vary in their reaction to a given shear; a light high-wing piston-engined aircraft
may react in a totally different way to a swept-wing four-engine jet. It is not easy to describe the
effects in general terms, as they do not apply universally. The notes which follow only attempt to
describe stylised windshears and their progressive effects. Windshear can, of course, be
encountered at any height and the effects will be similar. The windshear encounter at low level
which is a great hazard; it is this which must be borne in mind when the effects are described.
An understanding of windshear is difficult, unless the relationship of an aeroplane in a moving air
mass to its two reference points is appreciated. One reference is the air mass itself, the other is
the ground. In a windshear encounter it is not only the magnitude of the change of wind vector
that counts but the rate at which it happens.
For example, an aeroplane at 1000 ft agl may have a headwind component of 30 kt, but the
surface wind report shows that the headwind is only 10 kt on the runway. That 20 kt difference
may taper off evenly with the effect of a reasonable wind gradient. However, it may be noticed
that the 20 kt differential still exists at 300 ft and the change, when it comes, will clearly be far
more sudden and its effects more marked. Shear implies a narrow borderline and the 20 kt of
wind speed may well be lost over a vertical distance of 100 ft as the aircraft descends from 300 to
200 ft.
If the pilot wanted a stabilised approach speed of 130 kt, the power would be set according to
conditions, providing the required airspeed and rate of descent.
On passing through the shear line, the loss of airspeed is sudden, but the inertia of the aircraft at
first keeps it at its original groundspeed of 100 kt and power is needed to accelerate the aircraft
back to its original airspeed. This takes time; meanwhile the aircraft having lost 20 kt of airspeed,
sinks faster as a substantial amount of lift has also been lost.
19-4
Meteorology
Windshear and Turbulence
Chapter 19
The headwind was a form of energy and when it dropped 20 kt, an equivalent amount of energy
loss occurred. One source available to balance that loss is engine power; this arrests the
increased rate of descent and starts the process of accelerating back to the approach reference
speed.
The opposite effect can be illustrated using similar conditions, but seen from the point of view of
an aeroplane taking off. Initially take-off along the runway and into the second segment of the
climb, with a 10 kt headwind, the wind becomes a 30 kt headwind after encountering the shear
between 200 and 300 ft. Assuming a target climbing speed of 120 kt, the effect of a sudden
transition through the shear line into a 20 kt increase of headwind, increases the lAS by the same
amount until the momentum of the groundspeed is lost.
This is a case of temporary energy gain, with lift added so the aircraft climbs more rapidly. This
example shows the windshear as being positively beneficial and it is true to say that a rapid
increase in headwind (or loss of tailwind), because they are “energy gains,” temporarily enhances
performance.
It may help with understanding windshear to see it in terms of energy changes, when it is readily
apparent that the windshear which causes temporary loss of energy (sudden drop of headwind or
increase in tailwind, and downdraughts) is the main danger at low altitude.
The effect of a downdraught is not always easy to visualise, as we normally think of the aeroplane
in relation to airflow along the flight path even when climbing or descending. It is now necessary
to envisage flying suddenly from a horizontal flow into air with a vertical component.
In turbulent conditions, air in motion may strike the aeroplane from an angle and the situation may
be constantly changing. However, in thunderstorms, substantial shafts of air which can be moving
either up or down may be encountered with no warning; such shafts may be virtually side by side
and the shear very marked and violent.
Entering a vertical updraught or downdraught from a horizontal airflow, the aeroplane's
momentum at first keeps it on its original path relative to the new direction of flow. In addition to a
loss of airspeed, also realise that the shift of relative airflow affects the angle of attack of the wing,
Meteorology
19-5
Chapter 19
Windshear and Turbulence
which may result in either increased or decreased lift. A slight increase of angle may not cause
much concern. However, if the aircraft is already on the approach with a high angle of attack, an
increase might put the wing near the stall and any decrease will bring about a loss of lift.
Normally, below 1000 ft, the risk of a downdraught is more likely than an updraught.
Having described the combination of increasing headwind followed by downdraught followed by
increasing tailwind consider, that this is the sequence which might be encountered in a microburst
on the approach or following a take-off. This may be a rare occurrence in the United Kingdom or
Europe, but it needs to be appreciated by those flying to the USA. Even on this side of the
Atlantic, an encounter with a downburst, a headwind followed by downdraught, or a downdraught
followed by tailwind is possible and may cause problems.
19-6
Meteorology
Windshear and Turbulence
A
B
C
Chapter 19
Energy gain
Increasing headwind
Airspeed rising
Rate of descent reduced
Tendency to go high on glide path
Energy loss
Reducing headwind and downdraught
Airspeed falling
Rate of descent increased
Tendency to go low on glide path
Energy loss
Increasing tailwind
Airspeed still falling
Rate of descent checked by missed approach
Success depends upon power, height and speed reserves available
An aircraft, approaching on a 3° ILS glidepath, might see ahead an area of heavy rain. Ideally this
might alert the pilot to possible danger, and a missed approach could be executed in good time,
though even this might take the aircraft into the microburst. Then, however, the aircraft will have
gained precious extra height.
Given that the approach continues towards the microburst, the leading edge can produce a
rapidly increasing headwind; the airspeed increases and the aircraft goes high on the glidepath.
The likely reaction is to reduce power to increase the rate of descent and adjust attitude to reduce
airspeed. Then comes the downdraught when the rate of descent increases rapidly and the
aircraft passes through and below the glidepath, still possibly with the nose high and the power
low.
Power is re-applied, but it takes time to spool up the engines, meanwhile the aircraft passes from
downdraughts to increasing tail wind with the airspeed dropping. The rate of descent is not
checked and the nose is high while power increases.
No figures are attached to this description, merely the likely sequence of events. A very strong
microburst has a more pronounced effect on the rise and fall of airspeed and extremes of rate of
descent. The power reserves available and the rate at which they can be applied and built up to
give maximum thrust, determine the aircraft’s ability to counteract the energy loss of downdraught
and increasing tailwind.
Strong wind buffeting, the lashing of rain, and possibly blinding flashes of lightning may
accompany this dynamic sequence of events. If this is a black picture, it matches the descriptions
of those that have flown through a microburst and would probably be echoed by some who have
tried but failed to fly through one. The aim must be to avoid severe windshear at all costs.
It might be thought that an encounter with windshear from a microburst after take-off is likely to be
less hazardous than when approaching to land. The aircraft is at high power and is not
constrained by the need to hold a precise glide path. The temporary energy gain from meeting
the increasing headwind, with a burst of higher air-speed and rate of climb may seem positively
beneficial.
Meteorology
19-7
Chapter 19
Windshear and Turbulence
The transition to downdraught soon kills any rise in airspeed; it may even drop. The rate of climb
may lessen or even show a rate of descent enhanced by the shift to increasing tailwind, when the
airspeed (with the aircraft close to the ground) may drop further. Any benefits of high power may
be balanced by higher aircraft weight. There may be a small power reserve in hand and this may,
or may not, be sufficient to enable the aircraft to fly through the microburst or downburst, together
with other measures described later.
TECHNIQUES TO COUNTER THE EFFECTS OF WINDSHEAR
Windshear can vary enormously in its impact and effect. There is as yet no international
agreement on definitions for grading windshear, but clearly some shears are more severe and
consequently more dangerous than others. In discussing guidance on countering the effects of
windshear, one must inevitably deal with the “worse case” situation. If the golden rule of
avoidance fails for whatever reason, it is impossible to predict at the first stages of a windshear
encounter how severe it will be and it is not bad advice to suggest that recovery action should
anticipate the worst.
No pilot who studies the meteorological situation carefully in advance and updates his knowledge
with the latest reports during flight should be taken totally by surprise by windshear. If
thunderstorms are forecast in the vicinity of the planned destination and then are reported as
being active and are seen on the weather radar or visually, then a mental Windshear Alert should
register. At this stage, depending on the evidence, a diversion might be considered, as windshear
avoidance is the safest course.
If it is decided to continue to the destination, then the crew should consider a few basic measures
to anticipate a possible windshear encounter. One of these is to increase the airspeed on the
approach. The amount of airspeed increase to be recommended is less easy to assess, as what
might be suitable for a light twin-piston engined aeroplane might be quite inappropriate for a
swept-wing jet.
Rule of thumb guidance includes adding half the headwind component of the reported surface
wind to VAT, or, half the mean wind speed plus half the gust factor, in each case up to a maximum
of 20 kt. This may be satisfactory for a strong but turbulent wind, but may not meet the
thunderstorm case, where it is not uncommon for light and variable winds to precede the
onslaught of a gust front or downburst.
The unpredictability of windshear is such that, if it does not materialise, the aircraft can arrive at
threshold with excessive speed to be shed and that could be embarrassing on a short runway.
Because the amount of airspeed “margin” is related to the aircraft's acceleration potential, the
relatively slow propeller driven aircraft is probably at an advantage over a faster jet aircraft.
Remember that the rate of shear is important and the aircraft which penetrates the shear zone
slower experiences a lower rate of shear — the rapid response of propeller driven airflow over a
wing also helps.
The windshear encounter which produces a sudden increase in airspeed (temporary energy gain)
on the approach destabilises it to a greater or lesser extent, which calls for some control
adjustment. The normal reaction to the rise above the glidepath is to reduce power to regain the
glidepath and as the deviation was sudden, the power reduction will probably be more than just a
slight one. The pilot must then be alert to the need to re-appIy power in good time to avoid
dropping below the glidepath. If the wind component then stabilises, leaving the aircraft merely
with a stronger headwind, a further power adjustment will be needed to a higher setting than the
initial one which had given a stable airspeed and rate of descent.
19-8
Meteorology
Windshear and Turbulence
Chapter 19
When an aircraft on the glidepath in the later stages of an approach runs into an “energy loss”
windshear, it can be much more hazardous.
A building or line of trees obstructing the windflow might cause the shear, and the resulting drop
in the wind speed might bring about a very sudden drop in airspeed with a consequent increase
in the rate of descent. To avoid a heavy and premature landing, a rapid and positive increase in
power is needed. Another likely effect is for the nose to drop initially, requiring a check with an
increase in pitch attitude - but not so much that this causes a further loss of airspeed; as always
power and attitude adjustments must be coordinated. These actions may enable the aircraft to
regain the glidepath and continue the approach.
Anticipate the power reduction to avoid flying through the glidepath and expect to set slightly less
power than that originally used, to continue the approach. If the approach has been badly destabilised, full missed approach action may be the wiser and safer option, with a second
approach made with an airspeed “margin” to counter the anticipated windshear effect.
Vital Actions to counter loss of airspeed caused by windshear near the ground:
¾
¾
¾
¾
Briskly increase power (full go round power if necessary)
Raise the nose to check descent
Co-ordinate power and pitch
Be prepared to carry out a missed approach rather than risk landing from a destabilised approach
To counter the effect of a downburst or microburst on an approach or take-off calls for more
stringent measures. It must be stressed that any well-founded report of either phenomenon must
be treated seriously and the approach or take-off delayed until the danger has passed. If there is
an inadvertent encounter, the aircraft may be affected by wind from any flank by the descending
and outflowing column of air, but again the worst case will be considered - entry on one side,
through the centre and exit through the other side. It will be a turbulent and unpleasant
experience which can tax the abilities of the most skillful pilots.
The presence of thunderstorms should be known and obvious, so the increase in speed caused
by the rising headwind should be seen as the forerunner of a downburst or microburst; any hope
of a stabilised approach is abandoned and a missed approach is the only safe course of action the technique is to make it as safe as possible.
The initial rise in airspeed and rise above the approach path should be seen as a bonus and
capitalised. Without hesitation, increase to go-around power, being prepared to go to maximum
power if necessary, select a pitch angle consistent with a missed approach, typically about 15°
and hold it against turbulence and buffeting.
The next phase may well see the initial advantages of increased airspeed and rate of climb
rapidly eroded. The downdraught now strikes, airspeed may be lost and the aircraft may start to
descend despite the high power and pitch angle. It will be impossible to gauge the true angle of
attack, so there is a possibility that the stick shaker (if fitted) may be triggered; only then should
the attempt to hold the pitch angle normally be relaxed.
Meteorology
19-9
Chapter 19
Windshear and Turbulence
The point at which downdraught begins to change to increasing tailwind may well be the most
critical period. The rate of descent may lessen, but the airspeed may still continue to fall; the
height loss may have cut seriously into ground obstacle clearance margins. Given that maximum
thrust is already applied, as an extreme measure if the risk of striking the ground or an obstacle
still exists, it may be necessary to increase the pitch angle further and deliberately raise the nose
until stick shaker is felt. Then an easing forward of the control column to try and hold this higher
pitch angle should be made, until the situation eases with the aircraft beginning to escape from
the effects of the microburst.
When there is an indefinite risk of shear, it may be possible to use a longer runway, or one that
points away from an area of potential threat. It may also be an option to rotate at a slightly higher
speed, provided this does not cause undue tyre stress or any handling problems.
The high power setting and high pitch angle after rotate have already put the aircraft into a good
configuration should a microburst then be encountered. The aircraft is, however, very low where
there is little safety margin and the ride can be rough. If there is still extra power available, it
should be used without hesitation. Ignore noise abatement procedures and maintain the high
pitch angles, watching out for stick shaker indications as a signal to ease the controls forward.
In both approach and take-off cases, vital actions are:
¾
¾
¾
Use the maximum power available as soon as possible.
Adopt a pitch angle of around 15° and try and hold that attitude. Do not chase
airspeed.
Be guided by stick shaker indications when holding or increasing pitch attitude,
easing the back pressure as required to attain and hold a slightly lower attitude.
Windshear warning can be provided in several ways:
¾
¾
¾
¾
¾
Meteorological warning
ATS warning
Pilot warning
On board pre-encounter warning
On board encounter warning and/or guidance
ICAO DEFINITIONS
The following windshear reporting system is used to give pilots a common understanding of the
problem of windshear:
19-10
Intensity of
windshear
Vertical
windshear/100 ft
Horizontal
windshear/2000 ft
Up or down
draught
Effect on flight
altitude
Light
Moderate
Strong
Severe
0 – 4 kt
4 – 8 kt
8 – 12 kt
> 12 kt
0 – 4 kt
4 – 8 kt
8 – 12 kt
> 12 kt
0 – 4 kt
4 – 8 kt
8 – 12 kt
> 12 kt
Small
Significant
Hazardous
Highly dangerous
Meteorology
Windshear and Turbulence
Chapter 19
NATURE OF TURBULENCE
The small-scale vortices that constitute turbulence, form:
¾
¾
¾
When the air-flow is disturbed by an obstruction, (e.g. the ground surface).
When two air-flows of different direction and/or speed adjoin each other.
When the speed of the air changes rapidly within the same air-flow.
Turbulence transfers momentum from one volume of air to another by exchanging small amounts
of air. The wind speed, for example, can be accelerated or retarded.
To describe this we use the words gust and lull.
Gust is an increase of the wind speed of short duration.
Lull is a short-lived decrease of the wind speed.
TURBULENCE, METEOROLOGICAL FACTORS
Windshear caused by ascending and subsiding thermals, convection, results in the aircraft
bouncing along through the thermals, which creates thermal turbulence.
THERMAL TURBULENCE
Thermal turbulence is generated by heated thermals ascending through the air, causing a return
flow at the sides. During a flight, this causes severe bumps, and during the landing phase the upand downdrafts may disturb the approach.
Thermal turbulence is marked over warm surfaces, such as tarmac, concrete, mountains, sand,
or dark ground surfaces.
As a matter of fact, it is often a question of a combination of up- and down-winds with a clear local
character. Thermal turbulence occurs:
¾ Above land in the daytime and generally in association with convective clouds.
¾ In the autumn/winter above seas by day and night.
Except during the landing phase thermal turbulence does not constitute any major problem in
Northern Europe. In extreme cases, however, the aircraft can be bumped into exceptional flight
attitudes, and it may be rather uncomfortable to fly in areas with severe thermal turbulence
MECHANICAL/ FRICTIONAL TURBULENCE
Windshear and turbulence occur because of friction against the ground surface at high wind
speeds.
The mechanical effect depends on the structure of the surface and the wind speed, see the table
below. The consequence is very uncomfortable flight up to 2000 — 3000 ft above the terrain with
the aircraft being subjected to accelerations of several “g”.
Meteorology
19-11
Chapter 19
Windshear and Turbulence
Criteria of mechanical turbulence:
Surface
Wind< 15 kt
15-30 kt
>30 kt
Sea
Light
Moderate
Mod/severe
Plain
Only light
Moderate
Severe
Broken terrain
Light -moderate
Severe
Extreme
Mechanical turbulence occurs throughout the year, when the prevailing wind is high. The more
unstable the air, the more severe the turbulence - this applies to both thermal and mechanical
turbulence.
MOUNTAIN WAVES
FLIGHT OVER AND IN THE VICINITY OF HIGH GROUND
Air flow is more disturbed and turbulent over high ground than over level country and the forced
ascent of air over high ground often leads to the formation of cloud on or near the surface. This
sometimes extends through a substantial part of the troposphere if the air is moist enough.
Forced ascent also increases instability so that thunderstorms embedded in widespread layer
cloud may occur over high ground, even when no convective clouds form over low ground. When
the air is generally unstable, cloud development is greater, icing in the clouds is more severe and
turbulence in the friction layer and in cloud is intensified over high ground.
The air flowing over high ground may be so dry that, even when it is forced to rise, little or no
cloud is formed. The absence of cloud over high ground does not imply the absence of vertical air
currents and turbulence.
Strong down currents are caused by the air descending the lee slope and it is, therefore,
especially hazardous to fly towards high ground when experiencing a headwind.
On some occasions, the disturbance of a transverse airflow by high ground creates an organised
flow pattern of waves and large scale eddies in which strong up-draughts and downdraughts and
turbulence frequently occur. These organised flow patterns are usually called mountain waves
but may also be referred to as lee waves or standing waves. These can be associated with
relatively low hills and ridges as well as with high mountains.
CONDITIONS
Conditions favourable for the formation of mountain waves are:
¾
¾
¾
¾
A wind blowing within about 30° of a direction at right angles to a substantial ridge.
The wind must increase with height with little change in direction (strong waves are
often associated with jet streams).
A wind speed of more than 15 kt at the crest of the ridge is also usually necessary.
A marked stable layer (approaching isothermal, or an inversion), with less stable air
above and below, between crest level and a few thousand feet above.
Mountain wave systems may extend for many miles downwind of the initiating high ground.
Satellite photographs have shown wave clouds extending more than 250 nm from the Pennines
in the UK; 50 to 100 nm is a more usual extent of wave systems in most areas. Wave systems,
on occasion, extend well into the stratosphere.
19-12
Meteorology
Windshear and Turbulence
Chapter 19
The average wavelength of mountain waves in the troposphere is about 15 miles, but much
longer waves occur. Derive a good estimate of the wave length using the following formula:
Wavelength = mean troposphere wind ÷ 7
Disturbances in the stratosphere are often irregular features located very near or just over the
initiating mountains. When waves to the lee of the high ground are evident, their length is usually
greater than in the troposphere. A typical wavelength is 15 nm, but wavelengths of 60 nm have
been measured.
The amplitude of waves is much more difficult to determine. In general, the higher the mountain
and the stronger the airflow, the greater the resulting disturbance. The most severe conditions
occur when the natural frequency of the waves is tuned to the ground profile.
In the troposphere, the double amplitude (peak-to-trough) of waves is commonly 1500 ft with
vertical velocities about 1000 ft/min. However, double amplitudes of about 20 000 ft and vertical
velocities aver 5000 fpm have been measured.
VISUAL DETECTION OF MOUNTAIN WAVES
The clouds which owe their appearance to the nature of wave flow are a valuable indicator of the
existence of wave formation. Provided there is sufficient moisture available, the ascent of air
leads to condensation and formation of characteristic clouds. These clouds form in the crest of
standing waves and therefore remain more or less stationary.
They occur at all heights from the surface to cirrus level.
Meteorology
19-13
Chapter 19
Windshear and Turbulence
Lenticular Clouds provide the most unmistakable evidence of the existence of mountain waves.
They form within stable layers in the crusts of standing waves. Air streams through them, the
clouds forming at the up-wind edges and dissipating downwind. They have a characteristically
smooth, lens-shaped outline and may appear at several levels, sometimes resulting in an
appearance reminiscent of a stack of inverted saucers.
Lenticular clouds usually appear up to a few thousand feet above the mountain crests, but are
also seen at any level up to the tropopause and even above. Mother-of-pearl clouds, seen on rare
occasions over mountains, are a form of wave-cloud at an altitude of 80 000 ft. Air flow through
these clouds is usually smooth unless the edges of the cloud take on a ragged appearance,
which is an indication of turbulence.
Rotor Clouds, or roll clouds, appear as ragged cumulus or stratocumulus parallel to and
downwind of the ridge. On closer inspection, these clouds rotate about a horizontal axis. Rotor
clouds are produced by local breakdown of the flow into violent turbulence. They occur under the
crests of strong waves beneath the stable layers associated with the waves. The strongest rotor
normally forms in the first wave downwind of the ridge and is usually near or somewhat above the
level of the ridge crest. There are usually no more than one or two rotor clouds in the lee of the
ridge.
Cap Clouds form on the ridge crest. Strong surface winds which are commonly found sweeping
down the lee slope may extend the cap cloud down the slope.
Although cloud often provides the most useful visible evidence of disturbances to the airflow,
other cloud systems, particularly frontal cloud, sometimes obscures the characteristic cloud types.
TURBULENCE
TURBULENCE AT LOW AND MEDIUM LEVELS
A strong wind over irregular terrain produces low-level turbulence which increases in depth and
intensity with increasing wind speed and terrain irregularity.
In a well developed wave system, the rotor zone and the area below are strongly turbulent and
reversed flow is often observed at the surface. Strong winds confined to the lower troposphere,
with reversed or no flow in the middle and higher troposphere, produce the most turbulent
conditions at low levels. These are sometimes accompanied by rotor streaming, comprised of
violent rotors which are generated intermittently near lee slopes and move downwind. These lowlevel travelling rotors are distinct from the stationary rotors which form at higher levels in
association with strong mountain waves.
TURBULENCE IN THE ROTOR ZONE
Rotors lie beneath the crests of lee waves and are often marked by roll-cloud. The most powerful
rotor lies beneath the first wave crest down-stream of the mountains. Rotors give rise to the most
severe turbulence found in the air flow over high ground. On occasions it may be as violent as
that in the worst thunderstorms.
