Aviation Weather Hazards
Mark Sinclair
Department of Meteorology
Embry-Riddle Aeronautical University
Prescott, Arizona
Weather radar, observing
equipment and balloon
launching on roof
ERAU Academic Complex
Weather center
Talk Overview
• Survey of weather related accidents
• Turbulence
– Low-level turbulence and surface wind
– Thermal turbulence
– Microbursts
– Mountain wave turbulence
• IMC conditions
All weather related accidents
• The following data are from the FAA’s
National Aviation Safety Data Analysis
Center (NASDAC), Office of Aviation
Safety, Flight Standards Service and
are based on NTSB accident data.
• Data from all accidents, the majority
non-fatal
• http://www.asias.faa.gov/aviation_studi
es/weather_study/studyindex.html
Weather related accidents
Nearly 87% or 7 out of
8 of these involved
general aviation
operations
General Aviation
GA
Commuter
Ag
Air carrier
19,562 total accidents
4,159 (21.3%) weather related
Main cause = wind
GA weather-related fatalities
– a study by D.C. Pearson (NWS)
• http://www.srh.noaa.gov/topics/attach/html/
ssd02-18.htm
• Looked at NTSB data from 2,312 GA fatal
accidents in the US during 1995-2000
• Weather a factor in 697 or 30% of all GA
fatalities
• A similar study by AOPA showed an
average of 35% but declining
• Weather a bigger factor in FATAL accidents
than for non-fatal
GA weather related fatalities (cont.)
• NTSB cited NWS weather support to be a
contributing factor in only two (0.3%) of
the 697 weather-related fatal accidents.
• NTSB cited FSS support to be a factor in
only five (0.7%) of the accidents.
• NTSB cited inadequate ATC support only
nine times (1.3%)
• Combined, NWS, FSS and ATC = 2.3%
• Pilot error accounted for remaining 97.7%
– Continued flight into IMC the leading cause
of GA weather-related fatalities
Flight Safety and Weather
• Clearly, the responsibility for flight safety is
YOU, the pilot
• You need to brief (up to 41% don’t)
• Clear sky and light wind now does not
mean it will be that way
– One hour from now
– 50 miles from here
– 1,000 ft AGL
Fatal GA accidents
Causes of
Aviation Weather Hazards
• Surface wind is the major listed hazard
in in ALL weather related GA accidents
• Continued flight into IMC conditions
(reduced visibility and/or low ceilings)
the leading cause of FATAL GA
accidents
A. Turbulence
• “Bumpiness” in flight
• Four types
– Low-level turbulence (LLT)
– Turbulence near thunderstorms (TNT)
– Clear-air turbulence above 15,000 ft (CAT)
– Mountain wave turbulence (MWT)
• Measured as
– Light, moderate or severe
– G-load, air speed fluctuations, vertical gust
Turbulence in PIREPs
Turbulence Frequency
Turbulence Intensity
Turbulence
• Can be thought of as random
eddies within linear flow
+
Hi!
I’m an
eddy
Turbulence
• Linear wind and eddy components
add to gusts and lulls, up and down
drafts that are felt as turbulence
20 kt gust
15 kt wind
+
updraft
5 kt
eddy
10 kt lull
downdraft
Low-level Turbulence (LLT)
• Occurs in the boundary layer
– Surface layer of the atmosphere in which
the effect of surface friction is felt
– Typically 3,000 ft deep, but varies a lot
– Friction is largest at surface, so wind
increases with height in friction layer
– Vertical wind shear  turbulence
• Important for landing and takeoffs
• Results in pitch, yaw and roll
Low-level Turbulence (LLT)
Factors that make low-level
turbulence (LLT) stronger
• Unstable air – encourages turbulence
– Air is unstable when the surface is heated
– Air is most unstable during the afternoon
– Cumulus clouds or gusty surface winds
generally indicate an unstable atmosphere
• Strong wind
– More energy for turbulent eddies
• Rough terrain
• When LLT is stronger than usual, the
turbulent layer is deeper than usual
Low-level turbulence (LLT)
• Mechanical
– Created by topographic obstacles like
mountains, and by buildings and trees
– Increases with increasing flow speed and
increasing surface heating (afternoon)
• Thermal
– Occurs when air is heated from below, as on
a summer afternoon
– Increases with surface heating
Mechanical Turbulence
• Created by topographic obstacles in flow
• Increases in both depth and intensity with
increasing wind strength and decreasing
stability. Worst in afternoon
– Extends above 3000 ft for gusts more than 50 kt
• Strongest just downwind of obstacles
• Over flat terrain, mechanical turbulence
intensity is usually strongest just above
surface and decreases with height
Mechanical Turbulence (cont.)
