Title: Aviation Weather Hazards
1Aviation 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
2Talk Overview
- Survey of weather related accidents
- Turbulence
- Low-level turbulence and surface wind
- Thermal turbulence
- Microbursts
- Mountain wave turbulence
- IMC conditions
3All weather related accidents
- The following data are from the FAAs 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_studies/weather_
study/studyindex.html
4Weather related accidents
5Nearly 87 or 7 out of 8 of these involved
general aviation operations
General Aviation
GA
Commuter
Ag
Air carrier
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719,562 total accidents 4,159 (21.3) weather
related Main cause wind
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10GA weather-related fatalities a study by D.C.
Pearson (NWS)
- http//www.srh.noaa.gov/topics/attach/html/ssd02-1
8.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
11GA 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
12Flight Safety and Weather
- Clearly, the responsibility for flight safety is
YOU, the pilot - You need to brief (up to 41 dont)
- 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
13Fatal GA accidents
14Causes of
15Aviation 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
16A. 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
17Turbulence in PIREPs
Turbulence Frequency
Turbulence Intensity
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19Turbulence
- Can be thought of as random eddies within linear
flow
Hi! Im an eddy
20Turbulence
- Linear wind and eddy components add to gusts and
lulls, up and down drafts that are felt as
turbulence
21Low-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
22Low-level Turbulence (LLT)
23Factors 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
24Low-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
25Mechanical 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
26Mechanical Turbulence (cont.)
- Over flat terrain
- Maximum surface wind gusts are typically 40
stronger than the sustained wind - Moderate or greater turbulence for surface wind gt
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
27Thermal 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
28Thermal 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
29Deep summer convective boundary layer causes
thermal turbulence
(more stable air above)
up to 20,000 MSL
thermal
thermal
dust devil
Hot, dry, unstable air
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31Towering cumulus over Prescott Fall 2000 Photo by
Joe Aldrich
32Dry 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 dont reach the ground)
33Downburst (Phoenix, AZ) July 2003Photo by
Phillip Zygmunt
34Downburst (Prescott Valley, AZ) 1999Photo by
Jacob Neider
35The 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
36Strong turbulence during day means a deep layer
is stirred Mixing means 3,000 ft wind better
mixed down to surface Stronger turbulence,
reduced vertical wind shear
Reduced turbulence means only a shallow layer is
mixed Suppressed downward mixing means surface
wind falls to near zero at night Stronger
vertical shear
37Diurnal variation of surface wind
382. Mountain Wave Turbulence
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40In 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
41Air flow over mountains
Airflow
Upwind
Downwind
Strongest wind speed and turbulence on downwind
side, also warm and dry
Orographic cloud and possible IMC conditions on
upwind side
Desired flight path
Actual flight path
Splat!
Mountain
42Mountain wave turbulence (MWT)
- Produces the most violent turbulence (other than
TS) - Occurs in two regions to the lee of mountains
- Near the ground and
- 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
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44MWT (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 gt 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)
45Mountain wave terminology
Breaking waves
Wave clouds (altocumulus lenticularis)
Inversion
Fohn cloudwall
Hydraulic jump
rotor
46Mountain 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
47Breaking Wave Region
- Vertically-propagating waves with sufficient
amplitude may break in the troposphere or lower
stratosphere.
48Rotor cloud
cap cloud
Rotor cloud
49Lee 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.
50Lee 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
51Lee wave photos
Satellite photo of lee waves over Scotland
52Flow 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
53Mountain 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
54Example ExtremeMWT 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
55Turbulence PIREPs
56Web 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
57B. Instrument Meteorological Conditions
58Instrument Meteorological Conditions (Ceiling
and visibility below specified minimum values)
59IFR/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.
60Fog-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
61Cloud 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.
62D. C. Pearson, 2002
IFR conditions are a factor in over half of the
General Aviation weather related accidents
63Meteorological 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
641. Fog
65Fog
- Fog low cloud with base lt 50 ft AGL
- Generally reported when vis lt5 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)
66Mist
- 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."
67Fog
- 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. -
68Foggy Weather
69Fog types
- Radiation fog
- Air near ground cools by radiation to saturation
- Also called ground fog
- Needs clear night, light breeze lt 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
70Fog 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 gt 0C)
- Supercooled fog (-30C lt temp lt 0C)
- Ice fog (temp lt -30C)
71The COMET program
72Radiation Fog Near Ground in Valley
73Advection Fog over San Francisco
74Fog Formation over San Francisco
75Onshore Winds Advect Fog Inland
76Types of Fog - Upslope Fog
- Air is lifted by moving up to higher ground.
77Upslope Fog Example
78Types 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
79Questions 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?
80Climatology of IMC
- In west, highest frequency of IFR conditions
occur in - Pacific northwest - lots of cyclones fronts
- gt 40 in winter
- California coast - coastal upwelling fog
- LA basin - smog
- Elswhere in west lt 10 IFR conditions
- Higher frequency in east, particularly in midwest
and south - In IL, IN, OH, PA, gt 50 frequency in winter
- Also gt 40 along Gulf coast in winter
81Climatology of IMC, winter
10-40
10-40
40-50
lt 10
40-50
40-50
10-40
gt 50
lt 10
10-40
40-50
10-40
lt 10
10-40
40-50
10-40
82Identification of Current IFR Conditions
- AWC - Aviation Weather Center
- red dots IFR, magenta dots LIFR, blue dots MVFR
- Also shows Icing and Turbulence reports
83Other 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
84IFR 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.
85Sources 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.
86Area 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/f
a/ NWS plans to develop graphical Area Forecast
product in future.
87AIRMET
- 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
88Model 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.
89Forecasting LIFR is Difficult
LIFRLow IFR
PODProbability of Detection It happened - was it
forecast?
FARFalse Alarm Rate It was forecast but did not
occur.
About 75 of the time LIFR was forecast, it did
not happen.
Less than half of the observed LIFR conditions
were forecast correctly at TUL.
90Online 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
91Summary
- 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
92Talk 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
93Thank you Any questions?
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