TURBULENCE IN WAVES
Although flight in waves is often remarkably smooth, severe turbulence can occur. The transition
from smooth to bumpy flight can be abrupt. Very occasionally, violent turbulence may result,
sometimes attributed to the wave breaking.
19-14
Meteorology
Windshear and Turbulence
Chapter 19
TURBULENCE AT HIGH LEVELS (NEAR AND ABOVE THE TROPOPAUSE)
TURBULENCE NEAR THE JET STREAM
Turbulence in jet streams is frequently greatly increased in extent and intensity over high ground.
Strong vertical windshears are often concentrated in a few stable layers just above and below the
core of the jet stream. Distortion of these layers when the jet stream flows over high ground,
particularly when mountain waves form, can produce local enhancements of the shears so that
the flow in those regions breaks down into turbulence. Usually the cold side of the jet stream is
more prone to turbulence, but mountain waves may be more pronounced on the warm side.
TURBULENCE IN THE STRATOSPHERE
Flight experience shows that in the stratosphere, moderate or severe turbulence is encountered
over high ground about four times more frequently than over plains and about seven times more
frequently than over the oceans.
DOWNDRAUGHTS
Whether or not a well developed wave system exists, if the air is stable a strong surface air flow
over high ground produces a substantial and sustained downdraught and/or turbulence on the lee
side. Such downdraughts may, on occasion, be strong enough to defeat the rate of climb
capability of some aircraft. In a wave system, a series of downdraughts and updraughts exists,
the most powerful being those nearest the high ground.
ICING
Adiabatic cooling caused by the forced ascent of air over high ground generally results in a
lowering of the freezing level and an increase of liquid water concentration in clouds. Thus, when
extensive cloud is present, airframe icing is likely to be more severe than at the same altitude
over lower ground. This hazard is at a maximum a few thousand feet above the freezing level, but
in general is unlikely to be serious at altitudes above 20 000 ft except in cumulonimbus clouds.
FLYING ASPECTS
The effects of the airflow over high ground on aircraft in flight depends on the magnitude of the
disturbance to the airflow; in other words, the altitude and the aircraft’s speed and direction in
relation to the wave system. A broad distinction may be made between low-level hazards (below
about 20 000 ft) and high-level hazards (above 20 000 ft).
LOW ALTITUDE FLIGHT
The main hazards arise in low altitude flight from severe turbulence in the rotor zone, from
downdraughts and from icing. The presence of roll clouds in the rotor zone may warn pilots of the
region of most severe turbulence, but characteristic cloud formations are not always present or, if
they are present, may lose definition in other clouds. Similarly, the updraughts and downdraughts
are, in general, not visible. If an aircraft remains for any length of time in a downdraught, which
may be remarkably smooth (e.g. by flying parallel to the mountains in the descending portion of
the wave), serious loss of height may occur.
During upwind flight, the aircraft’s height variations are normally out of phase with the waves; the
aircraft is, therefore, liable to be at its lowest height when over the highest ground. The aircraft
may also be driven down into a roll-cloud over which ample height clearance previously appeared
to be available.
Meteorology
19-15
Chapter 19
Windshear and Turbulence
Downwind flight may be safer. Height variations are usually in phase with waves, but it must be
appreciated that the relative speed of an accidental entry into the rotor zone is greater than in upwind flight because the rotor zone is stationary with regard to the ground. Thus, the structural
loads imposed on the airframe when gusts are encountered are likely to be greater, and there will
probably be less warning of possible handling difficulties.
HIGH ALTITUDE FLIGHT
The primary danger at high altitude is that of a sudden encounter with localised disturbances
(i.e. turbulence or sudden large wind and temperature changes) at high penetration speeds. This
is particularly relevant at cruising levels above FL 300 where the buffet-free margin between the
Mach number for 1g buffet and the stall is restricted. In this respect, flight downwind is likely to be
more critical than flight up-wind, especially when the wind is strong.
As in the case of low altitude flight, the waves are stationary relative to the ground. The higher the
relative speed on accidentally encountering a standing wave while flying downwind, the greater
the likelihood of greater loads on the airframe. There is often no advance warning of wave activity
from preliminary variations in flight instrument readings, or from turbulence. Although
downdraughts are present, they are probably not hazardous and icing and rotor zone turbulence
are unlikely.
INVERSIONS
Inversions on the leeward side of a mountain range can prevent the down-slope wind from
reaching the ground. A very powerful shear is generated from about 300 ft up to 1500 ft above
the ground. When the downdraught moves over the inversion, a low level jet forms.
Fresh winds over a mountain but light winds at the airport on the leeward side of the mountain
indicate strong low-level windshear.
MARKED TEMPERATURE INVERSION
The marked temperature inversion occurs during cloudless nights due to terrestrial radiation. The
situation is enhanced if the aerodrome is situated in a valley. A pocket of cold air is trapped under
higher warm air. A low level jet can form just below the top of a strong radiation inversion on clear
nights.
At certain airfields, a warning of marked temperature inversion is issued when a temperature
difference of 10°C or more exists between the surface and any point up to 1000 ft above the
surface.
19-16
Meteorology
Windshear and Turbulence
Chapter 19
REPORTING TURBULENCE
CAT remains an important operational factor at all levels of flying but particularly above FL 150.
Pilots encountering CAT are requested to report time, location, level and intensity, and aircraft
type to the appropriate ATS unit. This is done as a Special Observations Report. The criteria
required are:
INCIDENCE
¾ OCCASIONAL — less than 1/3 of the time.
¾ INTERMITTENT — 1/3 to 2/3.
¾ CONTINUOUS — more than 2/3.
INTENSITY
LIGHT
¾ Light Turbulence — IAS fluctuates 5 - 15 kt, turbulence that momentarily causes
slight erratic changes in attitude and/or altitude.
¾ Light Chop — Turbulence that causes slight rapid rhythmic bumping without
appreciable changes in altitude or attitude. No IAS fluctuations.
¾ Reaction Inside Aircraft — Occupants may feel a slight strain against seat belts or
shoulder straps. Unsecured objects may be displaced slightly. Food service may be
conducted and little or no difficulty is encountered when walking.
MODERATE
¾ Moderate Turbulence — IAS fluctuates 15 - 25 kt, turbulence that is similar to light
turbulence but of greater intensity. Changes in altitude and/or attitude can occur but
the aircraft remains in positive control at all times.
¾ Moderate Chop — Turbulence that is similar to light chop but of greater intensity.
Rapid bumps or jolts without appreciable changes in altitude or attitude. IAS may
fluctuate slightly.
¾ Reaction Inside Aircraft — Occupants feel definite strains against seat belts or
shoulder straps. Unsecured objects are dislodged. Food service and walking are
difficult.
SEVERE
¾ Severe Turbulence — IAS fluctuates more than 25 kt; turbulence that causes large,
abrupt changes in altitude and/or attitude. The aircraft may be momentarily out of
control.
¾ Reaction Inside Aircraft — Occupants are forced violently against seat belts or
shoulder straps. Unsecured objects are tossed about. Food service and walking
impossible.
Meteorology
19-17
Chapter 19
19-18
Windshear and Turbulence
Meteorology
INTRODUCTION
Air pressure varies considerably between positions on the Earth’s surface. These pressure
differences are important to the Earth’s weather and winds. On the meteorological charts the
pressure pattern is shown by isobars, enclosing areas of different pressure.
LOW, CYCLONE OR DEPRESSION, AND TROUGH
For a low pressure system, the isobars are generally closely spaced which results in windy
weather. The centre of the low pressure system experiences calm winds. Convergence occurs
and air is forced upward and cools adiabatically. If the air is humid, condensation occurs and
clouds form.
Meteorology
20-1
Chapter 20
Non-Frontal Pressure Systems
In the Northern Hemisphere, the wind follows a left-hand circuit parallel to the isobars. Friction
acts as a brake on the wind in the friction layer, and the wind blows in toward the low pressure
centre. The result is a general lifting of the air within the low pressure area. For the Southern
Hemisphere the rotation is in the opposite direction.
In a low-pressure area, convective movement is strengthened, and CB are likely to form if the air
is unstable. If the air is stable but humid, clouds form. In this, extensive stratiform cloud layers
form. Visibility at low levels is generally better than in an anticyclone, due to a stronger mixing of
the air.
LOW PRESSURE TYPES
As in the case of the high pressures, there are two basic types of lows, warm and cold.
Dynamic Low
Thermal Low
Cold low, the low deepens at altitude and the winds are
increasing.
Warm low, the low weakens aloft and turns into a high pressure.
SECONDARY DEPRESSION
The secondary depression forms in the circulation of a larger primary depression. The secondary
depression can be frontal or non-frontal depending upon how it forms. Secondary depression
movement depends upon the hemisphere.
¾
¾
In the Northern Hemisphere, a secondary depression moves anti-clockwise around
the primary depression.
In the Southern Hemisphere, a secondary depression moves clockwise around the
primary depression.
The secondary depression can form:
¾
¾
¾
20-2
On the tip of an occlusion.
On unstable waves on a trailing cold front.
Inside the primary depression circulatory system.
Meteorology
Non-Frontal Pressure Systems
Chapter 20
The life cycle and weather patterns associated with secondary depressions are similar to those of
a primary depression. As the secondary depression deepens, the depression may become the
dominant feature. In this case, the old primary depression becomes the secondary depression
and starts circulating around the new primary depression. This process is known as “dumb
belling.”
The weather in a secondary depression is often more severe than in a primary depression.
The worst weather associated with a non-frontal secondary depression usually occurs on the side
of the secondary furthest from the primary.
ICELANDIC LOW
The Icelandic low, as shown on the January and July mean value surface pressure charts below,
is a dynamic system.
The adiabatic cooling (due to the expansion of the air) leads to extensive clouds in the low
pressure area. In temperate latitudes there is a transport of unstable cold air in the northern and
western areas of the low, while there is an airflow of more stable warm air in the southern and
eastern areas. Showers are more frequent in the north-western parts of the low. Apart from the
showers, visibility is good.
If the lifted air is humid, extended AS and AC with embedded areas of light rain can form.
Meteorology
20-3
Chapter 20
Non-Frontal Pressure Systems
JANUARY
JULY
20-4
Meteorology
Non-Frontal Pressure Systems
Chapter 20
THE ORIGIN OF LOW PRESSURES AND WEATHER
In aviation, dynamic and thermal classifications are rarely used. Normally depressions are
classes as Non-Frontal or Frontal:
Examples of non-frontal depressions are:
¾
¾
¾
¾
¾
¾
¾
Orographic lows
Thermal lows
Summer lows over land or
Monsoon low
Equatorial low or trough
Instability lows
Winter lows over sea
¾
¾
¾
¾
¾
¾
¾
Mediterranean low
Polar low
Baltic Sea low
Cold air pool
Tropical revolving storm
Easterly waves
Whirlwind or Tornado
The last three items are discussed in Chapter 27 — Tropical Storms and Tornadoes.
OROGRAPHIC OR LEE SIDE LOWS OR TROUGHS
If a current of air flows perpendicular to a mountain range, the barrier will force the air to
compress on the windward side and over the mountain. The air on the leeward side of the
mountain seems to “stretch.” There will be a tendency for anticyclonic curvature over the
mountain with closely spaced pressure surfaces, and on the lee-side there will be a clearly visible
cyclonic curvature.
Falling air pressure on the leeward side forms a depression. This is known as a lee-depression or
a lee-trough. The lee-trough is usually stationary if the airflow remains the same and no
deepening low forms.
The lee-low causes the pressure surfaces to slope down towards the mountain and become
closely packed over the mountain.
666
Meteorology
20-5
Chapter 20
Non-Frontal Pressure Systems
On the leeward side, foehn winds prevail, and the weather typically is fine. Humid air may be
sucked into a lee trough giving clouds and sometimes precipitation.
When a cold front encloses warmer air on the leeward side of the mountain, rapid development of
the system occurs. The low deepens and intense cumulonimbus clouds form.
A cold front may be activated/intensified when passing a range of mountains.
When the cold air sweeps around the sides of the mountain and across it, the warm air on the
leeward side acts as a warm sector and a wave forms on the front.
This wave normally develops rapidly which leads to an occlusion-like process, and storms move
away from the mountain.
The most severe Orographic lows that form over north-west Italy and affect the Mediterranean
form when the western Alps stop cold fronts.
¾
¾
¾
¾
The lower portion of the cold front is slowed by the mountains.
The slope of the frontal surface increases.
Eventually the cold air spills over into the warm air on the lee side of the Alps.
The warm air is undercut by the cold air causing severe instability.
Similar phenomena appear on the Skagerak when the Scandinavian mountains impede cold
fronts. In this particular case, air sweeps around the southern edge of the mountains, giving
strong winds. Humidity and temperature increase in the air that travels around the mountain at
low levels. Travelling over a relatively warmer water surface and causing increased instability,
rapid cyclonic development on the lee side occurs, often giving clusters of showers.
THERMAL DEPRESSIONS
Thermal depressions form over warm surfaces. The heated air rises through convection and
turbulence. A high pressure aloft is formed causing an outflow of air at height. The air pressure at
the surface begins to decrease and a circulation similar to the sea breeze occurs.
20-6
Meteorology
Non-Frontal Pressure Systems
Chapter 20
An influx of air occurs at the surface low and an ascending motion is generated, strengthening the
convective clouds in the area, if any.
The most predominant thermal depressions are:
¾
¾
¾
¾
The monsoon low pressures in Asia.
The Equatorial low pressure belt.
The summer lows in south-western USA.
The lows of north-east Africa.
Less intense thermal lows are common on the weather charts in the summer, especially over
France and Spain. These smaller cyclones are shallow and do not affect weather to any greater
extent.
In the winter, thermal depressions can form over “warm” water surfaces such as the Baltic Sea,
the Skagerak, the Black Sea, and the Mediterranean. These are referred to as instability lows.
If the air is dry, thermal lows bring good flying weather with some cloud and moderate to good
visibility.
If the air is humid, however, convective CB are likely to form, and heat thunderstorms or squalls
will also appear. This is a common feature in France and on the Iberian Peninsula. Thermal lows
generated in these areas may drift towards north-western Europe and Scandinavia.
INSTABILITY LOWS
If large scale organised convection occurs in an area where there is already a lee low, a
development may take place that looks similar to a thermal low. This is an instability low.
The same process that created the thermal low also influences the instability low. A significant
amount of the energy is derived from the released latent heat of the condensation process.
According to the hydrostatic equation, heating causes the distance between two pressure
surfaces to increase. As a result a high pressure is generated aloft resulting in an outflow of air
and falling pressure at the ground. If divergence already exists at height, the effect will be
strengthened and a rapid pressure fall can occur at the surface level. This generates a spiral flow
in toward the centre.
Instability lows can be very intense, particularly in the Tropics. In mid-latitudes the humidity
content is low and the lows are thus less intense.
MEDITERRANEAN LOW
A typical winter low that forms over the sea when cold polar air reaches the warm Mediterranean
water. A separate low forms in which clusters of convective cloud are found.
Meteorology
20-7
Chapter 20
Non-Frontal Pressure Systems
POLAR LOWS
Instability lows often form when cold polar or arctic air moves south over a gradually warmer sea
or major water area. Common from November to March in the Northern Hemisphere sea areas.
The air transforms due to an intense heating and vapour increase in the lower levels resulting in
intense convection caused by the southerly travel of the airmass.
Between the two highs there is a tendency to a cyclonic airflow and the formation of lee-lows off
the south-eastern coast of Norway. When the cold air reaches the warmer water, small intense
instability lows develop.
A similar type of instability low forms in the winter on the Bay of Genoa, generated when the cold
Mistral wind sweeps down over the warm Mediterranean.
BALTIC SEA CYCLONES
If energy is released in an area where a low pressure already exists, the low deepens or
intensifies. This is common in the autumn on the Baltic Sea, when lows from the continent in the
south and east move out over relatively warm water. It can also occur when a low has passed
Scandinavia from the west.
More precipitation and lower cloud bases than predicted in a forecast affect the Baltic Sea isles
and coasts. Heavy northerly squalls may develop.
CELLS OF COLD AIR ALOFT (COLD POOLS)
The general theories about the long waves encircling our globe, separating the cold polar from
the warm tropical air, are discussed in Climatology. The waves are usually zonal moving from
west to east along a latitude line. Cold air outbreaks can cut off from the main stream air and
generate a pool of cold air at height in a position south of the normal Polar front. This cold pool
can remain for several days constituting a potential area of instability at height.
In the summer, thermal lows form over the continents and may develop into instability lows. This
happens when cold air is carried in over the low (by the upper airflow) or when a cold pool
already exists at height. In these conditions, the atmosphere becomes unstable, and a major area
of thunderstorms may develop.
20-8
Meteorology
Non-Frontal Pressure Systems
Chapter 20
ANTICYCLONE OR HIGH, AND RIDGE OR WEDGE
A high pressure system is an area enclosed by isobars that decrease in value with distance from
the centre.
NATURE OF A HIGH
Isobars are normally well spaced resulting in light winds. Where a high is adjacent to a low the
pressure gradient can become steep, leading to moderate or strong winds.
In a high pressure cell, mass convergence at height and divergence at low levels creates
subsidence within the core of the anticyclone with an outflow at low level. The subsidence is
checked above the ground, due to the thermal mixing in the surface layer and a subsidence
inversion is formed. The height of the subsidence inversion depends on the intensity of the
anticyclone, the degree of thermal mixing, and the distance from the core.
Inversions form from 2000 to 5000 ft in cold anticyclones and up to FL 100 in warm anticyclones.
Above the friction layer, in the Northern Hemisphere, the wind blows in a right-hand circuit parallel
to the isobars. In the friction layer, friction slows the wind and it blows at an angle out from the
centre. The outflow at the bottom of the high leads to a sinking motion of air, which is compressed
and adiabatically heated. The subsidence inversion forms, the temperature rises significantly and
the humidity decreases. In the Southern Hemisphere, the rotation is reversed.
The air above the inversion is dry, while the air below may or may not be dry depending on the
circumstances that prevail. Air pollution collects below the inversion, and this leads to a drop in
visibility at the lowest levels. If the inversion persists, clouds can form in the inversion.
At high latitudes, the increased loss of terrestrial radiation due to the drying at height creates
nocturnal inversions at the surface. Large areas with SC and ST may form in maritime air
masses.
In the winter these clouds can persist for several days.
Where the humidity is high and the lower levels are cold, fog forms below the subsidence
inversion.
In the summer, or at lower latitudes, SC often dissipates during the day and returns at night. If the
air below the subsidence inversion is unstable or conditionally unstable, CU may form below the
inversion during the day.
In continental air masses, the humidity content is low, but visibility is still limited below the
inversion. If the air passes over a major water feature, moisture is rapidly absorbed and cloud
forms.
Maritime airmasses dry out with an extensive passage over a major land surface.
The weather above the subsidence inversion is normally fine; cloudless with good visibility.
Meteorology
20-9
Chapter 20
Non-Frontal Pressure Systems
HIGH PRESSURE SYSTEMS
There are two main high pressure systems, depending on whether they consist of warm or cold
air.
SUBTROPICAL HIGHS (WARM ANTICYCLONES)
Subtropical highs are formed by air from the equatorial regions travelling away from the equator
at altitudes around the Equatorial tropopause. They are deflected by Coriolis which generates a
subtropical jet stream and an accumulation of air around the 30º latitude. At low levels the air
pressure increases and there is an outflow of air from the system. In the subtropical high,
subsidence from aloft occurs. The subsidence inversion in these cells is sometimes called a
Trade Inversion.
These anticyclones are often stationary or move in a seasonal manner and are therefore referred
to as permanent highs.
Europe’s nearest subtropical anticyclone is the Azores High, which is the source region of
maritime tropical air.
The air below the subsidence inversion is humid and unstable; above it is dry and stable. CU
dominates the weather below the inversion.
The height to the inversion varies within the high pressure cell. The highest values are found in
the western areas nearest the Equator (5000 − 7000 ft) and the lowest in the north-eastern areas
(1500 − 2000 ft). Tropical showers are more likely to develop in the western part of an ocean than
in the eastern.
As the low level air travels away from the equator, the humidity increases. The sea temperature
decreases and the air is cooled from below. In winter, this frequently leads to vast areas of low
clouds, drizzle, and fog over NW Europe.
In summer, the anticyclone occasionally intensifies over the North Atlantic. This causes lows and
the associated rain areas to move in a wide arc north of Scandinavia, forming a blockage
(a “blocking high”) with dry and sunny weather over western Europe.
CONTINENTAL HIGHS (COLD ANTICYCLONES)
Consisting of polar air, the cold anticyclone forms over the cold continents in the winter. They
seldom reach higher than 700 hPa (FL 100), but the horizontal extension may be considerable.
Thermal highs are not as stable as dynamic highs, and travelling depressions can break them
down.
The Siberian and the Canadian highs consist of, and are the source regions of, continental polar
air. In midwinter they also constitute the source region of arctic air from within the Arctic and
Antarctic permanent cold anticyclones.
If the pressure system spreads over a coastal area, there will be convection and snow showers
over the open water surface with fog and mist below the inversion inland.
If the air is dry and there is no advection from open water, the weather can be cold, bright, and
cloudless. In clear and extremely cold areas, ice fog or diamond dust may form.
20-10
Meteorology
Non-Frontal Pressure Systems
Chapter 20
HIGH PRESSURES AND HIGH PRESSURE RIDGES (OR WEDGES) IN
SERIES OF TRAVELLING DEPRESSIONS
The third type of high pressure forms between the lows of a family of depressions.
The ridges, or temporary high, forms as cold air sweeps behind a frontal low. This type of high is
thermal, and as a consequence it is not visible on an upper air chart.
High pressure ridges follow low pressure systems in their movements and constitute a break in
the storms associated with the frontal systems of the lows.
The ridge can be subdivided into three weather zones:
¾
¾
¾
Ahead of the axis of the ridge (just behind the cold front)
Along the axis of the ridge
Behind the ridge (in front of the next warm front)
There is a high risk of showers, often troughs with low pressure and line squall
showers/thunderstorms well ahead of the ridge axis. CB turns to CU and SC closer to the ridge
axis. In winter, terrestrial radiation from the Earth is high, and nocturnal radiation fog is likely to
form if the wind is light. ST or SC form if the wind is stronger at the border of the ridge/cold high.
When the ridge passes, the air is humidified in the prevailing SW wind, which again leads to
increased cloud with CU and SC at lower levels while the frontal cloud deck thickens at height.