• Over flat terrain
– Maximum surface wind gusts are typically 40%
stronger than the sustained wind
– Moderate or greater turbulence for surface
wind > 30 kt
– When sustained surface wind exceeds 20 kt,
expect air speed fluctuations of 10-20 kts on
approach
– Use power on approach and power on landing
during gusty winds
– Sudden lulls may put your airspeed below stall
Thermal turbulence
• Produced by thermals (rising bubbles of
warm air) during day in unstable airmass
• Common on sunny days with light wind
• Stronger above sun-facing slopes in pm
• Turbulence intensity typically increases
with height from surface and is strongest
3-6,000 ft above the surface
Thermal turbulence (cont.)
• Generally light to moderate
– Commonly reported CONT LGT-MOD
• Usually occurs in light wind situations, but
can combine with mechanical turbulence
on windy days
• Often capped by inversion
– Top of haze layer (may be Sc cloud)
– ~3,000 ft, but up to 20,000 ft over desert in
summer
– Smoother flight above the inversion
Deep summer convective boundary
layer causes thermal turbulence
(more stable air above)
up to 20,000’ MSL
thermal
thermal
dust devil
Hot, dry, unstable air
Towering cumulus over Prescott
Fall 2000
Photo by Joe Aldrich
Dry microbursts from high
based thunderstorms
• When precipitation falls through unsaturated air,
evaporative cooling may produce dry microbursts
• Result in very hazardous shear conditions
• Visual clue: fallstreaks or virga (fall streaks that
don’t reach the ground)
Flight
path of
plane
45 kt
downburst
45 kt
headwind
45 kt
tailwind
Downburst (Phoenix, AZ)
July 2003—Photo by Phillip Zygmunt
Downburst (Prescott Valley, AZ)
1999—Photo by Jacob Neider
The nocturnal boundary layer
•
•
•
•
Clear nights, moderate flow
Shallow friction layer
Greatly reduced turbulence
Lack of mixing  possibility of strong
vertical shear
– Surface air decoupled from gradient flow in
free air above friction layer
– Surface flow often unrelated to pressure
pattern (and flow above friction layer)
• May have super-gradient flow and
turbulence at top of inversion
Friction layer during day
Friction layer during night
3,000 ft
Deep
turbulent
friction
layer
Shallow
nonturbulent
friction
layer
Strong turbulence during day
means a deep layer is stirred
Reduced turbulence means
only a shallow layer is mixed
Mixing means 3,000 ft wind
better mixed down to surface
Suppressed downward mixing
means surface wind falls to
near zero at night
Stronger turbulence, reduced
vertical wind shear
Stronger vertical shear
Diurnal variation of surface wind
Wind at 3,000 ft AGL
Wind speed (kt)
30
20
Surface wind is
stronger and
more turbulent
during afternoon
Surface
wind
10
0
Midnight
6am
noon
6pm
Midnight
2. Mountain Wave Turbulence
In mountainous terrain ...
• Watch for strong downdrafts on lee side
– Climb above well above highest peaks
before crossing mountain or exiting valley
• Intensity of turbulence increases with
wind speed and steepness of terrain
• Highest wind speed directly above crest
of ridge and on downwind side
• Maximum turbulence near and downwind
of mountain
Air flow over mountains
Upwind
Airflow
Orographic cloud and
possible IMC conditions
on upwind side
Downwind
Strongest wind speed and
turbulence on downwind
side, also warm and dry
Desired flight path
Splat!
