PILOT NAVIGATION
Senior/Master Air Cadet
THE WEATHER
Introduction
Previously you have learnt how the
weather affects the hill walker, and the
aviator in the circuit
We will now see how the weather
affects aircraft en route between
airfields
The Air
In order to understand the reactions involved
when the air is in motion we must consider
its constituents
The major variable is water as we shall see
Pure air consists of :
OXYGEN
20%
NITROGEN 79%
OTHER
1%
(POLLUTION,
OZONE,
CO 2)
Temperature & Pressure
From your physics you will remember
Boyles’s gas laws. If you don’t you should
know that up high in the atmosphere
(such as on top of a mountain) it is very
cold and climbers need to carry oxygen
This is because the air pressure is very
low at high altitudes
Temperature & Pressure
The air pressure at sea level is caused by the
weight of the air above us. The higher you go,
the less the weight becomes
If a gas is compressed it become hotter, &
so the less compressed it is the colder it is
Water Vapour
A certain volume of air, under fixed
conditions of pressure and temperature,
can only hold a certain amount of water
vapour, which is an invisible gas
If this air becomes cooler it will not be able to
hold as much moisture and will eventually
become saturated with water vapour
Water Vapour
When the air cools to this point
it is known as the dew point
It is at this point that dew , mist or fog will
form. If the temperature falls below
freezing then frost or freezing fog will
occur
Cloud
When unsaturated air rises the
temperature drops & the dew point is
reached, and cloud is formed
DP
AIR RISING
DUE TO
TRIGGER
ACTION
TEMPERATURE
DROPPING
Trigger Actions
There are four trigger actions that force
air to rise:
Turbulence - air currents coming together
Convection - heating of the air by the sun
??????
Trigger Actions
Orographic uplift (mountain effect) air
hitting mountain ranges
Trigger Actions
Frontal uplift - when 2 fronts meet
Warm Front
Warm air
Cool Air
The height of the cloud will depend on the
amount moisture in the air, and the
strength of the uplift
In extreme cases Cumulo Nimbus
thunder clouds form, and these are a
severe hazard to aircraft
At high levels the clouds (cirrus) consists of
ice crystals, but most other clouds are formed
by visible droplets of water
Thunderstorms
A large thunderstorm presents a variety of
hazards, both in the cloud and surrounding
areas, to aircraft and are best avoided
Precipitation - all types are present
- the most dangerous being hail
Icing - as previously covered
Lighting & thunder
the main effect is psychological.
Lighting often strikes aircraft
but the main danger is to electrical systemsradar, radios etc, and temporary blinding of
the crew !
Turbulence
the air in a thunder cloud can be in
vertical motion up or down in excess of
50 knots and can change from up to
down and back again in seconds. This
can destroy aircraft
Landing hazards
all of the above hazards exist under the
base of the thunderstorm - the most
significant being the risk of a severe down
drought just as the aircraft is landing
Most modern aircraft carry weather
radar’s for detecting thunderstorms &
turbulent air
Isobars
When watching the weather forecasts on
TV you will have noticed that the air
pressure changes from place to place
The normal range of pressure is
930 millibars
to 1050 millibars
The pressure is shown on the chart by
Linking areas of
equal pressure by a line called an Isobar
shown thus
1016
1016
L
1000
992
The isobars surround areas of high or
low pressure, and show us how the
wind is moving - the wind velocity
(WV)
The WV is always expressed as where it
is coming from in degrees and its
strength in knots
For example a WV of 200/25 is a
wind coming from 200° at 25 knots
In the northern hemisphere the wind
circulates clockwise around anti cyclones (high
pressure areas) & anti clockwise around
cyclones ( low pressure areas)
1016
L
1000
992
WIND
To remember this stand with your back
to the wind, and the area of low
pressure is to your left
This is reversed in the southern hemisphere
Isobars – facts and figures
Isobar patterns represent the
wind at 2000’ above the
surface
Isobars – facts and figures
The direction of the lines give the
direction the wind
The closer the lines the stronger the wind
Isobars – facts and figures
At the surface the wind is about 25% weaker
than at 2000’ as a result of surface friction.