Meteorology
20-11
Chapter 20
20-12
Non-Frontal Pressure Systems
Meteorology
TYPES OF SERVICE
PRE-FLIGHT BRIEFING
The primary method of meteorological briefing for flight crews is self briefing. An alternate method
for obtaining information is from the Meteorological Information Self Briefing Terminal (MIST).
Where the primary method is not available, then special forecasts are often provided.
If the personal advice of a forecaster is required, then information is only given on the
understanding that full use is made of all available information.
Note: Meteorological observations and forecasts have certain expected tolerances of
accuracy.
METEOROLOGICAL CHARTS
Meteorological information is available on various charts which are routinely transmitted over the
METFAX network to major aerodromes. They provide information under the following headings:
Low and medium level flights within the UK and to near Europe
Surface Weather Chart for: Surface − 15 000 ft amsl. (Form 215)
Spot Wind/ Temperature Chart for 1000 ft − 24 000 ft amsl, (Form 214)
Medium and high level flights to Europe and the Mediterranean
Significant Weather/Tropopause/Maximum Wind Charts for FL 100 − FL 450.
Upper Wind and Temperature Charts for FLs: 50, 100, 180, 240, 300, 340, 390
and 450.
High level flights to North America
Significant Weather/Tropopause/MaximumWind Chart for FL 250 − FL 630.
Upper Wind and Temperature chart for FLs: 180, 240, 300, 340, 390, 450 and
530.
Meteorology
21-1
Chapter 21
Meteorological Observations and Meteorological Services
High level flights to Middle/Far East
Significant Weather/Tropopause/MaximumWind Chart for FL 250 − FL 450.
Upper Wind and Temperature chart for FLs: 180, 240, 300, 340, 390, 450 and
530.
High level flights to Africa, The Caribbean and South America
Significant Weather/Tropopause/MaximumWind Chart for FL 250 and above.
Upper Wind and Temperature charts for FLs: 180, 240, 300, 340, 390, 450 and
530.
Other area charts or additional Flight Level information, which are not routinely available by
METFAX, are requested from the LONDON/Heathrow forecast office, subject to available
METFAX capacity. Amended charts are issued when forecast conditions change significantly.
BROADCAST TEXT METEOROLOGICAL INFORMATION
The following reports are broadcast by teleprinter:
METAR
Aerodrome meteorological reports. METARs are routinely broadcast every ½ hour during
aerodrome opening hours. Exceptionally they are sometimes broadcast every 1 hour.
TAF
Aerodrome Forecasts. FC denotes a TAF valid for a period less than 12 hours, usually 9
hours, which is issued every 3 hours. FT denotes a TAF valid between 12 and 24 hours
which is issued every 6 hours. Amendments are broadcast between routine times as
required.
SIGMET
Warnings of weather significant to flight safety these are available for areas within 1000
nm of the UK.
The following are additions that may be added to the METAR:
¾
¾
Short term landing forecasts (TREND), which are valid for 2 hours.
Information on runway state when weather conditions require and continue until
conditions cease. Special Aerodrome Meteorological Reports are issued when
conditions change through specific limits.
SPECIAL AERODROME METEOROLOGICAL REPORTS (SPECI)
Special Aerodrome Meteorological Reports are issued when conditions change significantly.
Selected Special Reports (SPECI) are defined as Special Reports disseminated beyond the
aerodrome of origin. The UK does not normally issue Selected Special Reports.
TERMINAL AERODROMES FORECAST (TAF)
TAFs are normally provided only for those aerodromes where official meteorological observations
are made. For other aerodromes, Local Area Forecasts are made. Amended TAFs or Local Area
Forecasts are issued when forecast conditions change significantly.
21-2
Meteorology
Meteorological Observations and Meteorological Services
Chapter 21
SPECIAL FORECASTS AND SPECIALISED INFORMATION
For departures from an aerodrome where the standard pre-flight meteorological briefing is
inadequate for the intended flight, a special forecast may be issued. Normally a Special Flight
Forecast is supplied from the last UK departure point to the first transit aerodrome outside the
coverage of standard documentation. By prior arrangement, forecasts are provided for other legs
if the initial ETD to final ETA does not exceed 6 hours and no stops longer than 60 minutes are
planned.
Forecast offices normally require prior notification for special forecasts. For flights up to 500 nm at
least 2 hours is required before the time of collection. For flights over 500 nm at least 4 hours is
required before the time of collection.
SIGMET SERVICE
Aircraft can be supplied with information in flight. MWOs are responsible for the preparation and
issue of SIGMETs to the appropriate ATC unit. Aircraft in flight are warned of the occurrence or
expectation of one or more of the following SIGMET phenomena for the route ahead, for up to
500 nm or 2 hours flying time:
a.
At Subsonic Cruising Levels (SIGMET)
i.
Thunderstorm (See Note)
ii. Heavy hail (See Note)
iii. Tropical cyclone
iv. Freezing rain
v. Severe turbulence (not associated with convective cloud)
vi. Severe icing (not associated with convective cloud)
vii. Severe mountain waves
viii. Heavy sand/dust storms
ix. Volcanic ash cloud
Note: Thunderstorm does not refer to isolated or occasional thunderstorms not embedded in
cloud layers or concealed by haze. This refers only to thunderstorms widespread, including if
necessary CB which is not accompanied by a TS, within an area:
With little or no separation
Along a line with little or no separation
Embedded in cloud layers
Or concealed in cloud layers or concealed by haze
FRQ
SQL
EMBD
OBSC
TS and tropical cyclones each imply:
Moderate or severe turbulence
Moderate or severe icing and hail
Heavy hail HVYGR is used as a further description of the TS as necessary
b.
Meteorology
At Transonic and Supersonic Cruising Levels (SIGMET SST)
i.
Moderate or severe turbulence
ii. Cumulonimbus cloud
iii. Hail
iv. Volcanic ash cloud
21-3
Chapter 21
Meteorological Observations and Meteorological Services
In general SIGMET messages are identified by the letters WS at the beginning of the header line.
Tropical Cyclones and Volcanic Ash will be identified by WC and WV respectively.
AIRCRAFT REPORTS
SIGMETs are not usually valid for more than 4 hours, except volcanic ash clouds where the
period is upwards of 12 hours. SIGMETs are sequentially numbered through the day
Flight levels for SIGMET SST are as follows:
¾ FL 250 − FL 600 London and Scottish UIRs
¾ FL 400 − FL 600 Shanwick OCA
ROUTINE AIRCRAFT OBSERVATIONS
Routine aircraft observations are not required in the London/Scottish FIR/UIR. In the Shanwick
OCA, aircraft are to conform with the requirements laid out in the ENR section or applicable
NOTAM.
SPECIAL AIRCRAFT OBSERVATIONS
Special observations are required in any UK FIR/UIR/OCA when:
a. Severe turbulence or severe icing is encountered.
or
b. Moderate turbulence, hail or cumulonimbus clouds are encountered during transonic
or supersonic flight.
or
c. Any factors which a pilot believes affects the safety of flight are encountered.
or
d. When requested by the meteorological office.
or
e. When there is an agreement between the meteorological office and the aircraft
operator.
21-4
Meteorology
Meteorological Observations and Meteorological Services
Chapter 21
CLEAR AIR TURBULENCE (CAT)
CAT remains an important operational factor at all levels of flying but particularly above FL 150.
Pilots encountering CAT are requested to report time, location, level, intensity, and aircraft type to
the ATS unit they are operating with. This is done as a Special Observations Report. The
criteria required are:
INCIDENCE
¾ OCCASIONAL — less than 1/3 of the time.
¾ INTERMITTENT — 1/3 to 2/3.
¾ CONTINUOUS — more than 2/3.
INTENSITY
LIGHT
¾ Light Turbulence — IAS fluctuates 5 − 15 kt, turbulence that momentarily causes
slight erratic changes in attitude and/or altitude.
¾ Light Chop — Turbulence that causes slight rapid rhythmic bumping without
appreciable changes in altitude or attitude. No IAS fluctuations.
¾ Reaction Inside Aircraft — Occupants may feel a slight strain against seat belts or
shoulder straps. Unsecured objects may be displaced slightly. Food service may be
conducted and little or no difficulty is encountered when walking.
MODERATE
¾ Moderate Turbulence — IAS fluctuates 15 − 25 kt, turbulence that is similar to light
turbulence but of greater intensity. Changes in altitude and/or attitude can occur but
the aircraft remains in positive control at all times.
¾ Moderate Chop — Turbulence that is similar to light chop but of greater intensity.
Rapid bumps or jolts without appreciable changes in altitude or attitude. IAS may
fluctuate slightly.
¾ Reaction Inside Aircraft — Occupants feel definite strains against seat belts or
shoulder straps. Unsecured objects are dislodged. Food service and walking are
difficult.
SEVERE
¾ Severe Turbulence — IAS fluctuates more than 25 kt; turbulence that causes large,
abrupt changes in altitude and/or attitude. The aircraft may be momentarily out of
control.
¾ Reaction Inside Aircraft — Occupants are forced violently against seat belts or
shoulder straps. Unsecured objects are tossed about. Food service and walking
impossible.
Meteorology
21-5
Chapter 21
Meteorological Observations and Meteorological Services
AIRFRAME ICING
Any pilot encountering unforecast icing is requested to report the time, location, level, intensity,
icing type, and aircraft type to the ATS unit they are operating with. The following are reporting
definitions, and are not necessarily forecasting definitions.
Trace
Ice becomes perceptible. Rate of accumulation slightly greater than the rate of
sublimation. It is not hazardous even though de-icing/anti-icing equipment is not used
unless ice is encountered for more than one hour.
Light
The rate of accumulation might create a problem if flight in this environment exceeds
1 hour. Occasional use of de-icing/anti-icing equipment removes/prevents
accumulation. It does not present a problem if anti-icing equipment is used.
Moderate
The rate of accumulation is such that even short encounters become potentially
hazardous and the use of de-icing/anti-icing equipment, or diversion, is necessary.
Severe
The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or
control the hazard. Immediate diversion is necessary.
AERODROME CLOSURE
The term SNOCLO is added to the end of an aerodrome report in a VOLMET radio broadcast
when it is unusable for take-off or landing due to heavy snow on the runways, or the runway is
blocked for snow clearance.
IN-FLIGHT PROCEDURES
An in-flight enroute service is available in exceptional circumstances by prior arrangement with
the meteorological office. Make applications for this service in advance stating:
1.
2.
3.
4.
The flight levels and route sector required.
The period of validity required.
The approximate time and position the request will be made.
The ATS unit the aircraft expects to be in contact.
Aircraft can obtain aerodrome weather information from any of the following sources:
¾
¾
¾
¾
21-6
VOLMET broadcasts.
Automatic Terminal Information Service (ATIS) broadcasts as described in the GEN
section.
By request to an ATC unit.
If an aircraft proposes to divert to an aerodrome for which no forecast is provided, the
commander may request the relevant information from the ATS unit serving the
aircraft.
Meteorology
Meteorological Observations and Meteorological Services
Chapter 21
ACCURACY OF METEOROLOGICAL MEASUREMENT OR OBSERVATION
The accuracies listed refer to assessment by instruments (except cloud amount). They are not
usually attainable in observations made without instruments.
Element
Accuracy of Measurement or Observation
Mean Surface Wind
Direction: ± 5°
Speed:
± 1 kt up to 20 kt, ± 5% above 20 kt
Variations from the mean
surface wind
± 1 kt
Visibility
± 50 m up to 500 m
± 10% between 500 m and 2000 m
± 20% above 2000 m up to 10 km
RVR
± 25 m up to 150 m
± 50 m between 150 m and 500 m
± 10% above 500 m up to 2000 m
Cloud amount
± 1 okta in daylight, worse in darkness and during
atmospheric obscuration
Cloud height
± 33 ft up to 3300 ft
± 100 ft above 3300 ft up to 10 000 ft
Air temperature
temperature
and
dew
Pressure value (QFE, QNH)
point ± 0.2° C
± 0.3 mb
MARKED TEMPERATURE INVERSION
At certain aerodromes a Warning of Marked Temperature Inversion is issued whenever a
temperature difference of 10° C or more exists between the surface and any point up to 1000 ft
above the aerodrome. This warning is broadcast on departure and arrival ATIS at aerodromes so
equipped, or in the absence of ATIS passed by radio to departing aircraft before take-off, and to
arriving aircraft as part of the report of aerodrome meteorological conditions.
AERODROME WARNINGS
Aerodrome warnings are issued as appropriate when one or more of the following occurs or is
expected to occur:
a.
b.
c.
d.
e.
f.
g.
Meteorology
Gales or strong winds agreed to locally agreed criteria (Gales — mean surface
wind >33 kt or gusts >42 kt).
Squalls, hail or thunderstorms.
Snow, including the expected time of beginning, duration and intensity of fall; the
expected depth of accumulated snow, and the time of expected thaw.
Amendments or cancellations are issued as necessary.
Frost warnings when any of the following are expected to exist.
i. A ground frost with air temperatures not below freezing point.
ii. The air temperature above the surface is below freezing (Air frost).
iii. Hoar frost, rime or glaze deposited on parked aircraft.
Fog (normally when visibility is expected to fall below 600 m).
Rising dust or sand.
Freezing precipitation.
21-7
Chapter 21
Meteorological Observations and Meteorological Services
SPECIAL FACILITIES
WINDSHEAR ALERTING
Forecasters at airports review the weather conditions on an hourly basis and monitor any aircraft
reports of windshear experienced on the approach or climb-out. Whenever a potential low level
windshear condition exists, a Windshear alert, based on one or more of following criteria, is
issued:
a. Mean surface wind speed at least 20 kt.
b. The magnitude of the vector difference between the mean surface wind and the gradient
wind (an estimate of the 2000 ft wind) of at least 40 kt.
c. Thunderstorm(s) or heavy shower(s) within approximately 5 nm of the Airport.
Note: Alerts are also issued based on recent pilot reports of windshear on the
approach or climb-out.
The Alert message is given in the arrival and departure ATIS in one of three formats:
a. Windshear Forecast (WSF)
When the meteorological conditions indicate that low level windshear on the approach
or climb-out (below 2000 ft) might be encountered.
b. Windshear Forecast and Reported (WSFR)
As above, supported by a report from at least one aircraft of windshear on the approach
or climb-out within the last hour.
c. Windshear Reported (WSR)
When an aircraft reports windshear on the approach or climb-out within the last hour, but
insufficient meteorological evidence exists for the issue of a forecast of windshear.
WINDSHEAR REPORTING CRITERIA
Pilots using navigation systems providing direct wind velocity readout should report the wind and
altitude/height above and below the shear layer, and its location. Other pilots should report the
loss or gain of airspeed and/or the presence of up or down draughts or a significant change in
crosswind effect, the altitude/height and location, their phase of flight, and aircraft type. Pilots not
able to report windshear in these specific terms are to do so in terms of its effects on the aircraft,
the altitude/height and location and aircraft type (e.g. Abrupt windshear at 500 ft on finals,
maximum thrust required, B747).
Pilots encountering windshear are requested to make a report even if windshear was previously
forecast or reported.
21-8
Meteorology
Meteorological Observations and Meteorological Services
Chapter 21
OBSERVING SYSTEMS AND OPERATING PROCEDURES — SURFACE
WIND
Surface wind sensors are positioned to give the best practical indication of the winds which an
aircraft encounters during take-off and landing within the layer between 6 and 10 m above the
runway. The surface wind reported for take-off and landing by ATS units supporting operations by
aircraft whose MTWA is less than 5700 Kg is usually an instantaneous wind measurement with
direction referenced to Magnetic North. At other designated airports the wind reports for take-off
and landing are averaged over the previous 2 minutes. Variations in the wind direction are given
when the total variation is 60° or more and the mean speed above 3 kt. The directional variations
are expressed as the two extreme directions between which the wind has varied in the past 10
minutes. In reports for take-off, surface winds of 3 kt or less include a range of wind directions
whenever possible if the total variation is 60° or more. Variations from the mean wind speed
(gusts) during the past 10 minutes are only reported when the variation from the mean speed
exceeds 10 kt. Variations are expressed as the maximum and minimum speeds attained.
Note: Surface wind measurement in a METAR and SPECI are referenced to true
north.
CLOUD HEIGHT
Information on cloud height is obtained by the use of:
a.
b.
c.
d.
e.
Ceilometers
Cloud searchlights
Alidades
Balloons
Pilot reports or observer estimation
At some aerodromes an additional cloud ceilometer is installed on the approach. The cloud
heights reported from an approach ceilometer are:
a. The most frequently occurring value during the past 10 minutes if the value is 1000 ft or
less.
b. If cloud is indicated at heights 100 ft or more below that indicated at (a) above then the
height of the lowest cloud is reported, prefaced by OCNL.
c. If the most frequently occurring value is above 1000 ft but the lowest value is 1000 ft or
below, then only the lowest value is reported.
TEMPERATURE
Temperature is reported in whole degrees Celsius, M indicates a negative value.
HORIZONTAL SURFACE VISIBILITY
Horizontal surface visibility is assessed by human observer. Visibility is reported in increments of:
a. 50 m up to 500 m
b. 100 m up to 5000 m
In METAR, SPECI or TAF the maximum value is "10 km or more."
Meteorology
21-9
Chapter 21
Meteorological Observations and Meteorological Services
When the lowest visibility is less than 1500 m and visibility in another direction is more than
5000 m, additionally, the maximum visibility and the direction in which it occurs is reported.
RUNWAY VISUAL RANGE (RVR)
RVR assessment is made by either human observer or by an Instrument RVR system (IRVR).
For the UK the standard RVR reporting increments are:
a. 25 m between 0 and 200 m.
b. 50 m between 200 and 800 m.
c. 100 m from 800 m.
Assessment and reporting in RVR begin when the horizontal visibility, or the RVR, is observed at
less than 1500 m.
RVR is passed to aircraft before take-off and during the approach to landing. Changes to the
RVR are passed throughout an aircraft's approach.
Where multi-site IRVR systems are installed, the procedure is for touchdown, mid-point and stop
end values of RVR to be given (e.g. RVR 600, 500, 550). Where only touchdown and one other
value is given, the RVR is given as RVR 500 stop end 500. Where a single transmitter fails and
the remainder of the system is serviceable RVR readings are not suppressed (e.g. RVR
touchdown missing, 600, 500). If two out of the transmissions fails, then the remaining value is
given provided that it is not the stop end value.
Aerodromes suppress mid-point and/or stop-end values when:
a. They are equal to or higher than the touchdown zone value unless they are less than
400 m.
Example: 300 350 350 All values are reported.
or
b. They are 800 m or more.
Example: 1000 900 900 Only the touchdown value is reported.
21-10
Meteorology
INTRODUCTION
A variety of weather messages are originated by Meteorological Observers at aerodromes. These
are collated and broadcast in text form to stations around the world. Aviators are usually able to
distinguish between the various message types and their uses.
AERODROME METEOROLOGICAL REPORT
Aerodrome Meteorological Reports (METAR) contain observations on the conditions that actually
exist at a station and are made every 30 minutes throughout the day.
¾
¾
Short term landing forecasts, valid for two hours (TREND), may be added to
METARS.
Information on runway condition is added to METAR when appropriate, until these
conditions cease.
SPECIAL AERODROME METEOROLOGICAL REPORTS
Special Aerodrome Meteorological Reports (SPECI) are issued when conditions change
significantly. Selected Special Reports (SPECI) are defined as Special Reports disseminated
beyond the aerodrome of origin.
TERMINAL AERODROME FORECASTS
Terminal Aerodrome Forecasts (TAF) are normally provided only for those aerodromes where
official meteorological observations are made. For other aerodromes, Local Area Forecasts are
made. Amended TAFs or Local Area Forecasts are issued when forecast conditions change
significantly.
ACTUAL WEATHER CODES
The content and format of an actual weather report are shown in the following table.
Report
Type
Location
Identifier
Date/Time
Wind
Visibility
RVR
METAR
EGSS
291250Z
31015G30KT
1400SW
6000N
R24/P1500
Present
Weather
Cloud
Temp/
Dew Pt
QNH
Recent
Weather
Wind
Shear
Trend
Rwy
State
SHRA
FEW005
SCT010CB
BKN025
10/05
Q0999
RETS
WS
RWY25
NOSIG
88290592
Meteorology
22-1
Chapter 22
Meteorological Messages
IDENTIFIER
The identifier has three components:
Report Type:
ICAO Indicator:
Date/Time UTC:
Example:
METAR or SPECI.
This is a four-letter group indicating the airfield
(e.g. EGPL, LFPB).
In a METAR or SPECI this is the date and time of the
observation in hours and minutes UTC (e.g. 091250Z).
METAR EGDL 211020Z.
Note: If a meteorological bulletin consists of a set of reports from one or more airfields the
codename METAR or SPECI is replaced by SA (Actual Report), or SP (Special Report) followed
by a bulletin identifier, date, and time of the observation.
SURFACE WIND VELOCITY
The first 3 figures indicate the wind direction (T) to the nearest 10°, followed by two figures
(exceptionally 3 figures) giving the mean windspeed during the previous 10 minutes. The
permitted units of speed are:
¾
¾
¾
KT indicating knots
KMH for kilometres per hour, or
MPS for metres per second.
Example: 30015KT.
These may be followed by a letter G and two more figures if the maximum gust speed exceeds
the average speed by 10 kt or more.
Example: 30015G30KT.
Variations in wind direction of 60° or more in the 10 minutes preceding the observation are shown
as 3 figures then the letter V followed by another 3 figures, but only if the speed is more than 3 kt.
Example: 270V330 meaning, the wind is varying in direction between 270°T and
330°T.
00000 indicates calm conditions, a variable wind direction is shown by VRB followed by the
speed.
HORIZONTAL VISIBILITY
When there is no marked variation in direction the minimum visibility is given in metres. The
minimum visibility with the direction is given when there is a marked variation with direction.
Example: 2000NE.
22-2
Meteorology
Meteorological Messages
Chapter 22
When the minimum visibility is less than 1500 metres and the visibility in any other direction is
greater than 5000 metres the maximum visibility and its direction is also shown.
Example: 1200NE 6000SW.
9999 indicates a visibility of 10 kilometres or more, 0000 indicates a visibility of less than 50
metres.
RUNWAY VISUAL RANGE (RVR)
Runway Visual Range is reported when the meteorological visibility falls below 1500 m. It has the
form R, followed by the runway designator, a diagonal and then the Touchdown RVR. If more
than one runway is in use, the RVR group is repeated. Parallel runways are distinguished by
adding C, L, or R to the runway designator.