Mountain
Mountain wave turbulence (MWT)
• Produces the most violent turbulence
(other than TS)
• Occurs in two regions to the lee of
mountains:
1. Near the ground and
2. Near the tropopause
– Turbulence at and below mountain top
level is associated with rotors
– Turbulence near tropopause associated
with breaking waves in the high shear
regions just above and below trop
Stratosphere
Rules of Thumb for Predicting Turbulence
Tropopause
Turbulent Layer 2
2kft above to 6kft below trop
Troposphere
Lenticular
Cloud
Roll
Cloud
Cap
Cloud
Turbulent Layer 1 - SFC-~7kft above peaks
Miles 0
2
4
6
8
10 12 14 16 18 20
Mountain Wave (> 25kt perpendicular component /stable air are key)
MWT (cont)
• Severity increases with increasing wind
speed at mountain crest
– For mountain top winds between 25 and 50
kt, expect mod turb at all levels between the
surface and 5,000 ft above the trop
– For mountain top winds > 50 kt, expect severe
turb 50-150 miles downstream of mountain at
and below rotor level, and within 5,000 ft of
the tropopause
– Severe turb in boundary layer. May be violent
downslope winds
– Dust may indicate rotor cloud (picture)
Mountain wave terminology
Breaking
waves
Fohn
cloud
wall
Wave clouds (altocumulus lenticularis)
Hydraulic
jump
rotor
Mountain Waves
• Mountain waves become more
pronounced as height increases and
may extend into the stratosphere
– Some pilots have reported mountain waves
at 60,000 feet.
– Vertical airflow component of a standing
wave may exceed 8,000 feet per minute
• Vertical shear may cause mountain
waves to break, creating stronger
turbulence
– Often happens below jet streak or near
front
Breaking Wave Region
• Vertically-propagating waves with
sufficient amplitude may break in the
troposphere or lower stratosphere.
cap
cloud
Rotor cloud
Wind
Rotor
cloud
Lee Waves
• Lee waves propagate horizontally because of strong
wind shear or low stability above.These waves are
typically at an altitude within a few thousand feet of
the mountain ridge crest.
Lee waves (cont.)
• Lee waves are usually smooth,
however, turbulence occurs in
them near the tropopause
– Avoid lenticular cloud with
ragged or convective edges
– Watch for smooth (but rapid)
altitude changes
Lee wave clouds in NZ
Lee wave photos
Satellite photo of lee
waves over Scotland
Flow over/around mountains
• Strongest flow near top and on downwind
side
• For stable air and/or lighter winds, air will
tend to go around rather than over
mountain
• For less stable air and strong winds, air
will go over mountain
Mountain Wave Accidents
• In 1966, a mountain wave ripped apart a
BOAC Boeing 707 while it flew near Mt.
Fuji in Japan.
• In 1992 a Douglas DC-8 lost an engine
and wingtip in mountain wave encounters
Example: Extreme
MWT encounter
• DC8 cargo plane over
Evergreen, CO 9 Dec 92
encountered extreme
CAT at FL 310
• Left outboard engine,
19 ft of wing ripped off
• 10 sec duration,
500 ft vertical
excursions, 20 deg
left/right rolls
• Safe landing at
Stapleton
Turbulence PIREPs
Web sites for turbulence information
• http://adds.aviationweather.gov/
– Hit the turbulence button
• http://www.dispatcher.org/brief/adfbrief.html
– Lots of aviation links to real time weather info
– Look down to turbulence section
• These are tools to help pilots better visualize
aviation weather hazards.
• Not intended as a substitute for a weather
briefing from a Flight Service Station
B. Instrument Meteorological
Conditions
Instrument Meteorological Conditions
(Ceiling and visibility below specified minimum values)
and/or
Category
VFR
(Visual flight rules)
MVFR
(Marginal VFR)
IFR
(Instrument flight
rules)
LIFR
(Low IFR)
Ceiling
vis
(feet AGL) (miles)
None or > > 5
3,000
1,000 to 3 to 5
3,000
500 to 1 to 3
1000
< 500
<1
IFR/MVFR/VFR
• VFR- Visible Flight Rules – Pilot must be able to
see the ground at all times.
