Aviation Weather Hazards

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Aviation Weather Hazards

• LT Clayton Martin
• NAS Patuxent River
• Air Operations

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Talk Overview

• Survey of weather related accidents
• Turbulence
• Low-level turbulence and surface wind
• Thermal turbulence
• Microbursts
• Mountain wave turbulence
• IMC conditions

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Flight 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

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Fatal GA accidents
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Causes of
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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

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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

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Turbulence

• Can be thought of as random eddies within linear
  flow

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Turbulence

• Linear wind and eddy components add to gusts and
  lulls, up and down drafts that are felt as
  turbulence

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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

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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

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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

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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

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Mechanical Turbulence

• 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

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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

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Thermal turbulence

• 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

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Summer Thermal Turbulence
(deep convective boundary more stable air above)
up to 20,000 MSL
thermal
thermal
dust devil
Hot, dry, unstable air
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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 dont reach the ground)

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Diurnal variation of surface wind
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Mountain Wave Turbulence
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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

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Mountain wave turbulence

• 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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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 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)

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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

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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

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Turbulence PIREPs
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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

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Instrument Meteorological Conditions
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VFR Minimums
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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.

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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
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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.

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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

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Fog

• 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)

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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."

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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.

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Foggy Weather
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Fog 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

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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 gt 0C)
• Supercooled fog (-30C lt temp lt 0C)
• Ice fog (temp lt -30C)

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Types of Fog - Upslope Fog

• Air is lifted by moving up to higher ground.

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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

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Questions to Ask Before Flight

• 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?

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Climatology 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

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Climatology 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
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WX Information Sources

• 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

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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.

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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.

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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/f
  a/ NWS plans to develop graphical Area Forecast
  product in future.

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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

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Online Wx info 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 Flight Weather Briefer

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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