Example: R24L/1200R24R/1100.
When RVR is greater than the maximum assessable value the prefix P is added followed by the
maximum value.
Example: R15/P1500.
The prefix M indicates the RVR is less than the minimum value that can be assessed.
Example: R15/M0050.
Tendencies are indicated by U for up, D for down, or N for no change. They show a significant
change (100 m or more) from the first 5 minutes to the second 5 minutes in the 10 minute period
prior to the observation.
Example: R25/1000D.
Variations are reported if the RVR changes minute by minute during the 10 minute period prior to
the report. The 1 minute minimum and maximum separated by V are reported instead of the 10
minute mean.
Example: R15L/0850V1000.
Meteorology
22-3
Chapter 22
Meteorological Messages
WEATHER
Each weather group may consist of the appropriate intensity indicators and abbreviations, making
groups of two to nine characters from the table below.
SIGNIFICANT PRESENT AND FORECAST WEATHER CODES
QUALIFIER
WEATHER PHENOMENA
Intensity or
Proximity
Descriptor
Precipitation
Obscuration
Other
1
2
3
4
5
- Light
MI
Shallow
DZ
Drizzle
BR
Mist
PO
Dust/sand
whirls
Moderate
(no qualifier)
BC
Patches
RA
Rain
FG
Fog
PR
Partial (Covering
part of Aerodrome)
SN
Snow
FU
Smoke
SQ
Squalls
+ Heavy
'Well developed
in the case of FC
and PO
DR
Drifting
SG
Snow Grains
VA
Volcanic Ash
FC
Funnel
Cloud(s)
(tornado or
water-spout)
VC
In the vicinity
(within 8 km of
aerodrome
perimeter but
not at
aerodrome)
BL
Blowing
IC
Ice Crystals
(Diamond Dust)
DU
Widespread
Dust
SS
Sandstorm
SH
Shower(s)
PE
Ice-Pellets
SA
Sand
DS
Duststorm
TS
Thunderstorm
GR
Hail
HZ
Haze
FZ
Freezing
Super-Cooled
GS
Small hail
(<5 mm diameter)
and/or snow
pellets
A mixture of weather is reported using up to three groups to indicate different weather types.
Examples: MIFG, VCBLSN, +SHRA, -DZHZ.
Note: BR, HZ, FU, IC, DU and SA will not be given in METAR or TAF when the
visibility is above 5000 m.
22-4
Meteorology
Meteorological Messages
Chapter 22
CLOUD
The cloud group usually consists of 3 letters and 3 figures. These show the cloud amount
followed by the height of the cloudbase, above airfield level, in hundreds of feet. The cloud
groups are given in ascending order of height.
Example: SCT015 or OVC080.
These groups are:
FEW indicating 1 − 2 oktas
BKN (broken) indicating 5 − 7 oktas
SCT (scattered) indicating 3 − 4 oktas
OVC (overcast) indicating 8 oktas.
The cloud group may have a suffix for significant convective cloud, CB for Cumulonimbus, or
TCU for Towering Cumulus. No other cloud types are reported.
Example: BKN015CB.
Layers are reported as:
First Group: Lowest individual layer of any amount.
Second Group: Next individual layer of more than 2 oktas.
Third Group: Next higher layer of more than 4 oktas.
Additional Group: Significant convective cloud not already reported.
SKC indicates no cloud to report when CAVOK does not apply.
Sky obscured is shown by VV followed by vertical visibility in hundreds of feet. When the vertical
visibility is not assessed the group reads VV///.
Example: VV003.
CAVOK
CAVOK is for use in place of groups 4, 5, 6, and 7 when all of the following conditions apply:
a. Visibility is 10 km or more.
b. There is no cloud below 5000 ft or below the highest Minimum Sector Altitude (MSA),
which ever is greater, and no CB.
c. No significant weather phenomenon at or in the vicinity of the aerodrome.
Minimum Sector Altitude is the lowest altitude that is allowed for use under emergency conditions
which provides a minimum clearance of 1000 ft above all objects located in an area contained
within a sector of a circle of 25 nm radius centred on a radio navigation aid. A sector is not less
than 45°.
AIR TEMPERATURE AND DEWPOINT
Air Temperature and Dewpoint are reported in degrees Celsius. M indicates a negative value.
Examples: 10/08, 01/M01.
Meteorology
22-5
Chapter 22
Meteorological Messages
SEA LEVEL PRESSURE (QNH)
QNH is reported in the form Q followed by a four-figure group. If the QNH is less than 1000 mb
the first figure is 0. QNH is rounded down to the nearest whole millibar.
Example: Q0995.
The pressure, when given in inches of mercury, is reported as A followed by the pressure in
hundredths of inches.
Example: A3037.
SUPPLEMENTARY INFORMATION
RECENT WEATHER (RE)
This is operationally significant weather observed since the previous observation (or in the last
hour, whichever is the shorter) but not occurring now. Up to three groups are used to indicate the
former presence of more than one weather type.
Example: RETS REGR.
WINDSHEAR (WS)
Windshear is inserted, if reported in the lowest 1600 ft of the take-off or approach paths.
Example: WS RWY27, WS ALL RWY.
TREND
A forecast of significant changes in weather expected within 2 hours of the observation time is
added to the end of a METAR or SPECI, if a qualified Forecaster is present.
Change Indicator:
BECMG (becoming) or TEMPO (temporary) which are followed by a time group in
hours and minutes UTC, and possibly followed by FM (from), TO (until), or AT (at)
followed by a four figure time group.
Weather:
Standard codes are used in this section. NOSIG is used when no significant changes
are expected to occur during the trend forecast period.
Example: BCMG FM1100 25035G50KT or, TEMPO 0630 TL 0830 3000 SHRA.
Only those elements of the above in which a change is expected are included. When no change
is expected, the term NOSIG is used.
22-6
Meteorology
Meteorological Messages
Chapter 22
RUNWAY STATE GROUP
An eight-figure Runway State Group is added to the end of the METAR or SPECI (following any
TREND) when there is lying precipitation or other runway contamination. The student requires the
ability to decode the first two digits (Runway designator) and last two digits (Braking Action). The
complete group consists of:
Runway Designator (First Two Digits)
27 = Runway 27 or 27L
88 = All runways
Runway Deposits (Third Digit)
0 = Clear and dry
1 = Damp
2 = Wet or water patches
3 = Rime or frost covered
(depth normally less than 1 mm)
4 = Dry Snow
77 = Runway 27R (50 added to the
designator to indicate 'right' Runway)
99 = A repeat of last message because no
new information received
5 = Wet Snow
6 = Slush
7 = lce
8 = Compacted or rolled snow
9 = Frozen ruts or ridges
/ = Not reported (e.g. due to runway clearance in progress)
Extent of Runway Contamination (Fourth Digit)
1 = 10% or less
2 = 11% to 25%
5 = 26% to 50%
9 = 51% to 100%
/ = Not reported (e.g. due to runway clearance in progress)
Depth of Deposit (Fifth and Sixth Digits)
The quoted depth is the mean number of readings or if operationally significant the greatest depth
measured.
00 = less than 1 mm
91 = not used
93 = 15 cm
95 = 25 cm
97 = 35 cm
01 = 1 mm through to 90 = 90 mm
92 = 10 cm
94 = 20 cm
96 = 30 cm
98 = 40 cm or more
// = Depth of deposit operationally not significant or not measurable
Meteorology
22-7
Chapter 22
Meteorological Messages
Friction Coefficient or Braking Action (Seventh and Eighth Digits)
The value, transmitted is the mean or, if operationally significant, the lowest value.
28 = Friction coefficient 0.28
or
91 = Braking action: Poor
93 = Braking action: Medium
95 = Braking action: Good
35 = Friction coefficient 0.35
92 = Braking action: Medium/Poor
94 = Braking Action: Medium/Good
99 = Figures unreliable (e.g. if equipment used does not measure satisfactorily in slush or loose
snow)
// = Braking action not reported (e.g. runway not operational, closed, etc.)
If contamination conditions cease to exist, the abbreviation CLRD is used.
Examples:
24CLRD93 = Rwy 24 cleared: Braking action; Medium.
88CLRD95 = All runways cleared: Braking action; Good.
'AUTO' AND 'RMK'
Where a report contains fully automated observations with no human intervention, it is indicated
by the code word 'AUTO', inserted immediately before the wind group.
The indicator 'RMK'(remarks) denotes an optional section containing additional meteorological
elements. It is appended to METARs by national decision, and is not disseminated internationally.
MISSING INFORMATION
Information that is missing from a METAR or SPECI is replaced by diagonals.
EXAMPLES OF METARS
SAUK02 EGLY 301220Z METAR
EGLY 24015KT 200V280 8000 -RA SCT010 BKN025 OVC080 18/15 Q0983 TEMPO
3000 RA BKN008 OVC020=
EGPZ 30025G37KT 270V360 1200NE 6000S +SHSN SCT005 BKN010CB 03/M01
Q0999 RETS WS LDG RWY27 BECMG AT 1300 9999 NSW SCT015 BKN100=
The METARs above are for 1220 UTC on the 30th day of the month. The decode in plain
language is:
EGLY: Surface wind: mean 240°True, 15 kt; varying between 200° and 280°
minimum visibility 8 km; slight rain; cloud: 3 − 4 oktas base 1000 ft, 5 − 7 oktas 2500
ft, 8 oktas 8000 ft; Temperature +18°C, Dew Point +15°C; QNH 983 mb; Trend:
temporarily 3000 m in moderate rain with 5 − 7 oktas 800 ft, 8 oktas 2000 ft.
EGPZ: Surface wind: mean 300°True, 25 kt; maximum 37 kt, varying between 270°
and 360°; minimum vis 1200 m (to northeast), maximum visibility 6 km (to south);
heavy showers of snow, Cloud: 3 − 4 oktas base 500 ft, 5 − 7 oktas CB base 1000 ft;
Temperature +3°C, Dew Point -1°C; QNH 999 mb; thunderstorm since previous
report; windshear reported on approach to runway 27; Trend: improving at 1300 UTC
to 10 km or more, nil weather, 3 − 4 oktas 1500 ft, 5 − 7 oktas 10 000 ft.
22-8
Meteorology
Meteorological Messages
Chapter 22
AERODROME FORECASTS (TAF) CODES
TAF describe the forecast of conditions at aerodromes and usually cover periods of not less than
9 hours, and not more than 24 hours. Those valid for less than 12 hours are issued every 3 hours
and those valid for 12 to 24 hours issued every 6 hours. TAFs prefixed FC are valid for periods of
less than 12 hrs. TAF's prefixed FT are valid for periods of 12 to 24 hours. An 18 hour forecast
normally starts 8 hours after the time of issue and normally accompanies a 9 hour TAF.
TAF CONTENTS AND FORMAT
The TAF uses the same code system as the METAR, with the following differences:
Validity Period
In the validity period the first two numbers indicate date of issue. The next 4 figures
the forecast period in whole hours UTC. If the TAF bulletin consists of forecasts for
one or more airfields, the codename TAF is replaced by FC or FT, followed by the
date and time of origin and neither codename nor time/date group appears in the
forecast.
Visibility
Same as METAR with only the minimum visibility forecast.
Weather
If no significant weather is expected, the group is omitted. After a change group if the
weather becomes insignificant then NSW (No Significant Weather) is used.
Cloud
If clear sky is forecast the cloud group is replaced by SKC (Sky Clear). If CAVOK and
SKC are not appropriate then NSC (No Significant Cloud) is used.
SIGNIFICANT CHANGES
FM followed by the time to the nearest hour and minute UTC, is used to show the
beginning of a self contained part in the forecast. All conditions given before this
group are superseded - they no longer apply.
Example: FM1220 27017KT 4000 BKN010.
BCMG followed by a four figure time group indicating the earliest and latest start
hours of an expected permanent alteration to the meteorological conditions. This
change can occur at a regular or irregular rate during the forecast change period. The
change does not start before the first time and it is complete by the second time given.
Example: BECMG 2124 1500 BR.
TEMPO followed by a four figure time group indicates the hours of a period of
changes in the conditions of a temporary nature which may occur at any time during
the period. These changes are expected to last less than one hour in each case and
in total for less than half of the forecast period indicated.
Meteorology
22-9
Chapter 22
Meteorological Messages
PROBABILITY of the occurrence of alternative forecast conditions are given as a
percentage but only 30% or 40% is used.
Example:
PROB30 0507 0800 FG BKN004.
PROB40 TEMPO 1416 TSRA BKN010CB.
OTHER GROUPS
Three additional TAF groups are often used in overseas and UK military TAF. They are used to
forecast temperature (Group indicator T), Icing (Group indicator 6), and turbulence (Group
indicator 5).
Example 9 hr TAF:
FCUK33 EGGY 300900Z
EGGW 301019 23010KT 9999 SCT010 BKN018 BECMG 1114 6000 -RA BKN012
TEMPO 1418 2000 RADZ OVC004 FM1800 30020G30KT 9999 -SHRA BKN015CB=
Decode:
Nine hour TAF issued at 0900 UTC on the 30th of the month at Luton, Valid from
1000 to 1900 UTC. Wind from 230°T at a speed of 10 kt. Visibility 10 kilometres or
more. Cloud amount 3 − 4 oktas, base 1000 ft, second cloud layer 5 − 7 oktas, base
1800 ft. Between 1100 − 1400 UTC a permanent change occurs. Visibility becomes 6
km in slight rain. with 5 − 7 oktas of cloud base 1200 ft. There are short term changes
between 1400 − 1800 UTC. Visibility decreases to 2000 metres in moderate rain and
drizzle and overcast at 400 ft. From 1800 UTC there is another permanent change.
Wind velocity becomes 300°T at 20 kt gusting to 30 kt. Visibility improves to 10 km or
more with slight rain showers and the cloud is 5 − 7 oktas of cumulonimbus base 1500
ft.
Example 18 hr TAF:
FTUK31 EGGY 102300Z
EGLL 110624 13010KT 9000 BKN010 BECMG 0608 SCT015 BKN020 PROB30
TEMPO 0816 17025G40KT 4000 TSRA SCT010 BKN015CB BECMG 1821 3000 BR
SKC=
Decode:
Eighteen hour TAF issued at 2300 UTC on the 10th for London Heathrow, valid from
0600 − 2400 UTC on the 11th. Wind from 130°T at 10 kt. Visibility 9 km. Cloud 5 − 7
oktas base 1000 ft. A permanent change occurs between 0600 − 0800 UTC to 3 − 4
oktas of cloud base 1500 ft and 5 − 7 oktas of cloud base 2000 ft. There is a 30%
probability that for short periods between 0800 − 1600 UTC the wind velocity
becomes 170°T speed 25 kt maximum to 40 kt with visibility of 4000 m in
thunderstorms with rain, cloud becoming 3 − 4 oktas base 1000 ft and 5 − 7 oktas
cumulonimbus base 1500 ft. A permanent change occurs between 1800 − 2100 UTC
the visibility becoming 3000 m in mist with clear skies.
22-10
Meteorology
Meteorological Messages
Chapter 22
VOLMET BROADCASTS
These are aerodrome weather reports, METARS, which are transmitted on VHF frequencies in
plain language in the following order:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Meteorology
Surface Wind Velocity (degrees True)
Visibility
RVR if applicable
Weather
Cloud
Temperature
Dewpoint
QNH
TREND if applicable, or NOSIG
The spoken word SNOCLO is added at the end of the aerodrome report when
the aerodrome is unusable for take-offs and landings due to heavy snow on
runways or runway clearance operations.
22-11
Chapter 22
22-12
Meteorological Messages
Meteorology
The Synoptic Chart
Chapter23
INTRODUCTION
The Synoptic Chart, Chart 1, is one of the tools used by the Meteorology Forecaster. For the
Meteorology exam there is a required working knowledge of:
a.
b.
c.
d.
Chart symbology
The synoptic situation
The likely future development of the situation shown on the chart
The implications of the weather situation to the pilot
Refer to Chart 1 for paragraphs a to f:
a. The chart is a Lambert Projection which covers both the UK and Northern
Europe.
b. To the bottom right of the chart is the Date/Time for which the chart is valid. In
this case January, 1200 UTC.
i.
The major synoptic charts are issued at 0000, 0600, 1200, and
1800 UTC.
ii. The minor synoptic charts are issued in the intermediate hours 0300,
0900, 1500, and 2100 UTC.
c. To the bottom left of the chart are two scales:
i.
The upper scale is a range scale in nautical miles. This scale is usually
for use when distances require measuring on the chart.
ii. The lower scale is a Geostrophic Wind Scale which is explained later.
d. The pressure pattern is shown by sea level isobars. The isobars join together
points of equal mean sea level pressure (QFF). The isobars are always plotted
as whole even hectopascalic pressure values, typically 2, 4, or 8 hPa depending
on the pressure gradient. How to give numeric values to the isobars is shown
later.
e. Fronts are shown in the standard way:
i.
In the warm sector the isobars are nearly straight and parallel and give a
good indication of the Geostrophic winds at 2000 ft (even though the
isobars are MSL values).
ii. Around the depression (55°N 012°30'W) the isobars are curved and the
value derived from the geostrophic wind scale need correcting to
produce a value for the gradient wind at 2000 ft. The direction is
measured directly from the chart.
Rule of thumb corrections to be applied for both depressions and anticyclones
are:
Around a low pressure
-5 kt
Around a high pressure
+ 5 − 10 kt
Meteorology
23-1
Chapter 23
The Synoptic Chart
f.
Observations are given for different reporting stations in a coded format, these
are called Station Circles. Using the information from the Station Circle, the
forecaster constructs the isobaric pattern.
Chart 1
23-2
Meteorology
The Synoptic Chart
Chapter 23
Note: The remainder of this chapter is for information and is not currently examinable.
THE STATION CIRCLE DECODE
The observation reports submitted by Meteorological Observation Stations are transcribed, in
standard form, on to synoptic charts as the Station Circle which forms the basis for a
Meteorological Forecaster's forecasts. It is necessary for aviators to have a good general
knowledge of the symbols on the station circle. The format for the arrangement of the station
circle has been internationally agreed.
High Cloud Type
Medium Cloud Type
Amount/Base Height
Air Temperature
Visibility
MSL Pressure
TOTAL
Present Weather
Dewpoint Temperature
CLOUD
Barometric Tendency and
Characteristic
COVER
Past Weather
Low Cloud Type
The wind symbol in a variable position
Amount/base height
PRESSURE (1 O'CLOCK)
QFF shown by three figures giving tens, units and tenths of a hectopascal (e.g. 721 = 72.1 hPa).
The expected QFF range is from 950 to 1050 hPa. The reader of the circle is required to prefix
the 3 figures by either a 9 or 10. In the above example, the full QFF is 972.1 hPa. Any figure
between 500 and 999 must be prefixed with a 9. Figures 000 to 500 with a 10 (e.g. 033 =
03.3 hPa which would be a QFF of 1003.3 hPa).
PRESSURE TENDENCY (3 O'CLOCK)
The pressure change in the past 3 hours is shown by two figures and a symbol. The symbols
show the type of change.
Overall Rise
Meteorology
Overall Fall
A rise followed by a small
fall
A fall followed by a rise
A rise and then steady
A fall and then steady
A steady rise
A steady fall
A small fall followed by a
rise
A small rise followed by a fall
23-3
Chapter 23
The Synoptic Chart
The two figures show the amount of the pressure change in units and tenths of a hPa
(e.g. 36/ indicates in the past three hours there was a steady rise of 3.6 hPa).
PAST WEATHER (5 O'CLOCK)
All weather is shown by symbols only. Weather in this context refers to precipitation, mist, haze,
fog, thunderstorms, and snowstorms. Certain symbols are common to both "Past Weather" and
"Present Weather" (9 o'clock position).
,
*
Fog or ice fog
Showers
Drizzle
Thunderstorms
Rain
Hail . The symbol is sometimes shown
black instead of open.
Snow
The arrangements of the common symbols and their combinations have different meanings for
Past and Present Weather.
Past weather refers to the past 6 hours for the Major Synoptic hours of 0000, 0600, 1200, and
1800 UTC. It refers to the past 3 hours for the Minor Synoptic hours of 0300, 0900, 1500, and
2100 UTC. If hourly charts are produced then the past weather for the last hour.
ADDITIONAL PAST WEATHER SYMBOLS
A single symbol means that particular weather occurred for part of the time. Two identical
symbols means that it is continuous. Two different symbols means that each occurred, the first
being predominant.
Other combinations similarly apply.
Note: Intensity of past precipitation is not shown.
23-4
Meteorology
The Synoptic Chart
Chapter 23
LOW CLOUD OR VERTICAL VISIBILITY (6 O'CLOCK)
The cloud type is shown by a symbol. The amount of sky cover in eighths and the cloud base in
hundreds of feet are shown in figures, positioned below the symbol. The figures for cloud amount
and base are separated by a slash. Cloud height is given with reference to airfield level
(e.g. 3/12 indicates 3 oktas of stratus base 1200 ft above airfield level).
VERTICAL VISIBILITY
These figures are for use in indicating the sky is obscured. The 9 shows that the sky is obscured.
The 2 figures after the slash show the vertical visibility in hundreds of feet:
9/00
9/01
vertical visibility less than 100 ft
vertical visibility 100 ft
DEWPOINT (7 O'CLOCK)
This is shown by 2 figures for units and tens of degrees Celsius. Thermometers are read to the
nearest 0.1 of a degree and then rounded up or down to the nearest whole figure with 0.5 always
allocated to the nearest whole odd number. Values are positive unless prefixed by a minus sign.
VISIBILITY (9 O'CLOCK OUTER POSITION)
The two figures are for use in showing visibility in metres or kilometres using the following range
system:
00 to 50
metres (x 100)
56 to 80
subtract 50 giving an answer in kilometres
81 to 89
subtract 80, multiply the result by 5 then add 30. This
gives an answer in kilometres
e.g. 85
85 − 80 = 5
x5
= 25
+ 30
= 55 km
Meteorology
23-5
Chapter 23
The Synoptic Chart
PRESENT WEATHER (9 O'CLOCK INNER POSITION)
Weather at the time of observation is shown by symbols only. Weather which has occurred during
the past hour, but not at the time of observation is also shown at this position.
23-6
Meteorology
The Synoptic Chart
Chapter 23
The above descriptions also apply for drizzle and snow symbols.
WEATHER IN THE PAST HOUR BUT NOT AT THE TIME OF OBSERVATION
The situation is provided by a square bracket around a precipitation symbol.