• MVFR – Marginal VFR conditions. Still legally
VFR but pilots should be aware of conditions
that may exceed their capabilities
• IFR – Instrument Flight Rules – Pilot has special
training and equipment to fly in clouds.
• LIFR – Low IFR.
Fog-Visibility IFR/MVFR/VFR
•
•
•
•
VFR – Visibility greater than 5 miles.
MVFR – Visibility 3-5 miles.
IFR – Visibility 1-3 miles.
LIFR – Visibility less than 1 mile.
Red IFR
Magenta LIFR
Blue MVFR
Cloud Ceiling IFR/MVFR/VFR
•
•
•
•
VFR - Ceiling greater than 3,000 ft.
MVFR – Ceiling 1,000 to 3,000 ft.
IFR – Ceiling less than 1,000 ft.
LIFR – Ceiling less than 500 ft.
• IFR may be cause by either (or both)
ceiling and visibility restrictions.
D. C. Pearson, 2002
IFR conditions are a factor in over half of the General Aviation weather related accidents
Meteorological Causes of
IFR Conditions
• Fog (radiation fog, advection fog)
• Precipitation (snow, heavy rain)
• Low Clouds (lifting, cooling)
• High surface Relative Humidity (RH)
common factor in all causes of IFR
1. Fog
Fog
• Fog = low cloud with base < 50 ft AGL
• Generally reported when vis <5 miles and
there is no precipitation reducing visibility
• Formed by condensation of water vapor on
condensation nuclei
• Longer-lived when layer of cloud above
• Need
– A cooling mechanism
– Moisture
• Either lower T (cool) or raise DP (add
moisture)
Mist
• Mist (BR) is reported as "A visible
aggregate of minute water droplets or ice
crystals suspended in the atmosphere that
reduces visibility to less than 7 statute
miles but greater than or equal to 5/8
statute mile."
Fog
• Can be considered as a low stratus cloud in
contact with the ground. When the fog lifts, it
usually becomes true stratus. This photo shows
fog over the Pemigewasset River basin with
clear skies elsewhere.
•
Foggy Weather
Fog types
• Radiation fog
– Air near ground cools by radiation to saturation
– Also called ground fog
– Needs clear night, light breeze < 5 kts and high
surface relative humidity at nightfall
• Advection fog
– Occurs when warm moist air moves over colder
bodies of water (sea fog), or over cold land
– Needs winds up to about 15 kt
– Occurs mostly near coasts, day or night
• California coast (+ other upwelling regions)
• Near Gulf coast in winter in southerly flow
Fog types (cont.)
• Upslope fog
– Occurs on windward side of mountains
– Moist air moves upslope and cools
• Precipitation fog
– Occurs with surface inversion during rain
– Occurs over land areas in winter
– Raindrops fall to cold ground and saturate
the air there first
• Three thermodynamic types
– Warm fog (temp > 0°C)
– Supercooled fog (-30°C < temp < 0°C)
– Ice fog (temp < -30°C)
The COMET program
Radiation Fog Near Ground in
Valley
Advection Fog over San
Francisco
Fog Formation over San Francisco
Onshore Winds Advect Fog Inland
Types of Fog - Upslope Fog
• Air is lifted by moving up to higher ground.
Upslope Fog Example
Types of Fog - Precipitation Fog
• Rain falling into layer of cold air
• Evaporation below cloud base raises
the dew-point and lowers the
temperature
• Typically occurs in winter when there is
a surface inversion
• The precipitation itself can also lower
visibility to below IFR criteria in heavy
snow or rain conditions
Questions pilots should consider
regarding fog before they take off:
1. How close is the temperature to the dew point?
Do I expect the temperature-dew point spread
to diminish, creating saturation, or to increase?
2. What time of day is it? Will it get colder and form
fog, or will it get warmer and move further from
saturation?
3. What is the geography? Is this a valley where
there will be significant cold air drainage?
Will there be upslope winds that might cool and
condense?
4. What is the larger scale weather picture? Will it
be windy, suppressing radiation fog formation?
Is warm, moist air moving over a cold surface?