It is also backed by about 25°
For example: a wind at 2000’ of 270/20
will be 245/15 at the surface
TAFs & METARs
The weather forecaster uses many charts &
symbols to convey the details of the weather
over the whole country
For an aviator, who receives info from radio, it
must be coded & is standardized world wide
(except Canada !)
There are 2 formats:
TAF
Terminal Aerodrome Forecast
M ETAR
Meteorological Actual Report
one records a forecast and the other
reports actual conditions
TAF
Is published for a 9 hour period,
and starts with 4 figures
eg 0615 introduces a forecast valid
from 0600 to 1500 hrs
TAF
TAFs do not include temperatures or pressures
But may include information on expected changes
TAF
These are prefixed :
BECMG - Becoming
TEMPO - Temporary
TAF
If the forecaster is unsure then he
gives a probability:
PROB 30TS
means 30% chance of thunderstorm
METAR
Gives the conditions at an airfield and is
recorded hourly
It is given to aircraft inbound
It is normally prefixed by a time - which is the
time the conditions where observed
METAR
If the weather changes rapidly a
SPECI – (special) report is issued
METARs & SPECIs Do not forecast
conditions, but do include pressure &
temperature
The Code
TAF
CRANWELL 0615 260/05 4000 HZ SCT030
BECMG CAVOK=
This translates as
Cranwell/ for 0600 - 1500 hrs/ surface
wind 260° true at 5 knots/visibility 4000
meters in haze/scattered cloud base at
3000’/becoming “cloud and visibility okay”
that is the visibility will be at least 10km
and no cloud below 5000’
CRANWELL 0615 260/05 4000 HZ SCT030
BECMG CAVOK=
The Code
METAR
CRANWELL 0900 250/07 8000 FEW 035
+17/+13 1028=
This translates as
CRANWELL/ AT 0900 Z HRS/ SURFACE
WIND 250° TRUE AT 7 KNOTS/VISIBILITY
8000 METERS/LOWEST CLOUD - few
(EIGHTS) AT 3500’/TEMPERATURE +17°C,
DEW POINT + 13°C/PRESSURE 1028
CRANWELL 0900 250/07 8000 FEW 035
+17/+13 1028=
In the USA temperature is give in °F
= Means End Of Message
Pressure is the QNH at the airfield
TAF and METAR codes
Time:
A 4 Figure Group In Hours/Minutes
Wind:
Wind Speed In Knots & Direction
In Degrees True. In gusty conditions a “G”
is added with A higher figure to indicate
range – 18G28
Visibility:
4 Figures From 0000 To 9999. This Is
The Visibility In Meters & Km
Weather:
2 letter groups to indicate type of weather
BR
Mist
FZ
Freezing
DZ
Drizzle
SN
Snow
FG
HZ
Haze
RA
Rain
SH
Shower
TS
Thunderstorm
-
Slight
+
Heavy
Fog
FU
Smoke
HZ
Haze
These can be mixed in any combination:
RASN
Rain & snow mixed
+SHRA
Heavy rain shower
-FZDZ
Light freezing drizzle
Cloud: 6 item code giving cloud amount
& height of cloud base
Cloud Amounts:
FEW
1 or 2 eighths coverage
SCT
Scattered 3 or 4 eighths
BKN
Broken 5 or 7 eighths
OVC
8 eighths coverage
Cloud Base:
3 numbers to indicate cloud base
height above airfield in hundreds of
feet, eg. 018 = 1800 feet
Check of understanding
What is a TAF?
Terminal Aerodrome Forecast
What is a METAR?
Meteorological Actual Report
If the weather changes rapidly a
SPECI – (special) report is issued
What is a SPECI?