Meteorology
23-7
Chapter 23
The Synoptic Chart
SURFACE AIR TEMPERATURE OR DRY BULB TEMPERATURE
(11 O'CLOCK)
This is shown by 2 figures for units and tens of degrees Celsius. Thermometers are read to the
nearest 0.1 of a degree and then rounded up or down to the nearest whole figure with 0.5 always
allocated to the nearest whole odd number. Values are positive unless prefixed by a minus sign.
MEDIUM LEVEL CLOUD (12 O'CLOCK LOWER POSITION)
The cloud type appears by symbol.
The cloud amount in eighths is sometimes given below the cloud symbol followed by a slash and
then two figures indicating the height of the cloud base. These two figures have a range of
56 − 80. The cloud base is then given in thousands of feet by subtracting 50.
In this example we have 4/8 of thin As with a base of 12 000 ft above airfield level.
23-8
Meteorology
The Synoptic Chart
Chapter 23
HIGH LEVEL CLOUD (12 O'CLOCK UPPER POSITION)
The cloud type appears by symbol.
The cloud amount and cloud base are shown if there is no medium level cloud reported. Higher
cloud bases than the figure 80 (-50 = 30 000 ft) are catered for as follows.
Range of codes 81 − 89, subtract 80 and multiply the result by 5 and then add 30. This gives the
cloud base in thousands of feet.
In this example there is a 7/8 Cirrus, not increasing with a base of 40 000 ft above
airfield level.
Meteorology
23-9
Chapter 23
The Synoptic Chart
TOTAL CLOUD COVER (SHOWN IN THE CENTRE OF THE CIRCLE)
This is indicated in eighths of the total sky covered by cloud.
Sky clear
5/8ths
1/8th
6/8ths
2/8ths
7/8ths
3/8ths
8/8ths
4/8ths
Indicates obscured
sky— usually by fog,
smoke or sand/dust
SURFACE WIND
Shown by a straight line from the periphery of the circle. This line indicates the direction from
which the wind is blowing (090° (T) in the examples below). The speed is shown by the feathers
at the end of the line.
Calm
20 kt, further additions up to 45 kt
1 to 2 kt
50 kt
5 kt
60 kt
10 kt
65 kt, further additions as
necessary
15 kt
Note: The feathers on the wind arrows conform with Buys Ballot's Law. The feathers indicate the
low pressure side. Left in the Northern Hemisphere, right in the Southern Hemisphere.
23-10
Meteorology
INTRODUCTION
Charts for the middle and upper levels must now be considered. These vary in coverage from FL
100 to FL 630 depending upon the area covered. The layout, and symbology used on these
charts is similar to ones already taught in previous sections. Other symbology includes:
SYMBOLS FOR SIGNIFICANT WEATHER
Notes
1. In flight documentation for flights operating up to FL 100 this symbol refers to a squall
line.
2. The following information referring to the symbol should be included in the side of the
chart.
¾
Volcanic eruption
¾
Name of volcano
¾
Latitude and longitude
¾
Date and time of the first eruption
¾
Check SIGMET for volcanic ash
3. This symbol does not refer to icing due to precipitation coming into contact with an
aircraft at a very low temperature.
Meteorology
24-1
Chapter 24
Upper Air Charts
FRONTS AND CONVERGENCE ZONES AND OTHER SYMBOLS
Where the cold front, warm front, occlude front and quasi-stationary front symbols are not filled in
then the front is above the surface.
In the above diagram a cold front above the surface.
CLOUD ABBREVIATIONS
CI
Cirrus
AS
Altostratus
ST
Stratus
CC
Cirrocumulus
NS
Nimbostratus
CU
Cumulus
CS
Cirrostratus
SC
Stratocumulus
CB
Cumulonimbus
AC
Altocumulus
CLOUD AMOUNT
CLOUDS EXCEPT CB
SKC
FEW
SCT
BKN
OVC
24-2
Sky clear
Few
Scattered
Broken
Overcast
0
/8 (0 oktas)
/8 to 2/8 (1−2 oktas)
3
/8 to 4/8 (3−4 oktas)
5
/8 to 7/8 (5−7 oktas)
8
/8 (8 oktas)
1
Meteorology
Upper Air Charts
Chapter 24
CUMULONIMBUS ONLY
ISOL
OCNL
FREQ
EMBD
Individual CBs (isolated)
Well separated CBs (occasional)
CBs with little or nor separation (frequent)
Thunderstorm clouds contained in layer of other clouds
(embedded)
WEATHER ABBREVIATIONS
DZ
LOC
K
COT
WDSPR
SH
FZ
MAR
Drizzle
Locally
Thunderstorm
At the coast
Widespread
Showers
Freezing
Over the sea
GEN
LYR
BLW
SEV
General
Layer
Below
Severe
LINES AND SYMBOLS ON THE CHART
SPEED OF A FRONT IN KNOTS
15
SLOW
SPEED OF THE FRONT CAN BE DEPICTED IN WORDS
BOUNDARY OF AREA OF SIGNIFICANT WEATHER
BOUNDARY OF AREA OF CLEAR AIR TURBULENCE
THE CAT AREA MAY BE MARKED BY A NUMERAL INSIDE A SQUARE AND
A LEGEND DESCRIBING THE NUMBERED CAT AREA MAY BE ENTERED
IN A MARGIN
- - - 0° C: FL120 - - - ALTITUDE OF THE 0°C ISOTHERM IN FLIGHT LEVELS
Meteorology
24-3
Chapter 24
Upper Air Charts
SIGNIFICANT WEATHER CHART
The chart on the next page is an example of a high level chart issued by London. The chart
covers a considerable area of Europe, the Middle East, North Africa, and North America. These
charts are issued in advance of their valid times, which are 0000, 0600, 1200, and 1800 UTC.
The validity of this chart is 1200 UTC on 17 August. A Polar Stereographic or Mercator projection
is used for all middle and upper air significant weather charts.
Note:
Take great care when measuring direction on all small scale meteorological charts. Use a
square navigation protractor.
a.
b.
c.
d.
e.
f.
24-4
The bottom right hand corner of this chart gives a box which:
i.
Indicates the issuing station.
ii. The type of chart – Significant Weather.
iii. The depth of the atmosphere covered. In this case FL 250 - 630 this
is also given in hPa.
iv. The chart is a fixed time chart for 1200 UTC, 17 August.
v. The units used on the chart are Pressure Altitude (Hectofeet), knots,
and °C .
The bottom right box indicates that all heights are Flight Levels. Tropopause
heights are shown in boxes on the chart (Indicated by A on the chart). The
symbols and CB imply moderate or severe turbulence and icing.
The vertical distance at which phenomena are expected are indicated by flight
levels, top over base or top followed by base . 'XXX' means the phenomenon is
expected to continue above or below the vertical coverage of the chart (Indicated
by B on the chart).
The surface positions together with the direction and speed of movement of
pressure centres and fronts are denoted as shown on the chart. Where slow is
used this indicates movement of less than 5 kt (Indicated by C on the chart).
Dashed lines denote areas of CAT. These areas are numbered and are
associated with the decode box on the chart in the bottom right corner (Indicated
by D on the chart). (e.g. Area 4 Moderate turbulence FL 370 to FL 300).
On lower charts the 0°C Isotherm is also shown as a dotted line with the FL
indicated (e.g. - - - - - - - 0°C:FL130 - - - - - - - -).
Meteorology
Upper Air Charts
Meteorology
Chapter 24
24-5
Chapter 24
Upper Air Charts
UPPER WIND AND TEMPERATURE CHARTS
Issued in conjunction with the significant weather chart, these charts give spot winds from 700
hPa (FL100) up to 200 hPa (FL390). Spot values of wind and temperature are shown at regular
intervals of latitude and longitude. The temperatures given are assumed negative unless prefixed
by PS. The wind arrow symbology is exactly the same as that for the synoptic chart.
The chart on the next page is again issued by London and is for Upper Wind and Temperature.
Remember that the maximum wind is contained on the significant weather chart. The chart is for
FL 340 and has the same validity as the upper wind chart. At the bottom is the time of issue 1200
UTC on the 16 August.
24-6
Meteorology
Upper Air Charts
Meteorology
Chapter 24
24-7
Chapter 24
Upper Air Charts
AVERAGING WIND VELOCITIES
Apply common sense. For instance, if there is an east/west track with a wind velocity of
310°/20 kt to the north and 270°/20 kt to the south then the average wind is at 290°/20 kt.
Numerical averaging is the common sense way of approaching the problem.
Example: As well as taking spot winds and temperatures from specific points, winds, and
temperature might require averaging over a route.
Because of the time limitations of the Flight Planning examination the rule of KISS (keep
it simple stupid) applies.
Temperature
STEP 1 Along the route add up the temperatures and numerically average the
sum total.
Temperature 48°C
The above system is quite a simple way of arriving at the mean temperature. To
average the wind velocity over a route is not as simple.
STEP 1 Look at the wind directions involved at approximately 10° spacing.
80W
320/20
70W
250/35
60W
240/50 (average between the two velocities spanning the track)
50W
270/15
40W
020/65
30W
020/20
20W
210/70
10W
290/30 (average between the two velocities spanning the track)
0E/W 270/50
The winds are predominantly westerly. Ignore the two north-easterly
winds as they distort the figures.
Direction 265°
STEP 2 For the speed use the same principle as the direction. Give westerly
winds a + configuration and easterly winds a – configuration.
Speed 25 kt
Time may mean that you are not able to make these calculations. If not, try to come to a sensible
wind by inspection.
24-8
Meteorology
INTRODUCTION
Climatology is the long term study of the behaviour of the weather carried out by looking at the
average weather of the various areas of the Earth. Much of the information is open to discussion.
Remember, these are the ideals not the actual.
Climatology needs a basic knowledge of the location of countries, major cities, the Tropic of
Cancer (Northern Hemisphere), and the Tropic of Capricorn (Southern Hemisphere).
IDEAL GLOBAL CIRCULATION
Initially, assume that the Earth has a uniform surface, that it is not rotating, and it is not tilted.
ell
yC
d le
Ha
H
Surface Flow
Equator
H
The circulation of the air resembles a large scale sea breeze.
Meteorology
25-1
Chapter 25
Climatology-The World Climate
The Equator receives more insolation than the poles. This insolation causes the Equator to have
a higher temperature than at the poles.
The air at the surface is warmed, expands, and rises. This rising air creates a high pressure at
altitude over the Equator. This flow starts an outflow of air from the high pressure at height. A low
pressure system is formed at the surface which draws air in.
At the poles the low temperature causes a high pressure system at the surface and subsidence
occurs. The subsidence allows a low pressure system to form at height drawing air from the
Equator.
ROTATION OF THE EARTH
North Pole
30°
Equator
30°
Because of the Earth’s rotation, take into account the geostrophic force or Coriolis.
In the upper levels as the air travels towards the poles from the equator it comes under the
influence of Coriolis. In the:
Northern Hemisphere
Southern Hemisphere
The air is deflected to the right
The air is deflected to the left
This movement takes place at approximately 30° from the Equator.
The deflection means that the flow is eastwards in both hemispheres. The air is cooled as it
moves parallel to the Equator and eventually subsides to the surface. The falling of the air causes
a high pressure to form at the surface. Known as the Sub-Tropical High these are recognisable
on the average pressure charts.
At height, strong westerly winds form the sub-tropical jet stream.
25-2
Meteorology
Climatology-The World Climate
Chapter 25
IDEALISED PRESSURE ZONES
The Coriolis effect forms the first of three cells in the idealised circulation. This first cell is known
as the Hadley Cell.
Polar High
Polar
Cell
Polar Front
H
Horse 60° N
Latitudes
30° N
Doldrums
L
H
H
0°
L
Equatorial
Lows
30° S
Ferrel
Cell
L
H
Polar Easterlies
Su b p o lar L o w
Hadley
Cell
Westerlies
30°
Subtropical High
L
H
L
60°
NE Trade Winds
I n te
r tr o p ic
CZ
al Convergence Zone (IT
)
0°
SE Trade Winds
30°
Subtropical High
60° S
H
Westerlies
Subpolar Low
Polar Easterlies
60°
The sub-tropical high pressure system has an outflow of air to both the Equator and towards the
poles.
The flow of air from the poles and the flow from the sub-tropical high meet in the temperate
latitudes. Convergence occurs and air rises. A surface low pressure forms with a high pressure
area at height.
The three distinctive cells are:
¾
¾
¾
The Hadley Cell
The Mid-Latitude Cell (Ferrel Cell)
The Polar Cell
THE EARTH’S TILT
The Earth is tilted by 23° 27’. The sun travels to the Tropic of Cancer in the Northern hemisphere
summer and the Tropic of capricorn in the winter. As the sun moves across the Earth’s surface so
does the Equatorial Low pressure belt.
This general picture ignores certain features such as the irregular surface of the land and the
different characteristics of the land and sea surfaces.
These features do have a major influence on the climate and the weather.
When the pressure patterns are discussed later in the chapter it is apparent that the general
circulation discussed so far does in reality exist.
Meteorology
25-3
Chapter 25
Climatology-The World Climate
PRESSURE ZONES
EQUATORIAL LOW (TROUGH)
The Equatorial Low is an area of convergence at the surface created by the outflow of air in the
upper atmosphere. The surface winds travel in towards the Low pressure area. In the Northern
Hemisphere these winds are deflected to the right, and to the left in the Southern Hemisphere.
These are the Trade Winds:
¾
¾
In the Northern Hemisphere they are North Easterly
In the Southern hemisphere they are South Easterly
In Sea areas, occassionally there is an area known as the Doldrums, where the trade winds are
light and variable. Over land the winds can get up to 25 kt.
SUB-TROPICAL HIGHS
Created by the upper level air from the Equator being deflected by the geostrophic force.
Eventually the flow is near parallel to the parallels of latitude at 30°N/S. The sub-tropical highs
are seen in both hemispheres in summer and winter.
The source area for the Trade Winds.
TEMPERATE LOW
The area where the sub-tropical and polar airmasses meet in each hemisphere. An area of
convergence, the temperate low is defined by travelling depressions forming on the polar front.
These travelling depressions appear on the pressure chart with sufficient frequency for them to
appear permanent.
The circulation described is bounded by low temperatures at the poles and high temperatures at
the Equator. To maintain a temperature balance the air must move between the poles and the
Equator. At irregular periods North-South surges do occur which distort the climatological pattern.
The best example of this is “El Nino.”
POLAR HIGH
The polar high is apparent in both hemispheres. The Antarctic polar high is similar to the ideal
circulation and is a near constant feature. The Arctic polar high is not so permanent. The area is
surrounded by land and suffers from regular travelling depressions which remove the high
pressure system.
PREVAILING SURFACE WINDS
WESTERLY WINDS
The outflow of air on the polar side of each sub-tropical high is deflected by the geostrophic
forces. These forces create mainly westerly winds in both hemispheres. These winds are often
strong, especially in the Southern Hemisphere between 40°S and 60°S where they are known as
the “Roaring Forties.”
25-4
Meteorology
Climatology-The World Climate
Chapter 25
EASTERLY WINDS
The outflow of air from the polar high pressure regions results in the formation of easterly winds
at the surface in high latitudes. In reality the polar highs are less well defined than the idealised
circulation. The polar easterlies are therefore variable.
CLIMATIC ZONES
Because the zones are essentially the product of solar heating, the pressure zones change
latitude with the seasonal movement of the sun. These pressure zones in turn produce climatic
zones. The pressure zones are complex and depend upon the nature of the surface of the Earth
and the movement of the sun.
Because rising air cools, in the temperate and equatorial low pressure areas large amounts of
cloud and precipitation are found. At the poles and sub-tropical high belts subsiding air disperses
any cloud and dry areas occur.
EQUATORIAL CLIMATE (0° TO 10° LATITUDE)
This area is also known as humid tropical and occurs up to 10° either side of the Equator. Over
the sea areas, light winds, high temperature, and high humidity are apparent all year. Convective
activity prevails giving heavy showers and TS. The slack pressure gradient causes strong sea
breezes on coasts. The zone has two rainy seasons which occur at the equinoxes in March and
September when the sun crosses the Equator.
Neither rainy season is a distinct feature, the days are wetter than normal.
TROPICAL TRANSITION CLIMATE (10° TO 20° LATITUDE)
Also known as the Savannah. Zones occur in both hemispheres. The area has a marked wet and
dry season associated with the passage of the sun. In the area nearest the Equator there is a
possibility of two wet seasons. The edge of the area nearest the pole only has one wet season.
The amount of rain decreases as the latitude increases. In winter the area is one of dry trade
winds. In summer there are belts of equatorial trade winds. Temperatures are fairly high
throughout the year. Annual and diurnal temperature ranges increase with latitude.
ARID SUB-TROPICAL (20° TO 35° LATITUDE)
Also known as the Steppe. The edge of the equatorial low pressure area is not well defined. This
area is always under the influence of the sub-tropical high pressure belt. The subsiding air is
cloudless and hot. In the summer there are large diurnal and annual temperature ranges. This
area contains most of the Earth’s deserts:
¾
¾
Meteorology
In the Northern Hemisphere
In the Southern Hemisphere
Sahara, Arabia, Arizona
Kalahari, Australia
25-5
Chapter 25
Climatology-The World Climate
Trade winds flow and are consistent in direction. In the desert interior there is little or no rain.
Bordering the desert is the Steppe. An area of treeless plains with short rainy seasons. In the
Northern hemisphere this area is the region north of the deserts in winter, south of the deserts in
summer. Steppe regions include:
¾
¾
¾
Algeria
The Veldt of South Africa
Central and southern Russia
The upper winds are westerly sub-tropical jet streams.
MEDITERRANEAN CLIMATE (35° TO 40° LATITUDE)
A warm temperate transition zone, which exhibits the following:
In Winter
¾
¾
¾
¾
¾
Disturbed temperate climate
Travelling frontal depressions
Prevailing westerly winds
Cloud and precipitation
Cool and unsettled
In Summer
¾
¾
¾
¾
¾
Dry sub-tropical climate
Anticyclonic in nature
Hot fine sunny weather
Land and sea breezes on the coasts
Westerly upper winds
The areas of the world include California, the Mediterranean, Central Chile, and the cape area of
South Africa.
DISTURBED TEMPERATE (40° TO 65° LATITUDE)
The weather is controlled by travelling frontal depressions. Less frequently high pressure systems
may affect the area. The winds are normally westerly. There is no dry season. In winter the polar
front lows are more frequent. Winters are cold and in Western Europe wet. Gale force winds are
experienced at any time.
The areas of the world include Western Europe and New Zealand
25-6
Meteorology
Climatology-The World Climate
Chapter 25
POLAR CLIMATE (65° TO 90° LATITUDE)
Around the edge of the polar zone is the Tundra. The mean temperature of this area rises above
0°C for only a few months of the year. Subsoil temperatures remain below 0°C permanently,
giving the term perma frost. No trees are found in this area. The vegetation consists of grass,
lichens, and moss.
The area is subject to 24 hours darkness for 3 months in winter and 24 hours daylight for 3
months in summer. The area is generally anticyclonic which is occasionally replaced by travelling
depressions. The travelling depressions are more common in the Northern Hemisphere.
MODIFICATIONS TO THE IDEALISED CIRCULATION
The seasonal movement of the sun distorts the pressure zones. To allow for this movement
climatology takes two extremes, the months of January and July.
The Earth is assumed to have a uniform surface. In the standard pressure and temperature
variations for January and July the complications of large land and sea masses become
apparent.
GLOBAL TEMPERATURE DISTRIBUTION
All climate and weather, energy, and movement are caused by solar energy. The weather is
related to the temperature distribution over the Earth. In the ideal case the temperature
decreases from the Equator to the poles evenly. In the Southern Hemisphere this is nearly the
case and in the temperature distribution charts for January and July the isotherms nearly follow
the lines of latitude. In the Northen Hemisphere the distribution is distorted by the large land
masses.
MEAN SEA LEVEL TEMPERATURES — JANUARY
This is the Southern hemisphere summer.
The highest temperatures occur between 10°S to 20° over the land areas. At these latitudes the
sea temperatures are lower than land temperatures at the same latitude. For example, at 20°S
the temperature over the land is 25°C to 30°C, while over the sea it is 20°C to 25°C.
Meteorology
25-7
Chapter 25
Climatology-The World Climate
The coldest temperatures are found over the Northern Hemisphere land masses. Note the
extremes are found in Canada and Siberia. The isotherms are distorted by the land masses in
both hemispheres. In the Northern Hemisphere the relatively warm temperatures of the North
Atlantic and Pacific contrasts to the colder land temperatures.
MEAN SEA LEVEL TEMPERATURE — JULY
The Northern Hemisphere summer.
The highest temperatures are over the land between 20°N and 40°N. Sea temperatures are
slightly lower than the land temperatures. Note that the extremes occur over the land masses of
South America, Africa, and South East Asia.
Over the Southern hemisphere oceans there are no land masses to distort the isotherms and
they parallel the lines of latitude.
On both charts the highest isotherm value is emboldened. This marks the position of the heat
equator, or the equatorial low pressure area. This belt is known as the Inter Tropical
Convergence Zone (ITCZ). This is discussed in the next chapter.
SEASONAL VARIATIONS IN TEMPERATURE
In tropical regions the mean temperature only varies by about 5°C throughout the year. This is
more apparent over large ocean areas. The largest temperature variations are found over the
large land masses such as Northern USA and Siberia.
In the Southern Hemisphere the lack of large land masses mean little temperature variation
throughout the year.
The polar fronts are more apparent in the summer than the winter. In the North Atlantic the polar
front lies:
In Summer
In Winter
25-8
Newfoundland to the North of Scotland
Florida to South West England
Meteorology
Climatology-The World Climate
Chapter 25
UPPER AIR TEMPERATURE DISTRIBUTION
The upper air temperature is controlled by the surface temperature. Because the height of the
tropopause is higher over the Equator than the poles the 2°C lapse rate applies over a greater
atmospheric depth. The diagram below gives an illustration of the temperature deviation in the
upper atmosphere. The temperature at the tropical tropopause is likely between -75° to -80°C. At
the poles -55°C.
WORLD PRESSURE DISTRIBUTION
Temperature variations found across the world can link to the January and July Pressure charts.
MEAN SEA LEVEL PRESSURE — JANUARY
The high and low pressure areas are very apparent on the chart.