Climatology of IMC
• In west, highest frequency of IFR
conditions occur in
– Pacific northwest - lots of cyclones & fronts
• > 40% in winter
– California coast - coastal upwelling & fog
– LA basin - smog
– Elswhere in west < 10% IFR conditions
• Higher frequency in east, particularly in
midwest and south
– In IL, IN, OH, PA, > 50% frequency in winter
– Also > 40% along Gulf coast in winter
Climatology of IMC, winter
10-40
40-50
< 10
10-40
40-50
40-50
10-40
> 50
< 10
10-40
10-40
10-40
40-50
< 10
40-50
10-40
Identification of Current IFR Conditions
• AWC - Aviation Weather Center
– red dots IFR, magenta dots LIFR, blue dots MVFR
• Also shows Icing and Turbulence reports
Other Sources of Current IFR Conditions
• AWC Standard Brief – Satellite with AFC
AWC - Standard Brief
• ADDS (Aviation Digital Data Service – run by
AWC) Metar regional plots are color coded for
IFR conditions ADDS – METARs
• ADDS Interactive Java tool using sky cover
ADDS - METARs Java Tool
• NCAR-RAP Surface Observations (similar to
ADDS site) RAP Real-Time Weather
IFR Forecast Products
• Terminal Area Forecast (TAF) – Text product
issued by WFOs for selected airports. Hourly
resolution of prevailing and temporary surface
conditions for up to 24 hours into the future.
• TAF provide visibility and cloud ceilings, which
can be related to IFR conditions
• TAF has standard format so can be decoded
and displayed as graphics or plain text.
Sources of TAF Forecasts
• ADDS – TAFs – Available as plotted maps for a
single time for a given region for prevailing or
tempo conditions. Also available in text form in
raw or translated formats for a given single
station (need to know 4 letter ID).
• ADDS - TAFs Java Tool – Mouse over map for
raw TAF data at any station.
• Aviation Weather Center (AWC) - TAF Graphics
–Mouse over times and data types showing US
prevailing or tempo conditions (3 hour
resolution) in graphical form for IFR conditions.
Area Forecasts
• Text product generated by AWC. Covers
state or part of state VFR conditions for
12 hours into future with 6 hour outlook.
• Coded format not decoded into
graphics.
• Available at
http://aviationweather.gov/products/fa/
NWS plans to develop graphical Area
Forecast product in future.
AIRMET
• AIRMET regularly issued for IFR or
Mountain Obscuration conditions covering
at least 50% of an area.
• 6 hour forecast with 6 hour outlook
• Text product with graphical products
generated from decoding of “from” lines.
• Available at ADDS - AIRMETs
Model Guidance
• NCEP Short Range Ensemble (multiple model
runs which generate probabilities). Aviation
products at SREF Aviation Products. Available
for 3 ½ day outlooks.
• TDL Model Output Statistics (MOS) (statistical
relationship of model parameters and observed
conditions) for visibility and ceiling probabilities
and most likely conditions. Available at MAV
MOS Graphics. Available for 3 ½ day outlooks.
Forecasting LIFR is Difficult
LIFR=Low IFR
POD=Probability of Detection
It happened - was it forecast?
FAR=False Alarm Rate
It was forecast but did not occur.
Less than half of the observed
LIFR conditions were forecast
correctly at TUL.
About 75% of the time LIFR
was forecast, it did not happen.
Online Weather information and
Forecasts – to reiterate:
• These are tools to help pilots better
visualize aviation weather hazards.
• Not intended as a substitute for a weather
briefing from a Flight Service Station
Summary
• Issues to do with low-level wind are the
main weather hazard facing GA
– Probably includes cross winds, low-level
turbulence, mountain effects and shear
• Continued flight into IMC conditions the
main cause of GA fatalities
• Get a weather brief from your FSS
• Get a weather brief from your FSS
• Get a weather brief from your FSS
Talk Web site
• http://meteo.pr.erau.edu/aviation_weather
_hazards.ppt
• Embry-Riddle Aeronautical University has
a degree program in Meteorology.
• Check us out at http://meteo.pr.erau.edu
Thank you
Any questions?
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