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									       Aviation Weather Hazards
      Mark Sinclair
      Department of Meteorology
      Embry-Riddle Aeronautical University
      Prescott, Arizona

Weather radar, observing
equipment and balloon                    ERAU Academic Complex
launching on roof

                                                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
• http://www.asias.faa.gov/aviation_studi
Weather related accidents
Nearly 87% or 7 out of
8 of these involved
general aviation
operations                       General Aviation



                         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/
• 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
• 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
 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
• Can be thought of as random
  eddies within linear flow

                  +     I’m an
• 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

                                10 kt lull
 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
  – 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
  – 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)

                                      45 kt
    Flight                          downburst
    path of          45 kt
     plane         headwind
                                                  45 kt
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

Strong turbulence during day            Reduced turbulence means
means a deep layer is stirred           only a shallow layer is mixed
Mixing means 3,000 ft wind              Suppressed downward mixing
better mixed down to surface            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
                                                  Surface wind is
                                                    stronger and
Wind speed (kt)

                                                   more turbulent
                  20           Surface            during afternoon


                  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                       Downwind
 Orographic cloud and          Strongest wind speed and
possible IMC conditions         turbulence on downwind
    on upwind side              side, also warm and dry

                                   Desired flight path


Mountain wave turbulence (MWT)
• Produces the most violent turbulence
   (other than TS)
• Occurs in two regions to the lee of
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
               Rules of Thumb for Predicting Turbulence
                               Turbulent Layer 2
Troposphere               2kft above to 6kft below trop



                                  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     Wave clouds (altocumulus lenticularis)

cloud              Hydraulic
wall               jump

          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
  – Often happens below jet streak or near
     Breaking Wave Region
• Vertically-propagating waves with
  sufficient amplitude may break in the
  troposphere or lower stratosphere.
        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
• For stable air and/or lighter winds, air will
  tend to go around rather than over
• 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
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
Instrument Meteorological Conditions
  (Ceiling and visibility below specified minimum values)

    Category               Ceiling     vis
                          (feet AGL) (miles)
        VFR               None or > > 5
  (Visual flight rules)     3,000
       MVFR                1,000 to 3 to 5
   (Marginal VFR)           3,000
         IFR                500 to 1 to 3
  (Instrument flight        1000
        LIFR                < 500      <1
      (Low IFR)
• 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 = 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
• 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."
• 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
Advection Fog over San
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
• 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
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
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
        40-50                  10-40
                       < 10
                                                  > 50
                        < 10                                 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
• Available at
  NWS plans to develop graphical Area
  Forecast product in future.
• 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       FAR=False Alarm Rate
It happened - was it forecast?     It was forecast but did not occur.

Less than half of the observed     About 75% of the time LIFR
LIFR conditions were forecast      was forecast, it did not happen.
correctly at TUL.
  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
• 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
• 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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