Meteorology
25-9
Chapter 25
Climatology-The World Climate
The simplified diagram on the previous page shows low and high pressures that relate to the
idealised circulation. In January the sun is overhead the Tropic of Capricorn in the Southern
Hemnisphere. The warm air over Australia, Africa, and South America creates surface low
pressure areas. These low pressure areas break up the sub-tropical high pressure belts between
20°S to 40°S. In the Northern Hemisphere the sub-tropical high is apparent over the oceans at
30°N. The land masses distorting the picture because of the well established cold anticyclones.
The Siberian high is the dominant feature of the Eurasian land mass.
There are two mean low pressure areas:
North Atlantic
North Pacific
The Icelandic low
The Aleutian low
Neither of these pressure areas is permanent. Travelling depressions are so common that they
show up as a permanent low pressure area on the chart.
MEAN SEA LEVEL PRESSURE — JULY
The sun is now overhead the Tropic of Cancer.
In the Southern Hemisphere the sub-tropical high is well established. The pressure system
moved to approximately 30°S. This picture is near the ideal pattern discussed earlier.
The sub-tropical high pressure areas in the Northen Hemisphere moved north to 35°N. These
areas are now more dominant than in January. The Siberian High is replaced by a low pressure
area which extends over the land masses of India and the Gulf States. The Monsoon Low, or
Baluchistan Low, dominates the area.
The low pressure areas in the North Atlantic and the North Pacific are now weaker and retreat
northwards as the high pressure systems move north.
25-10
Meteorology
Climatology-The World Climate
Chapter 25
UPPER WINDS
The temperature decreases in the troposphere from the tropics to the poles. The thermal wind
component in both hemispheres is therefore westerly. The mean circulation is also westerly.
The wind circulates around the upper air depressions at each pole.
Southern Hemisphere
Outside the tropics the winds follow the ideal circulation. The prevailing westerlies at
temperate latitudes increase with height.
Northern Hemisphere
The same applies, with westerly winds increasing with height.
The higher wind speeds associated with jet streams are transient in the temperate latitudes. The
sub-tropical jet streams are a normal feature of the meteorological chart.
MEAN UPPER WIND — JANUARY
80°
Westerly
Winds
55°
Polar Front Jet Stream (70 to 200 Knots)
Westerly Winds
40°
Sub-Tropical Jet Stream (70 to 200 Knots)
(Up to 300 Knots)
25°
Westerly Wind
10°
0°
Easterly Wind (Maximum 40 Knots)
20°
Westerly Wind
40°
45°
Sub-Tropical Jet Stream
(70 to 200 Knots)
Polar Front Jet Stream
55°
Westerly Wind
Westerly Wind
180°
0°
180°
The normal flow is westerly. South of the Equator is an easterly flow. This flow is never greater
than 40 kt, normally 15 to 25 kt.
The mean position of the sub-tropical jet stream is:
Northern Hemisphere
North Africa to Japan passing over the Persian Gulf. The highest wind speeds in the
world are found along the Chinese/Japanese Coast.
Southern Hemisphere
Approximately 40°S
The polar front jet streams do not appear on mean wind charts normally as they are a transient
feature. Strong wind speeds occur along the east coast of North America.
Meteorology
25-11
Chapter 25
Climatology-The World Climate
The strong winds over the coastal areas of the USA and China are caused by the strong
temperature gradient found between the cold polar air over the land and the warm tropical
maritime air over the sea.
MEAN UPPER WIND — JULY
The sub-tropical jet stream moves north in the Northern Hemisphere to 40°N to 45°N.
Temperature gradients are now weaker in this hemisphere and the mean speeds reduce.
In the Southern Hemisphere the sub-tropical jet moves north to 30°S. The highest speeds being
towards Australia and the South Pacific
Away from the Equator the winds are generally westerly. Easterly winds do affect the Equatorial
region with the possibility of an easterly jet stream over India up to 70 kt at 50 000 ft. The polar
front jet stream in the Northern Hemisphere is less evident although strong winds are
experienced over the eastern seaborad of the USA.
In the Southern Hemisphere the polar front jet is approximately 50°S.
80°
Westerly
Winds
65°
Polar Front Jet Stream (70 to 200 Knots)
45°
40°
Westerly Winds
Sub-Tropical Jet Stream (70 to 200 Knots)
Westerly Wind
20°
Easterly Wind (30 to 50 Knots)
10°
Westerly Wind
25°
Sub-Tropical Jet Stream
(70 to 200 Knots)
Polar Front Jet Stream
(70 to 200 Knots)
40°
Westerly Wind
55°
180°
0°
180°
Note the position of the easterly winds:
In January
In July
Between 10°N and 20°S
Between 20°N and 10°S
INTER TROPICAL CONVERGENCE ZONE (ITCZ)
The Equatorial low moves across the Earth’s surface with the movement of the sun. The
Equatorial low is fed with trade winds from the two sub-tropical high pressure belts and because
of these convergent winds is termed the ITCZ.
25-12
Meteorology
Climatology-The World Climate
Chapter 25
ITCZ — JANUARY
Maximum heating of the land mass is in the Southern Hemisphere.
The effect of heating the land mass moves the ITCZ well south of the Equator over the land
areas. Over the sea areas the ITCZ is just north or follows the line of the Equator.
ITCZ — JULY
The ITCZ is moved north of the Equator by the heating of the land masses.
The general position of the ITCZ is well north of the Equator. The most northerly position is over
China at 45°N. There is little travel over the sea areas, and over the Atlantic and the Pacific the
ITCZ lies between 10°N and 15°N.
Meteorology
25-13
Chapter 25
Climatology-The World Climate
STABILITY AND MOISTURE CONTENT OF THE ITCZ
The trade winds that flow into the ITCZ are from a relatively dry and stable area, originating from
the sub-tropical high pressure belt. The passage of the air over warmer seas towards the ITCZ
produces instability due to heating from below; this coupled with a rapid increase in moisture
content due to evaporation in to the air at lower levels means convective cloud formation.
ITCZ WEATHER
There are wide variations in the weather along the ITCZ. Over the land the ITCZ is often very
narrow and resemble a temperate latitude cold front. Over the sea the ITCZ varies between 30 to
300 nm wide. The cloud varies between fair weather CU to CB.
INTER TROPICAL FRONT (ITF/FIT)
Most of the Equatorial region is water. The converging airstreams in these areas are very similar
in both moisture content and temperature. This gives rise to lines of CU and CB. The approach is
the same whether from the north or the south.
On approach into the ITCZ the weather is:
¾
¾
¾
¾
Fair weather cumulus
Due to heating CU with great depth form. There is usually an inversion from between
3000 ft to 8000 ft.
CB form with the cloud tops possibly over 50 000 ft.
If there are stable layers at mid-levels then CU build up ceases and extensive
Stratiform layers can form.
The main aspect of the ITCZ is the potential for the warm moist air to produce heavy cloud and
heavy precipitation.
If the air stream has a continental track then the change in moisture content and temperature is
often quite marked. Normally, when the ITCZ travels over the land the term ITF/FIT is used.
25-14
Meteorology
Climatology-The World Climate
Chapter 25
LOW LEVEL WINDS
LOW LEVEL WINDS — JANUARY
Outside 40°S.40°N the winds are generally westerly. The greatest deviation is over the Northern
Hemisphere.
The Southern Hemisphere flow is similar to the ideal flow. At approximately 40°S the “Roaring
Forties” blow. Because the ITCZ is south of the Equator in places, the north east trade winds
cross the Equator. As they cross the Equator they are influenced by the geostrophic force in the
Southern Hemisphere and become the north west monsoon winds of the southern hemisphere.
Monsoon Winds
Monsoon is derived from the Arabic for season. The monsoon winds blow quite steadily
for long periods near the ITCZ. The monsoon winds are often the trade winds. The trade
winds are considered to exist up to 10 000 ft.
Outflow of air from the Siberian High moves over China and Japan. The winds become North
easterly and follow the chinese coast to the coast of Malaysia.
High pressure over the north west indian plain results in air flowing down the ganges valley. This
air meets with the North easterly monsoon from the Siberian High.
Meteorology
25-15
Chapter 25
Climatology-The World Climate
LOW LEVEL WINDS — JULY
The ITCZ moves over the Asian continent, as far as 45°N over China. The south east trade winds
become the south west monsoon winds as they cross the Equator.
Outside 40°S/40°N the winds are predominantly westerly.
25-16
Meteorology
Climatology-The World Climate
Chapter 25
CLIMATIC SUMMARY
Climate
Weather Summary
Polar climate
Over the arctic anti-cyclonic regions, including NE Canada and
the most northern Russia.
Warmest month always below 10°C.
Sporadic influences from travelling cyclones.
Cold temperate
climate
or
Moist mid-latitude
climates with cold
winters
Warm summer months, usually above 10° C, winter months
usually below – 3° C.
Subdivided into two regions:
Sub-arctic Canada, N. Sweden, Finland towards Siberia
Humid Continental Sweden, Eastern Europe, SE. Russia,
N Japan and NE USA.
Warm temperate
climate
or
Moist mid-latitude
climates with mild
winters.
The coldest month is below 18°C but never lower than –3°C,
distinct summer and winter seasons are present.
Subdivided into three groups:
Mediterranean climate In the Mediterranean area but
also in California, SW Australia, and SW South Africa.
East coast or humid subtropical climate China, S
Japan, SE USA, Argentina, SE South Africa, E Australia.
West coast or marine Western Europe, NW-coast USA,
SE Chile, New Zealand.
Arid (dry) climates
Minimal precipitation most of the year.
Divisions include:
Arid desert North Africa, The Middle East towards
Himalayas, the interior of Australia, from N Mexico into
SW USA, the west coast of South America, and Africa
outside the equator area.
Steppe Great plains in USA, Interior of Asia north of the
Himalayas, around the deserts in South America, Africa
and Australia.
Tropical moist
climates
Temperatures above 18° C year round, significant rainfall usually
more than 1500 mm.
Subdivisions include:
Tropical rain forest The Amazon lowland, the Far East
islands from Sumatra to New Guinea and the Congo river
basin in Africa.
Tropical monsoon The coasts of Southeast Asia, India
and NE South America.
Savannah climate Central America, south central and
eastern Africa, in parts of India, Southeast Asia and in N
Australia.
Meteorology
25-17
Chapter 25
25-18
Climatology-The World Climate
Meteorology
INTRODUCTION
This chapter looks at the prevailing winds of the world and the effect of ocean currents on the
climate.
EUROPE AND THE MEDITERRANEAN
MISTRAL
Meteorology
26-1
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
A cold wind in the winter and early spring. The wind blows down the Rhone Valley in the South of
France in to the Gulf of Lions. The wind is a combination of three factors:
¾
¾
¾
Katabatic effect
Ravine effect
The holding of a suitable pressure system over the Gulf of Genoa
The wind is Northerly, cold and of gale force. At certain times the winds can reach 70 to 80 kt.
As the wind blows over the sea it becomes unstable and CB may form.
BORA
A strong katabatic wind of up to 100 kt, that blows down the Balkan Plateau and Dalmation coast
in winter.
Bora
Over the Balkan Plateau the wind is dry and cloudless but is still strong and turbulent.
26-2
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
The wind is north-easterly and of gale force. Like the Mistral, as this cold wind blows over the
warm sea instability occurs producing CB.
The wind is enhanced by some ravine effect and the possibility of depressions which are
apparent at this time of year in the Adriatic.
ETESIAN WIND
A summer wind which blows from the north over Greece and the Aegean. A similar wind blows
over Turkey and is known as the Meltemi.
High
Etesian
or
Meltemi
Low
The wind is northerly and cool. The blowing of this wind can bring relief from the normal heat
wave conditions which are apparent in this region at this time of year. The wind regularly blows
between 10 to 30 kt, gusting to 40 kt at times.
The wind is caused by the meeting of the air from the Azores High and the Baluchistan Low.
Meteorology
26-3
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
GREGALE
A strong north easterly wind which blows over the Ionian Sea and the Mediterranean in the
second half of the year. The wind can reach gale force and last for 2 to 3 days. The prevailing
conditions are low cloud, rain, and poor visibility.
Gregale
The wind is not especially cold.
LEVANTER
A humid easterly wind which blows over Gibraltar when there is anticyclonic weather over Spain.
The air is generally moist after its sea track. It is not a strong wind but its passage is
characterised by the cap cloud that covers the Rock of Gibraltar.
26-4
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
High
Levanter
The wind can blow at any time of year but is more prevalent during June to October
VENDEVALE
A south westerly wind that affects the Straits of Gibraltar at the beginning and end of winter. The
wind brings heavy rain.
Vendevale
Meteorology
26-5
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
SIROCCO
A southerly wind that blows in the winter months. The wind is hot and dusty as it blows in
advance of travelling depressions moving from west to east in the Mediterranean.
Sirocco
Ghibli
Khamsin
Low
Low
Low
As the wind progresses over the sea it becomes hot and humid. The moistening of the air cools
and stabilises the wind. Eventually low stratus, drizzle, or advection fog are formed.
A similar wind blows over the Libyan desert and is called the Ghibli.
KHAMSIN
The Khamsin is similar to the Siroccco.
Originating over the desert the wind is supposed to blow for 50 days (the Arabic for 50 is Kham).
A southerly wind of late winter and Spring in Egypt occurring ahead of travelling depressions.
The wind is more persistent than the Sirocco because traveling depressions tend to slow down as
they reach the Eastern Mediterranean basin. It is hot and dry.
26-6
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
AFRICA
HABOOB
Hab is Arabic for blow. This wind occurs in the Sudan in the afternoons and evenings between
May and September when the ITCZ is to the North.
Haboob
Moist air flows in from the Indian Ocean at both low and upper levels and convection produces
large CB. Ahead of the CB the squally winds raise dust storms to great heights.
As it approaches the Haboob is associated with an increase in wind speed and reduction in
visibility.
The dust storm is followed by torrential rain and conditions begin to improve.
Meteorology
26-7
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
HARMATTAN
The Harmattan is a north or north easterly wind over West Africa and the Gulf of Guinea. The
wind is dependent on the position of the ITCZ.
In the Northern Hemisphere summer the ITCZ is well north of the Equator and the Gulf of Guinea
and West Africa are subject to the trade south westerly flow of the trade winds that cross the
Equator. The weather is typically equatorial.
In the Northen Hemisphere winter the ITCZ retreats over the Equator and West Africa is subject
to a hot, dry, dusty wind from the Sahara. It is known as the “Doctor” by Europeans because of its
dry characteristics rather than the humid tropical climate of the summer period.
Temperatures can reach 40°C with dew points as low as 7°C.
Dust carried by the wind can cause serious deteriorations in visibility up to 5000 ft.
SIMOON
The Simoon originates in the desert in the heat of the afternoon in Africa and the Middle East.
Simoon literally means poison, which sums up the characteristics of this wind.
26-8
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
Thermal Low
The thermal lows formed carry large amounts of sand and dust. A summer and autumn
phenomena which can last up to about 20 minutes.
ASIA
NORWESTER
Violent convective squalls which occur in Bengal/Assam. They are named after the direction from
which they come. Normally a summer phenomena.
The storms can occur as frequently as every 3 days.
Norwester
Meteorology
26-9
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
SHAMAL
A north to north westerly wind that blows over Iraq during the summer months. The wind is
persistent during the day carrying a large amount of sand and dust in its wake. Visibility in this
wind is very poor.
Baluchistan
Low
At night the visibility may improve but in strong Shamal conditions the visibility may remain
throughout the 24 hour period.
SUMATRAS
The Sumatras are strong squalls with violent CB. The winds blow at night during the south west
monsoon in the Malacca Straits between Sumatra and Malaysia.
The high ground in Sumatra and Malaysia allows a katabatic flow to start at night. As the cold air
flows over the warm sea convection brings large CB.
The Sumatras are characterised by the formation of arches over the Malacca Straits when the
anvils of adjacent CB meet.
26-10
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
THE INDIAN MONSOON
THE WINTER MONSOON
Starting from December to February, the monsoon is fully established over India between
January and February. High pressure is centred to the north west of the continent with an outflow
of air.
The Siberian High is the dominant
feature with North Westerly winds
flowing over China
The air is warm and dry so the weather is fine with little cloud and moderate to good visibility.
These conditions hold good to the lee of the landmasses over the sea. There is considerable
modification in other parts.
The winds flowing down the Ganges valley are turned to become the north east monsoon of the
Bay of Bengal. Due to the long sea passage over a warm sea a large amount of moisture is
picked up resulting in the south east coast of India and Sri Lanka experiencing considerable rain
with CU and CB giving TS. Over the low lying areas fog may form but this clears once the sun
rises.
Occasionally depressions originating in the Mediterranean penetrate across India and Pakistan.
The number and paths of these depressions vary considerably from year to year. They do seem
to depend on the intensity of the Siberian high.
In some parts of northwest India the winter rain is associated only with the passage of these
disturbances.
Meteorology
26-11
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
THE HOT SEASON
The hot season is the inter-monsoon period between March and June. It includes mainly light and
variable winds with scattered TS. The TS is associated with depressions from the west. These
storms tend to become more frequent as the season advances. The feature of this season is the
thermal lows which form in the north west of India leading to:
¾
¾
¾
Very high temperatures
Quite frequent sand and dust storms
Severe wind squalls associated with the above as with TS
Between March and May in the vicinity of Calcutta there are violent storms known as the
Norwester.
THE SUMMER MONSOON
The summer monsoon is from June to September. The monsoon winds reach India after a long
sea passage where ocean temperatures are 27°C.
The South Easterly Trade Winds
cross the Equator and become
South Westerly with a long
maritime track
Summer
The air is moist and unstable.
The instability and the nature of the land mass, especially near the coast leads to considerable
orographic and convectional rain. The heaviest rain is in East Bengal and Bangladesh during this
monsoon.
Places to the lee of the mountain masses have a lighter rainfall. The southwest monsoon is
periodic where there are a few days of strong winds and bad weather interspersed with short
periods of fine weather. A feature of the onset of the southwest monsoon of India is the sudden
way in which it is established and the regularity of its onset.
26-12
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
THE RETREATING MONSOON SEASON
The retreating monsoon season is from September to December. By the second half of
September the southwest monsoon is retreating south. The winds are generally light and variable
with TS at times. These are less severe than those of the southwest monsoon. In the north fine
weather is soon established.
The fine weather spreads gradually south until by December it covers the whole of the Indian
sub-continent.
SEASON OF MAXIMUM CYCLONE ACTIVITY
The Bay of Bengal is the most affected area where most storms move north towards the Ganges
valley. Associated with these storms is a wide band of cloud and rain which affects the coastal
areas of Madras. Tropical cyclones do occur in the Arabian Sea but are less frequent. They can
occur in the March to June period.
THE FAR EAST MONSOON
CHINA, JAPAN, SOUTH EAST ASIA, INDONESIA, AND MALAYSIA
Meteorology
26-13
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
THE WINTER MONSOON
The winter monsoon is also known as the north easterly monsoon. This winter monsoon is
normally fully established by mid-October and lasts until late March/early April. The air flowing out
from central Asia is very cold and dry. As this air flows towards the equator and over the South
China Sea it is warmed and hence picks up moisture. It is during this season that we get the
Crachin.
¾
¾
¾
Drizzle, low ST, mist, and fog between January and April
Forms in the South China Sea and in the coastal area between Cape Cambodia and
Shanghai
The Crachin is caused by the interaction between the tropical maritime and polar
maritime air circulating round the eastern side of the Asiatic anti cyclone
Further inland the cool and dry northeast monsoon is experienced in southern China, Burma and
Thailand. To the north the mountains of Japan create orographic instability producing rain and
snow.
As the air moves south of 20°N the surface warming increases the degree of instability and the
humidity of the air. Over Malaysia and to the northeast this causes development of CU and CB
with the resultant heavy showers and TS.
FROM APRIL TO MID — JUNE
The north east monsoon degenerates as the Siberian high collapses. The winds become
variable, however, in May there is a tendency for south or south westerly winds. Frontal
depressions frequently affect the north of the area.
In the south, because of the moist tropical air the weather is warm and humid with CU type
clouds. Associated with the cloud are showers and TS.
THE SUMMER MONSOON
The southwest monsoon is fully established in the Far East in June and lasts until August. Over
China and Japan this monsoon is fully established in July and August. The weather is hot and
humid with heavy rain and TS near and over the land. Over the sea where there is shelter from
the land the conditions are better. Periods of broken Cu with quiet weather alternating with
showery periods. Morning mist and fog may affect Japan. Singapore is affected by thundery
weather. Many early morning storms are due to a build up in the Straits of Malacca, or the
Sumatras.
SUMATRAS
Violent, thundery squalls where the CB have taken on a characteristic arched shape. They form
at night due to the katabatic wind flowing down the mountains of Sumatra and the hills of
Malaysia with the winds meeting over the sea. Convergent lifting occurs. By dawn these storms
reach their maximum development but clear as the sun warms the land and the katabatic flow
ceases.
During this season, the seas and coasts north of 15°N are potentially affected by typhoons with
the main activity period being between July and September.
26-14
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
FROM SEPTEMBER TO MID — OCTOBER
The southwest monsoon is receding and the north east monsoon develops as the Siberian high
develops. This period shows an increasing number of fair periods. These periods are interrupted
particularly in the north by the passage of active cold fronts which are usually narrow belts of
thundery rain and squalls from a northerly direction.
Towards the end of October there is usually a fairly abrupt change to the northeast winds. This is
the definite onset of the northeasterly monsoon.
NORTH AMERICA
BLIZZARD
A blizzard is comprised of strong to gale force winds that are accompanied by falling or drifting
snow that is whipped up by the strong surface wind. It is prevalent in Northern USA and Canada.
Siberia has a similar wind called the Buran.
CHINOOK
A warm dry wind that is also known as the “Snow Eater”. This foehn wind produces a rapid rise in
temperature on the lee side of the rocky mountains. The wind blows in Alberta and Colorado.
SOUTH AMERICA
PAMPERO
The Pampero is a strong cold wind that develops behind cold fronts blowing at latitudes around
40°S. At this latitude the weather is influenced by the passage of depressions and anti-cyclones
moving to the east.
Meteorology
26-15
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
Warm
Humid
Air
Cold
Dry Air
Warm humid air is drawn from the north ahead of a depression. The passage of the depression
then sees violent line squalls in association with the cold polar air flow from the south or west.
This wind is most frequent in summer but can flow any time of the year.
ZONDA
The South American equivalent of the Chinook. The wind blows off the lee slopes of the Andes.
AUSTRALIA
BRICKFIELDER
A summer wind which is hot dry and dusty and affects the areas of New South Wales and
Victoria.
26-16
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
SOUTHERLY BUSTER
The Southerly Buster is similar to the Pampero occurring at latitudes around 40°S.
The wind blows most frequently in summer between travelling summer anticyclones when cold
unstable polar air moves behind a cold front which trails well to the south. The contrast between
the cold air and hot summer air is marked. Active line squalls form with strong winds. Low cloud
and poor visibility.
OCEAN CURRENTS
In the diagram below, the sub tropical high pressure systems give rise to warm water currents on
the west side of oceans and to cold water currents on the east side of oceans.
In an area of persistent offshore winds an upswell of cold water can be developed from the ocean
beds. This increases the effect of the cold water current and decreases the effect of the warm
water current.
Meteorology
26-17
Chapter 26
Climatology-Prevailing Winds and Ocean Currents
COLD WATER COAST
Due to the low rate of evaporation from the nearby cold ocean the air has a low vapour content.
Little cloud or precipiation forms. The cold water coasts bound the desert regions of the world.
During the night, cooling can produce advection fog or low stratus which disperses once the sun
is up.
WARM WATER COAST
Over both land and sea the air is humid due to the rapid evaporation from the warm ocean.
Over the land by day and the sea at any time the temperature is relatively high which results in
CU forming thunderstorms.
At night the diurnal variation of the surface temperature can cause CU clouds to disperse or form
SC. These clouds redevelop once insolation starts again.
Over the sea by night the CU persist because of the relatively constant temperature. It is these
areas that are suitable for tropical revolving storms to form.
SUMMARY OF THE LOCAL WINDS OF THE WORLD
Wind
Location
Season
Brief Description
Bora
Dalmatian Coast of
Croatia
Winter
North Easterly
Cold
Strong gale force wind
Brickfielder
Australia
Summer
Northerly
Hot and Dusty
Blows from the interior
Chinook
North America
Anytime
Foehn wind blowing
over the Rockies
Etesian
Aegean Sea, Greece
Summer
Northerly
Fine and clear
Foehn
Alps
Anytime
Warm, dry stable wind
to the lee of mountains
Ghibli
Libya
Late summer
Southerly
Hot and damp when
over the sea
Blows ahead of a
depression
Gregale
Malta and environs
Winter
North Easterly
Gales and squalls
Persistent
Haboob
Egypt and Sudan
Anytime
Sandstorm ahead of
advancing
thunderstorms
Harmattan
West Africa
November to March
East North Easterly
Hot, dry and dusty
from the desert
26-18
Meteorology
Climatology-Prevailing Winds and Ocean Currents
Chapter 26
Wind
Location
Season
Brief Description
Khamsin
Egypt
Late summer
From the Sahara
Hot and dry
Levanter
Straits of Gibraltar
March to Summer
Easterly
Hot and damp
Light winds
Meltemi
Turkey
Summer
North Westerly to
North Easterly
Fine and clear
Mistral
Rhone Valley,
France
Anytime, but more
predominant in late
autumn to winter
Northerly
Cold, gale force wind
Often clear conditions
Pampero
Argentina
Winter
South Westerly
Gales with line squalls
Shamal
Persian Gulf
Summer
North Westerly
Hot and dusty
Cloudless, calm nights
Simoon
Palestine and Syria
Summer and autumn
Southerly to South
Easterly
Hot, dry and dusty
Sumatra
Malacca Straits
between Sumatra
and Malaysia
South West Monsoon
onset
Squally
Thundery and wet
Southerly Buster
Australia
Anytime, more
marked in January
and February
Squall line
Cool wind
Vendevale
Gibraltar and Eastern
Spain
Spring and autumn
South Westerly
Strong squally wind
Thunderstorms
Zonda
Argentina
Anytime
Foehn wind
Meteorology
26-19
Chapter 26
26-20
Climatology-Prevailing Winds and Ocean Currents
Meteorology
TROPICAL REVOLVING STORM (TRS)
The TRS is a very confined region of low pressure where the Isobars are tightly packed together.
TRSs mainly occur on the western side of an ocean during the summer and autumn period for
that hemisphere.
Typhoon
Hurricanes
231/2° N
Equator
July to
October
June-October
June, October
and November
Cyclone
231/2° S
January to
Tropical Cyclone March
January to
March
Note that there are no TRSs in the Southern Atlantic. This is probably because the ITCZ never
travels into the South Atlantic and one of the requisites of these storms is that they require
intense heating and low pressure. The water temperature is, therefore, too low. In the Atlantic
TRSs are called hurricanes, in the Indian Ocean they are called cyclones, and in the Western
Pacific they are known as typhoons.
The mechanism of the TRS is not fully understood, but they seem to breed in the vicinity of the
ITCZ. The ITCZ provides the convergence that provides high instability and high humidity.
The storms tend to follow an elliptical path, firstly moving westward. If the westward path is
maintained then the storm runs aground and peters out. If the storm moves in an elliptical path
then it turns toward the east and as it moves into higher latitudes loses its vigour.
It is rare for a TRS to form within 5° of the Equator where Coriolis is near to zero.
Meteorology
27-1
Chapter 27
Climatology-Tropical Revolving Storms and Tornadoes
CHARACTERISTICS
The TRS forms in defined locations between the ITCZ and the sub tropical high pressure belt.
The formation is in the trade wind belt where the weather is normally fine with fair weather CU.
Occasionally in this area a weak trough forms which moves slowly westward in the trade wind
drift. These are known as Easterly Waves.
The convection in the wave is normally checked by the Trade Wind inversion. If the heat and
humidity at low level are sufficiently high and the wind profile favourable, convection breaks
through the inversion. The sea temperature has to be above 26°C. During its motion eastward the
trough is amplified; its convection is intensified; Coriolis force starts a cyclonic airflow, and a
“comma” cloud can form.
27-2
Meteorology
Climatology-Tropical Revolving Storms and Tornadoes
Chapter 27
After this development the TRS can form.
The convergence in the trough encourages the development of bands of large CB and CU with
their associated precipitation.
As the trough grows, the convergence and convection become organised and the pressure at the
surface begins to fall rapidly.
Meteorology
27-3
Chapter 27
Climatology-Tropical Revolving Storms and Tornadoes
A deep depression is formed which is characterised by a central pressure between 900 to 960
hPa (870 hPa is the lowest recorded value). High winds develop between 50 to 100 kt with CB
and torrential rain.
Much of the energy obtained from the latent heat of condensation is released in the atmosphere
as the high humidity is lifted.
The structure of the cloud is still under investigation. However, it is known that the isobars are
roughly circular with the depression having a diameter of ⊄ 350 nm. A mid latitude depression
has a diameter of approximately 1000 nm and so the smaller diameter reflects a steep pressure
gradient. The winds are strong below 10 000 ft and tend to spiral inwards giving the highest
speeds 10 to 20 nm from the centre of the storm.
Above 25 000 ft the winds spiral outwards carrying with them an extensive cloud table. Other
outward spirals of lesser extent are found at medium and low levels. These can also form cloud
tables.
The storm has great vertical extent with the CB in excess of 40 000 ft. The centre of the storm is
always marked by “the eye” which is a roughly circular area with a radius between 10 to 20 nm.
The area is one of subsidence which gives light winds and broken clouds.
27-4
Meteorology
Climatology-Tropical Revolving Storms and Tornadoes
Chapter 27
A wall of CU and CB surrounds the eye; these are formed in patterns which mark the spiralling
nature of the wind.
Stratiform tables appear out of the sides of the storm forming cloud tables. At low level they
consist of SC, at medium level they consist of AC or AS, and at high level they consist of CI or
CS.
Heavy showers accompanied by TS and severe squalls accompany the main wall of cloud.
The most severe weather is just outside the ring of the strongest surface winds which exist just
outside the eye. Satellite imagery can clearly depict the eye and the extent of the cloud.
The storm moves at approximately 15 kt. The speed changes frequently, slowing down as the
path or movement curves. The TRS can then accelerate as it passes 30° latitude where speed of
movement is up to 50 kt.
At lower or higher latitudes cold air is pulled into the system. The TRS develops into a very active
tropical depression. Over Western Europe these depressions can bring Hurricane force winds
with the associated weather of a depression.
If the moisture content of the storm is cut off then the storm dies out. This normally happens when
the storm travels over land.
The warning of the approach of the TRS is now done by satellite. These predictions are not totally
accurate, as the storm tends to move in an erratic manner.
Meteorology
27-5
Chapter 27
Climatology-Tropical Revolving Storms and Tornadoes
VISUAL INDICATIONS OF THE ADVANCE OF THE TRS
Surface Pressure
The diurnal variation in pressure in the tropics is suppressed. The pressure has a
tendency to fall.
Ocean Swell
At coastal sites an abnormally heavy swell can be seen. This is a result of the strong
winds that spread out from the centre of the disturbance.
Cloud
Extensive tables of CI can be detected up to 600 nm from the storm.
TORNADO
The term is applied to disturbances that are also known as whirlwinds. They are common in the
USA and Australia. Even though the Australian continent can have up to 150 disturbances a year
they are rarely reported, as they are much less severe than the storms in the USA.
The storm consists of a violent circular whirlpool of air shaped like a funnel between
100 to 1000 m in diameter. It is when the funnel reaches the surface that the storm becomes
destructive.
The extremely low central pressure makes the Tornado the most destructive storm known.
27-6
Meteorology
Climatology-Tropical Revolving Storms and Tornadoes
Chapter 27
The storm has limited dimensions and is difficult to assess accurately. However, the following is
typical.
¾
¾
¾
¾
¾
Wind speeds can range between 100 to 300 kt.
The Tornado has a twisting appearance due to the strong winds.
The pressure can change by 100 hPa in as little as 50 m.
The advance is between 15 to 20 kt.
Like a TS the Tornado lasts approximately 2 hours.
The Tornado forms in association with a marked trough of low pressure along which there is
marked instability. These troughs are:
¾
¾
¾
Linked to a frontal depression
A single cold frontal trough
A non-frontal trough
These troughs are generated when cold dry air from the western plateau overrides the tropical
maritime air. Instability is generated and this allows the trough to form overland.
If the Tornado forms over the sea it is known as a waterspout. This storm is much less violent and
lasts in the region of 20 minutes.
Meteorology
27-7
Chapter 27
Climatology-Tropical Revolving Storms and Tornadoes
TROPICAL REVOLVING STORM AREAS
Area
Formation
West Indies
5 to 10 hurricanes per year
The storms originate in or east of the Caribbean. Movement is
then westerly or northwesterly. Some affect the USA. Most
curve across the islands in the Caribbean or Florida passing
into the North Atlantic.
June to October
West and Central Pacific
Tropical Cyclones which form in the area of New and Old
Caledonia
January to March
Western North Pacific
and the China Sea
Typhoons which affect the Philippines, Hong Kong, and Taiwan
July to October
Bay of Bengal
Cyclones occurring in advance of the SW Monsoon in June and
during the retreat of the monsoon in October and November.
Arabian Sea
Cyclones form over the sea to the east of Oman to Bombay.
Associated with the ITCZ and the SW monsoon.
Times are the same as those for the Bay of Bengal.
South Indian Ocean
Tropical Cyclones in the Madagascar area
December to April
North West Australian
Coast
Tropical Cyclones, or “Willy Willys”.
Be careful with this second name, as the Willy Willys are really
an inland dust storm.
These storms occur NW of Darwin but originate in the Timor
Sea.
These storms can flow down the Coral Sea to Brisbane.
Occur between January to March
West African Tornado
Occur in the Gulf of Guinea and are severe TS.
These are not technically TRS but an extensive line squall
which affects the area twice a year.
March to May
October to November
27-8
Meteorology
EUROPE
The area lies in the same climatic zone as the North Atlantic and is considered a disturbed
temperate.
In Europe, the flow of weather is determined by the travelling depressions from the Atlantic. In
winter a dominant Siberian high can make the flow change. Norway is the exception where the
coastal mountains, which run north/south, cause a block to east-west flow.
The changes in temperature and weather conditions from summer to winter are less extreme than
the larger continents of Asia and North America.
NORTH WEST EUROPE
The climate is affected by the prevailing south westerly winds which transport warm air from the
North Atlantic drift to the land. The absence of any major topographical barriers allows the
maritime influence to extend deep into Europe. As depressions move into the land mass they
tend to dry out so the rainfall in the east of Europe is only about half that in the west.
The position of the polar front over the North Atlantic has a strong influence over European
weather. Depressions travelling east along it progress well into the continent; especially because
there are few mountains to oppose their progress. The only major topographical barrier is the
Alps which impedes the progress of cold fronts. These fronts slow down and cause widespread
cloud and rain. The final movement of easterly moving depressions is often dictated by the
position of the Siberian High in winter.
The Siberian High can become a dominant feature on the European weather map during the
winter months causing depressions to track around it.
In summer, the low pressure over Siberia and Asia is less dominant and the weaker and less
frequent depressions continue without deflection and follow the line of the polar front.
Meteorology
28-1
Chapter 28
Climatology-Regional Climatology
TEMPERATURE DISTRIBUTION
JANUARY
The isotherms run north/south indicating the temperature contrast between the mild waters of the
North Atlantic drift and the colder continent. The temperature gradient is much shallower than that
found on the east coast of the USA where the contrast is not as significant except that in
Scandinavia where the Norwegian mountains separate the mild ocean from the severe winter
temperatures of Siberia.
JULY
The isotherms conform to the lines of latitude and there is little contrast between the land and
water temperatures.
PRESSURE DISTRIBUTION
JANUARY
The general pressure distribution is as follows:
¾
¾
¾
28-2
Icelandic Low to the north approximately 1000 hPa.
Azores high (Sub-tropical high) to the south approximately 1020 hPa at 30°N.
The Siberian high to the east normally around 1035 hPa.
Meteorology
Climatology-Regional Climatology
Chapter 28
The ridge of the Azores high extends eastward over the cold lands of southern Europe and the
Icelandic low deepens. The Siberian high is intensified by the snow covered terrain of
Scandinavia and Eurasia and is the other major influence
JULY
The Icelandic low intensifies in pressure to 1010 hPa and the Azores high which moves north to
35°N is 1025 hPa.
The Siberian high is now replaced by the Baluchistan or monsoon low of India.
The Azores high has deepened and moved north and the Icelandic low has weakened and
moved north.
UPPER WINDS
JANUARY
The upper winds are westerly, normally 40 to 60 kt with frequent jet streams reaching 150 kt
associated with fronts. The jet stream direction is variable because of their positioning in relation
to the travelling depressions.
JULY
The upper winds weaken but are still westerly at 20 to 40 kt. Jet stream speeds are decreased
due to the weaker temperature gradients found in the summer period. Speeds of 100 to 150 kt
are seen.
SURFACE WINDS
JANUARY
The prevailing winds are from the south west. Winds from the east can persist for several days or
even longer when the Siberian high becomes well established over Scandinavia.
JULY
The prevailing winds are still from the west but are weaker.
Meteorology
28-3
Chapter 28
Climatology-Regional Climatology
HEIGHT OF TROPOPAUSE AND 0°C ISOTHERM
JANUARY
Tropopause
0°C Isotherm
35 000 ft
2000 ft
Tropopause
0°C Isotherm
40 000 ft
12 000 ft
JULY
CLOUD
In winter, like the North Atlantic region, the average cloud cover is 6/8. Cloud types are those
associated with frontal depressions and their respective warm and cold air masses.
In summer the cloud cover reduces slightly to an average of 5/8. Frontal depressions travel the
area less frequently and this small reduction in cloud cover is due to the high incidence of thermal
lows over the continent.
ICING
WINTER
The 0°C is low, often at the surface especially in central and eastern Europe. Conditions are
therefore favourable for icing in the extensive cloud of the travelling depressions. High ground in
the region can cause the icing to become severe in warm fronts or the convective clouds which
form in the unstable polar air.
SUMMER
The 0°C isotherm rises and the incidence of icing is reduced. With the travelling depressions that
travel across Europe icing can still be a problem at times during the summer months.
PRECIPITATION
The annual rainfall in the west is about double that in the east because of the drying out of the air
as it travels east. Normal rainfall in the west is 1000 mm against 500 mm in the east.
The western coastal parts of the region have the heaviest rainfall in winter. Elsewhere, the
wettest period is late summer and the driest period late winter or early spring.
Precipitation is liable to be snow in winter particularly in the east and south east where the ground
is occasionally snow covered for long periods.
VISIBILITY
WINTER
Greatest problem in Europe is poor visibility due to the high frequency of fog and very low cloud.
Both occur very readily in the maritime air masses and little cooling being required to produce
condensation.
In anticyclonic conditions fog may become widespread and dense, aggravated further by
industrial smoke in the Eastern European states.
28-4
Meteorology
Climatology-Regional Climatology
Chapter 28
SIGNIFICANT WEATHER
JANUARY
Frontal weather associated with the depressions travelling in from the west. When the pressure is
high then the visibility is often severely reduced by radiation fog.
Frost and severe wintry weather are frequently associated with an easterly flow from the
dominating Siberian high.
Advection fog is occasionally expected during periods of thaw and in coastal areas in spring and
early summer when the sea temperatures are at their lowest.
JULY
Some frontal depressions can still be expected though they are fewer in number. Less vigorous
than in the winter period the depressions still bring typical frontal weather but on a reduced scale.
SPECIAL FEATURES OF EUROPE
WINTER
If a trailing cold front is held up by the Alps in the south of France it can produce a belt of rain and
cloud on the northern slopes. However, more important is the possibility of waves forming on the
front which develop into vigorous secondary depressions which can move rapidly north east with
their associated weather.
Lee side orographic depressions can be formed in Northern Italy.
Occasionally, lows form in the Danube basin, in the south east of the region, due to the incursion
of warm air from the Mediterranean. These low pressure areas give rise to extensive low cloud
which can extend as far as eastern England. Associated precipitation, which may fall as snow, is
frequently heavy.
A low pressure over Scandinavia can bring Arctic air to the west of the region which again brings
snow.
SUMMER
Occasionally, large-scale thermal depressions form over the continent and these lows give rain
and thunder with extensive masses of cloud. The thermal low is most evident over the continental
areas of France and Spain.
Meteorology
28-5
Chapter 28
Climatology-Regional Climatology
MEDITERRANEAN
An area with a transitional climatic zone:
¾
¾
To the north is the disturbed temperate climate of Europe.
To the south are the arid sub-tropical regions of North Africa.
The weather in the Mediterranean is noted for its marked seasonal variations. The Mediterranean
is a sea surrounded by land. Remember that water warms up and cools down much slower than
land surfaces.
TEMPERATURE DISTRIBUTION
JANUARY
The sea is relatively warm but is surrounded by cold land.
JULY
The sea is relatively cool surrounded by warm land.
PRESSURE DISTRIBUTION
JANUARY
The sub-tropical high moves south and the disturbed temperate weather of northwest Europe
penetrates to the Mediterranean.
The water is warm compared to the land which leads to low pressure over the sea.
Incursions of cold air over the warm sea, in the western basin of the Mediterranean, help to
create, or enhance, the depressions that reach that area from the Atlantic.
Depressions can enter the Mediterranean via:
¾
¾
¾
The Carcassonne Gap which is between the Pyrenees and the Masif Centrale.
The Straits of Gibraltar.
From the orographic or lee depressions that form over the Gulf of Lyons, Gulf of
Genoa, and the northern Adriatic as a cold front advances from the north or with a
broad northerly airstream over Europe.
Saharan depressions in the lee of the Atlas Mountains travel from the western end of the
Mediterranean to the eastern basin where they:
¾
¾
¾
28-6
Slow down
Are sometimes regenerated by polar continental air from Russia and the Balkans
Sometimes continue to the Arabian Gulf
Meteorology
Climatology-Regional Climatology
Chapter 28
In winter, a depression passes through the Mediterranean in approximately 10 days. The warm
fronts associated with these depressions are not very active but the cold fronts can be quite
vigorous.
Depressions following a path close to the northern shores of the Mediterranean cause Italy and
the Balkans to have similar weather to that experienced over the UK. Where depressions follow
the southern coast, there is less cloud and precipitation as the air in the warm sector is from the
Sahara and thus very dry.
Ahead of the warm front the surface wind is southerly or south easterly and is often strong
enough to lift sand and dust off the desert. The obvious result is sand and dust storms causing
hot dusty winds over the Mediterranean (e.g. Sirocco, Ghibli and Khamsin).
JULY
The sub-tropical high-pressure belt moves north and now the sea is colder than the surrounding
land.
The Azores high extends over the area giving fine or fair weather. Occasionally, the north west
experiences a depression.
UPPER WINDS
JANUARY
The marked contrast in temperature between cold air from Siberia and warm air from North Africa
leads to a steep temperature gradient over the North African coast at the eastern end of the
Mediterranean.
This sub-tropical jet stream reaches speeds of 100 kt over Cairo and is about 80 kt over Cyprus.
At the western end of the Mediterranean the winds are westerly with a mean speed of 40 kt.
JULY
The upper winds are westerly at approximately 30 kt, due to the sub-tropical jet stream moving
north and reducing in speed.
Meteorology
28-7
Chapter 28
Climatology-Regional Climatology
SURFACE WINDS
JANUARY
At the eastern end of the Mediterranean, the surface wind is generally westerly to north westerly
but it can be variable. The wind speeds are moderate but can be increased to gale force when
depressions are reinvigorated by cold air from the Siberian high.
In the western basin the winds are moderate westerly to north westerly, but can be gale force
when associated with depressions. Both Mistral and the Bora winds occur.
JULY
The surface winds are predominantly from the north, such as the Etesian. Local sea breezes are
also evident during the day.
SIGNIFICANT WEATHER
JANUARY
The most noticeable features are the winter depressions with their attendant unstable squally
weather. Vigorous cold fronts on the depressions have attendant CU and CB with strong winds
and heavy rainfall. The visibility can deteriorate significantly especially when a Sirocco or
Khamsin is blowing from the south in advance of the depression.
JULY
The pressure is generally high which means warm cloudless conditions. Occasional TS can
generate near high ground.
NORTH ATLANTIC AND NORTH AMERICA
TEMPERATURE DISTRIBUTION OVER THE NORTH ATLANTIC
The temperature is regulated by both warm and cold water currents.
GULF STREAM
The warm water Gulf Stream from the Caribbean flows up the eastern seaboard of the USA. It
then turns east around the sub tropical high pressure zone and then divides into two distinct
currents. One element, the North Atlantic drift, fetches up against north west Europe and
Scandinavia. It is this current that keeps the coast of Norway, north of the Arctic Circle, ice free
throughout the year.
The second element flows eastward and eventually turns south around the east side of the subtropical high pressure zone (The Azores High).
28-8
Meteorology
Climatology-Regional Climatology
Chapter 28
CANARIES CURRENT
The cold water current emanates from the more northerly latitudes towards the northwest coast of
Africa. Hence the typical cold water coast of Morocco with a tendency for fog formation over the
Canary Islands and North West coast of Africa.
THE LABRADOR CURRENT
The Labrador Current is the cold water current emanating from high latitudes and flowing south
between Labrador and Greenland. When air, warmed by passing over the warm waters of the
Gulf Stream moves north over this current, advection fog forms. This is a well known feature of
the area known as the Grand Banks off the east coast of Newfoundland. The fog forms frequently
between May and August and can persist for several days at a time.
TEMPERATURE DISTRIBUTION OVER NORTH AMERICA
Land masses heat up and cool down comparatively rapidly. This is certainly noticeable with the
large North American landmass.
JANUARY
The land cools down rapidly, and when looking at a chart of isotherms at surface level you can
see they are tightly packed over the Eastern seaboard. This distribution occurs because of the
considerable temperature difference between the cold land mass and the warm waters of the Gulf
Stream. The steep temperature gradient is typical of the western sides of oceans in winter. The
temperature gradients form because in low latitudes the ocean currents circulate around the subtropical high pressure areas and the flow of warm water from equatorial regions is on the west
side of oceans.
This steep temperature gradient over the eastern seaboard produces a strong thermal wind
component from the south west. Steep temperature gradients and the accompanying strong
thermal wind component form in winter whenever a cold land mass is adjacent to a warm ocean
current.
JULY
The North American landmass is warm. The steeper temperature gradient is now to be found on
the west coast. The clockwise circulation of ocean currents around the North Pacific sub-tropical
high creates a cold water current flowing south from the Aleutian Islands. This results in a cold
water current off the coast of California and the formation of advection fog when the warm moist
air from the Pacific drifts over the cold Californian current. This is very prevalent in the region of
San Francisco.
Meteorology
28-9
Chapter 28
Climatology-Regional Climatology
PRESSURE DISTRIBUTION OVER THE NORTH ATLANTIC
JANUARY
The general pressure distribution is shown below:
¾
¾
¾
¾
Icelandic Low to the north 1000 hPa.
Azores high (Sub-tropical high) to the south 1020 hPa at 30°N.
High pressure to the west over the USA.
The Siberian high well to the east, 1035 hPa.
A large number of depressions pass over Iceland in winter creating the mean Icelandic low which
dominates the temperate latitudes. Families of travelling depressions move eastward. The large
landmass of North America allows cold polar air to move well south before meeting warm tropical
air from the Azores high around Florida and Bermuda.
Depressions form and run along a line roughly from Florida to south west England. These
travelling depressions are interspersed with ridges of high pressure.
Polar air depressions (Polar Lows) can form in the polar air as it moves into an area of the North
Atlantic to the North West of the UK. The general movement of these depressions is west to east.
JULY
¾
¾
28-10
The Azores high intensifies and moves to 35°N 1030 hPa.
The Icelandic low is reduced in size with a pressure rise to 1010 hPa.
Meteorology
Climatology-Regional Climatology
Chapter 28
The Icelandic Low is less of a dominant feature. The two continental high-pressure zones from
winter are replaced by low-pressure with the monsoon low of India being the predominant feature.
The Polar Front over the North Atlantic, along which we get the travelling depressions changes
position to run from Newfoundland to north of Scotland. Ridges of high pressure and anticyclones
last longer as the contrast in temperature between polar air and tropical air masses is reduced.
Fronts have less marked features and higher sea temperatures reduce the incidence of polar air
depressions.
GENERAL
Depressions that form on the polar front are more frequent in winter with 12 to 14 depressions
travelling per month. In summer, there are fewer depressions and they are much less vigorous:
¾
¾
To the north of the area 6 to 8 per month.
To the south of the area 1 to 2 per month.
PRESSURE DISTRIBUTION OVER NORTH AMERICA
JANUARY
Because of the cold land mass, the area becomes a centre for high pressure.
JULY
The heated land mass now becomes a centre of low pressure Upper Winds.
UPPER WINDS
JANUARY
The predominant feature is the south west sub-tropical jet stream formed by the large
temperature difference between the cold land and the warm sea.
Strong upper westerly winds prevail in the mid-latitudes normally in conjunction with travelling
depressions on the polar front. The wind direction varies from south west to north west and is
often of jet stream proportions, the speed increasing with height to an average 50 to 60 kt. Jet
stream speeds are between 100 to 200 kt.
JULY
Upper winds are still westerly but the speed decreases to between 40 and 50 kt. Jet streams
become less frequent, however the speeds are still between 100 to 200 kt.
LOW LEVEL WINDS
Over the ocean, the winds are westerly of a moderate speed, stronger in winter than summer.
The winds circulate anti-clockwise around the depressions so that to the north of a depression
exists an easterly flow, whilst to the south of a depression exists a westerly flow.
The north east trade winds blow in the southern part of the area.
Meteorology
28-11
Chapter 28
Climatology-Regional Climatology
OVER NORTH AMERICA
In winter the winds blow from the north. In summer, the winds blow from the south, except in
Canada where they blow from west to north.
TROPOPAUSE AND 0°C ISOTHERM
JANUARY
Tropopause
0°C Isotherm
56 000 ft in the south
30 000 ft in the north
10 000 ft in the south
Close to the surface in the north
ICING
The 0°C isotherm is low and on the surface on the eastern seaboard of the USA swinging
north to lie to the north of the UK. Conditions are favourable for icing in frontal clouds and
the CU and CB found in the polar air.
PRECIPITATION
Widespread and continuous ahead of warm fronts, showery at, and behind the cold
fronts.
The stable conditions found in the warm sector usually give drizzle. Snow can reach the surface
in the north and north west of the area when the surface temperatures become less than 4°C.
JULY
Tropopause
0°C Isotherm
55 000 ft in the south
35 000 ft in the north
15 000 ft in the south
10 000 ft in the north
ICING
The 0°C isotherm is higher so the incidence of icing is less. It still may present a major
problem.
CLOUD
In winter, an average of 6/8 cover with the cloud types varying dependent on the air mass and
frontal system.
Frontal
Frontal clouds are extensive both horizontally and vertically. They can sometimes extend
from the surface to the tropopause.
Polar Air
The convective cloud behind cold fronts is usually scattered, but often extensive in active
polar air depressions.
28-12
Meteorology
Climatology-Regional Climatology
Chapter 28
Tropical Air
In the warm sector of frontal depressions, widespread SC exists with tops not above
4000 ft. In anticyclones there is low SC over both sea and land.
In summer, the basic cover remains 6/8 due to the extensive SC in the tropical air.
SIGNIFICANT WEATHER
NORTH AMERICA
Winter
In winter, depressions moving from west to east produce most of the weather. Gale force winds
can produce blizzard conditions with minimal visibility. Precipitation, often in the form of snow,
accompanies the depressions. As spring arrives then the Chinook is a feature to the lee side of
the Rocky Mountains.
Summer
TS can build up over the mountains and also form when moist air from the Gulf of Mexico is over
ridden by cold dry air from the high mountain plateaus. The instability that results encourages the
formation of tornadoes in the mid-west especially in spring and early summer.
NORTH ATLANTIC
Over the ocean, the season for tropical revolving storms in the low latitudes is June to October.
About 3 to 5 hurricanes occur per year. These storms form in the low latitudes near the ITCZ
initially moving west then turning north and finally curving to the north east. If the TRSs reach the
higher latitudes they take on the characteristics of a severe temperate latitude depression. These
often reach northwest Europe bringing the wet windy conditions normally associated with a
travelling low in winter.
Advection fog occurs between May and August on the east coast of Canada and in the south
west approaches to the English Channel during spring and early summer
AFRICA
With most of the continent lying within the tropics there is no defined winter or summer period.
The most important aspect of the weather is the ITCZ and its seasonal movement. Because of
this movement there are clearly defined wet and dry seasons over the continent.
The northern area borders the Mediterranean and experiences the weather and temperature
changes of that zone.
The extreme south is outside the Tropic of Capricorn so can also be said to experience
Mediterranean style weather.
In January the land mass to the south of the equator receives the greatest amount of heat and
therefore has the higher temperatures. In July the thermal equator lies to the north.
PRESSURE DISTRIBUTION
The most distinctive feature is the ITF/FIT, which is directly influenced by the sun, which, in turn,
creates a low pressure convergence zone by heating up the land mass.
Meteorology
28-13
Chapter 28
Climatology-Regional Climatology
Over the adjacent oceans the ITCZ is less marked as the water does not respond as readily to
the sun’s heating.
JANUARY
ITF 5°N in West Africa
20°S in South Africa
Travelling depressions affect the Mediterranean.
JULY
ITF 20°N
Travelling depressions affect the Cape of Good Hope.
UPPER WINDS
JANUARY
To the north of the area, the winds are westerly with speeds up to 50 kt. These decrease to 10 to
20 kt in the lower latitudes. In the equatorial regions these winds become easterly at 10 to 20 kt.
Once in the southern hemisphere the winds increase from the west.
28-14
Meteorology
Climatology-Regional Climatology
Chapter 28
JULY
The upper winds are westerly in the higher latitudes both north and south of the equator. In the
lower latitudes the wind is easterly with speeds up to 60+ kt over the Guinea coast at the 200 hPa
level.
HEIGHT OF THE TROPOPAUSE AND 0°C ISOTHERM
JANUARY
Tropopause
0°C Isotherm
Equator
Higher Latitudes
Equator
Higher Latitudes
56 000 ft
50 000 ft
18 000 ft
12 000 ft
Equator
Higher Latitudes
Equator
Higher Latitudes
55 000 ft
50 000 ft
18 000 ft
12 000 ft
JULY
Tropopause
0°C Isotherm
SURFACE WINDS
JANUARY
Over the adjacent oceans, the trade winds blow from the north east and south east. In the Gulf of
Guinea the trade winds are deflected by Coriolis and so blow from the south west.
West Africa
The Harmattan blows from the north east as a hot dusty dry wind from the Sahara. The
resultant visibility is poor because of dust haze.
North African Coast
The winds are generally from the west.
East Africa
This region is affected by the trade winds.
South Africa
The winds are from the south west having circulated around the southern hemisphere
sub-tropical high pressure area in the south Atlantic.
Meteorology
28-15
Chapter 28
Climatology-Regional Climatology
JULY
North Africa
Northerly winds from the Mediterranean.
West Africa
The ITF has moved north and the West African monsoon from the south west brings
warm humid air in from the Gulf of Guinea.
Sudan
The warm humid air is drawn in from the Indian Ocean. This is the season for frequent
Haboobs.
South of the Equator
The south east trades blow in towards the equator.
South Africa (Cape Province)
The winds are often from the west with travelling depressions providing stormy
conditions.
SIGNIFICANT WEATHER
Over the continent in both January and July, the weather is occasionally severe in the vicinity of
the ITF which lies across some part of Africa throughout the year. Typical ITF weather is TS with
CB cloud extending to the tropical tropopause with the attendant rain and squally winds.
Long and short rains occur annually where there is a double passage of the ITF, usually at
locations close to the equator. One example is Nairobi/Seychelles where the long rains occur
when the sun moves north and the short rains occur when the sun moves south.
JANUARY
The Harmattan blows over West Africa where visibility is reduced to 4000 m and at times can be
as low as 1000 m. The dust carried by this wind can extend to considerable altitudes. Tropical
cyclones occur in the Mozambique Channel from January to March.
JULY
Haboobs form in the East African desert regions. In West Africa the south west monsoon moves
in behind the ITF. On the front there are often severe TS and heavy rain. The Guti affects
Zimbabwe and sometimes the Transvaal. The Guti is formed when moderate to strong south
easterly winds bring moist air from the Mozambique Channel. Conditions associated with this
wind are very low St and Sc. The wind occurs in spells of 1 to 5 days especially in the dry season
from April onward.
West African tornadoes, a line of TS moving westward, are a feature of spring and autumn.
28-16
Meteorology
Climatology-Regional Climatology
Chapter 28
ASIA
TEMPERATURE DISTRIBUTION
JANUARY
The vast Asian land mass is cold north of the Himalayas. This means that a steep temperature
gradient forms at this mountain barrier and the eastern seaboard of the continent.
The warm Kuro Siwo current running up the Chinese coast to Japan creates a strong
temperature gradient similar to that on the eastern seaboard of the USA
JULY
In the summer the sun migrating north of the equator heats the land mass. The isotherms now
conform to the lines of latitude except on the eastern seaboard where the land has warmed up
more than the ocean and the isotherms parallel the coast.
PRESSURE DISTRIBUTION
JANUARY
The Siberian high is the dominant pressure system affecting the continent. The cold is intense
where temperatures of –40°C can be reached.
The pressure reaches values which can be in the order of 1070 hPa. Air flows out from this high
pressure which gives rise to the winter monsoon. Winds in northern China are westerly, but
further south the winds become northerly. Finally, they become north easterly to become the
north east monsoon of south east Asia and Indonesia.
India is cut off from the Siberian high by the Himalayas and it develops its own high pressure
system centred in north west India and Pakistan. The resultant wind from this high pressure
system flows out along the Ganges valley and eventually joins the north east monsoon over the
Bay of Bengal.
Over China, Japan, and East Asia the air is cold, warming up as it flows toward the equator.
Meteorology
28-17
Chapter 28
Climatology-Regional Climatology
JULY
The high land temperatures create the monsoon low. Low pressure is over the continent, while
high pressure is over the oceans. The resultant airflow is from sea to land.
The air may come from the southern hemisphere as the south east trade winds may be turned to
from the south west monsoon winds by Coriolis. The south west monsoon has its direction
changed as it reaches the land masses. For example, it is diverted to flow from the south east up
the Ganges valley toward the low centred in north west India.
UPPER WINDS
JANUARY
The sub-tropical jet stream blows parallel to the steep temperature gradient created by the
Himalayan barrier and the Kuro Siwo current in the east. It blows from the west over northern
India to the south of the Himalayas. From the south west over Japan, the jetstream reaches
speeds of over 100 kt. At low latitudes, an easterly jetstream at 10 to 15 kt prevails.
JULY
Westerly winds prevail at higher latitudes. Between 20°N and the equator the winds are easterly
above 20 000 ft increasing in speed with height until at 30 000 ft they are 40 to 50 kt. At the 200
hPa level they become the equatorial jet stream at 80 kt.
HEIGHT OF THE TROPOPAUSE AND 0°C ISOTHERM
JANUARY
Over the Persian Gulf and India:
Tropopause
0°C Isotherm
28-18
56 000 ft
12 000 ft
Meteorology
Climatology-Regional Climatology
Chapter 28
Further north over China and Japan:
Tropopause
0°C Isotherm
49 000 ft
8000 ft
JULY
Over the Persian Gulf and India:
Tropopause
0°C Isotherm
55 000 ft
18 000 ft
Further north over China and Japan:
Tropopause
0°C Isotherm
52 000 ft
10 000 ft
SURFACE WINDS
JANUARY
The north east monsoon dominates much of the area. Over central Asia the winds circulate
around the Siberian high. This circulation produces northerly winds over eastern Siberia, Japan,
and Korea. Towards the north and west of Asia the flow is south westerly.
JULY
The situation is reversed where much of the area is under the influence of the south west
monsoon. The flow is modified over China and Japan where the monsoon is from the south and
south east. The northern and eastern areas of the continent experience a northerly flow.
AUSTRALIA AND THE PACIFIC
TEMPERATURE DISTRIBUTION
JANUARY
With the absence of any significant land mass in the South Pacific, apart from Australia, the
isotherms conform to the lines of latitude. Over Australia the land heats up to a greater extent
than the surrounding sea. Over parts of Australia temperatures can exceed 30°C.
JULY
The isotherms conform to the lines of latitude. There is some distortion over Australia where the
southern half of the continent is slightly cooler than the ocean.
PRESSURE DISTRIBUTION
JANUARY
A zone of low pressure forms in the centre of Australia due to the high temperatures. This
contrasts to the sub-tropical zones of high pressure which occur in both hemispheres.
Meteorology
28-19
Chapter 28
Climatology-Regional Climatology
1005
L
The ITCZ extends from the north of Australia north east to the equator and then across the north
Pacific to Columbia. Low pressure zones occur in both hemispheres in the temperate latitudes. In
the northern hemisphere this is known as the Aleutian low, the equivalent to the Icelandic low.
JULY
The ITCZ moves as far north as Hong Kong in the west Pacific basin.
1010
1020
H
1020
H
Over the ocean, the ITCZ follows a similar line to its January alignment across to South America.
The sub-tropical high and temperate low pressure zones are still apparent in their respective
hemispheres.
In addition, there is now a high pressure zone over Australia similar to the sub-tropical high
pressure zone over the oceans.
UPPER WINDS
JANUARY
Temperate latitude westerlies with jet streams in the vicinity of travelling depressions occur in the
north Pacific. Over Australia and the South Pacific westerlies at speeds of 60 to 70 kt blow. In the
equatorial regions the upper wind is easterly at 20 to 30 kt.
JULY
Temperate latitude westerlies still blow in the north Pacific and South Pacific. These winds reach
jet stream proportions in association with mid-latitude travelling depressions. Upper easterlies still
prevail in the equatorial regions.
28-20
Meteorology
Climatology-Regional Climatology
Chapter 28
HEIGHT OF THE TROPOPAUSE AND 0°C ISOTHERM
JANUARY
Tropopause
0°C Isotherm
47 000 ft
14 000 ft
Tropopause
0°C Isotherm
45 000 ft
10 000 ft
JULY
SURFACE WINDS
In both hemispheres, the surface winds diverge from the sub-tropical zones of high pressure to
form the trade winds.
Circulation causes the mid-latitude westerlies to merge with the “Brave West Wind” in the
southern hemisphere to form the Roaring Forties. These winds blow consistently in the southern
hemisphere because there is no land mass to interrupt their flow.
JANUARY
In the Pacific the monsoon blows from a northerly direction on the eastern seaboard of Asia and
the island archipelagos. The south east trade winds and the southern coast affect Australia,
especially Queensland, by south westerly winds.
JULY
The west Pacific basin and Japan are under the influence of the south west monsoon. South
Australia has mainly westerly winds associated with the travelling depressions of the midlatitudes.
SIGNIFICANT WEATHER
JANUARY
The northerly monsoon of the west Pacific is generally dry. However, after its long sea track it
acquires moisture before arriving over the island archipelagos of east and south east Asia and
Australia where it combines with the north east trades. Typical trade wind weather is CU with
accompanying showers.
The ITCZ has CU and CB with TS in varying intensity. Tropical revolving storms, cyclones, are
found off Queensland and Fiji. Off the Northern Territories these storms are termed the Willy
Willys and occur from January to March.
Note: The real Willy Willys are dust storms in central Australia.
South Australia occasionally experiences the Brickfielder, a hot dusty wind which originates in
the central Australian desert. These winds bring poor visibility in haze and occur during the
summer months of the southern hemisphere.
Near Sydney a strong southerly wind, Southerly Buster, brings dense CU cloud and heavy rain.
This usually signifies the passage of a vigorous cold front and is accompanied by a noticeable
drop in temperature.
Meteorology
28-21
Chapter 28
Climatology-Regional Climatology
In the north Pacific the families of travelling depressions are a feature of the temperate latitudes.
JULY
The west Pacific basin is influenced by the moist southerly monsoon. For Japan, the wettest
period is in June and July where the skies are overcast and produce continuous rain.
Typhoons occur from July to October from the south Philippines to Japan. On the opposite side of
the ocean the moist winds over the cold Californian current create advection fog mainly in the
vicinity of San Francisco. In the South Pacific, at the temperate latitudes, travelling depressions
march along the South Australian coast.
SOUTH AMERICA AND THE CARIBBEAN
TEMPERATURE DISTRIBUTION
JANUARY
The east west alignment of the isotherms is considerably distorted by the South American land
mass due to the sun being in the southern hemisphere during the summer months.
In the more southerly latitudes of the continent there is a considerable temperature gradient on
the west coast. It is here that the Humboldt Current travels north along the coast.
JULY
There is little variation in the temperature distribution from that in January. The temperature
gradient in the higher latitudes is shallower due to the cooling of the land in the southern
hemisphere winter and the sea being approximately the same temperature.
PRESSURE DISTRIBUTION
Because there is little variation in temperature through the year the pressure variation is minimal.
The equator effectively passes through the centre of the area.
JANUARY
The ITCZ advances south into the Amazonian rain forests of Brazil.
28-22
Meteorology
Climatology-Regional Climatology
Chapter 28
JULY
The ITCZ is aligned east west across Columbia and Venezuela.
UPPER WINDS
These winds are mainly equatorial easterlies flanked on either side by westerlies.
JANUARY
The zone of easterlies is south of the equator.
JULY
The easterlies lie mainly above the equator.
HEIGHT OF THE TROPOPAUSE AND 0°C ISOTHERM
JANUARY
Caribbean
Tropopause
0°C Isotherm
54 000 ft
14 000 ft
Central Brazil
Tropopause
0°C Isotherm
52 000 ft
16 000 ft
JULY
Caribbean
Tropopause
0°C Isotherm
52 000 ft
16 000 ft
Central Brazil
Tropopause
0°C Isotherm
51 000 ft
14 000 ft
Meteorology
28-23
Chapter 28
Climatology-Regional Climatology
SURFACE WINDS
JANUARY
The north east trade winds circulating around the Bermuda high affect the Caribbean and the
northern part of South America. These winds blow behind the ITCZ deep into the Amazonian rain
forest. The south east trade winds touch the coast of north Brazil.
The west side of the continent has a cold water current and offshore winds are a prominent
feature of the sub-tropical latitudes. In the temperate latitudes westerly winds predominate.
JULY
The north east trade winds affect the Caribbean and only the very northern part of South
America. The south east trade winds move further north along the east coast. At mid-latitudes,
further south, the temperate westerlies are still a persistent feature.
SIGNIFICANT WEATHER
The ITCZ lies across the South American continent throughout the year. The typical weather
consists of CU and CB with the attendant TS.
In the Caribbean the typical trade winds prevail but hurricanes can occur between June and
October.
To the south of the continent temperate latitude travelling depressions occur with the typical
weather associated. A typical wind that blows with the fronts associated with the depressions is
the Pampero.
The Zonda of northwest Argentina is a Foehn wind which blows down the eastern slopes of the
Andes.
28-24
Meteorology