Aviation Weather HazardsMark SinclairDepartment of MeteorologyEmbry-Riddle Aeronautical UniversityPrescott, Arizona

Weather center

Weather radar, observing equipment and balloon launching on roof

ERAU Academic Complex

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_studies/weather_study/studyindex.html

Weather related accidents

Nearly 87% or 7 out of 8 of these involved general aviation operations

Commuter

Ag

Air carrier

GA

General Aviation

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 ALLALL weather related GA accidents

• Continued flight into IMC conditions (reduced visibility and/or low ceilings) the leading cause of FATALFATAL 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

+15 kt wind5 kt eddy

10 kt lull

20 kt gust

downdraftupdraft

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 Deep summer convective boundary layer causes thermal turbulencelayer causes thermal turbulence

up to 20,000’ MSL

(more stable air above)

thermal thermal

Hot, dry, unstable air

dust devil

Towering cumulus over PrescottFall 2000Photo by Joe Aldrich

Dry microbursts from high Dry microbursts from high based thunderstormsbased 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

Strong 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

Friction layer during day

3,000 ft

Deep turbulent friction layer

Friction layer during night

Shallow non-

turbulent friction layer

Diurnal variation of surface wind

Wind at 3,000 ft AGL

Win

d sp

eed

(kt)

0

10

20

30

Midnight 6am 6pmnoon Midnight

Surface wind

Surface wind is stronger and

more turbulent during afternoon

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

Mountain

Strongest wind speed and turbulence on downwind side, also warm and dry

Air flow over mountains

Orographic cloud and possible IMC conditions

on upwind side

Upwind

Downwind

Airflow

Desired flight path

Actual flight path

Splat!

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

Turbulent Layer 1 - SFC-~7kft above peaksTurbulent Layer 1 - SFC-~7kft above peaks

Turbulent Layer 2 2kft above to 6kft below trop

Turbulent Layer 2 2kft above to 6kft below trop

TropopauseTropopause

RollCloud

LenticularCloud

CapCloud

0 2 4 6 8 10 12 14 16 18 20Miles

TroposphereTroposphere

StratosphereStratosphere

Mountain Wave (> 25kt perpendicular component /stable air are key)

Rules of Thumb for Predicting Turbulence

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

Fohn cloudwall

Hydraulic jump

rotor

Breaking waves

Inversion

Wave clouds (altocumulus lenticularis)

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.

Rotor cloud

Wind

Rotor cloud

cap 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

Satellite photo of lee waves over Scotland

Lee wave photos

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: 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

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)

Category Ceiling(feet AGL)

and/or

vis(miles)

VFR(Visual flight rules)

None or >3,000

> 5

MVFR(Marginal VFR)

1,000 to3,000

3 to 5

IFR(Instrument flight

rules)

500 to1000

1 to 3

LIFR(Low IFR)

< 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 IFRMagenta LIFRBlue 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

> 50

40-50

40-50

40-50

40-50

40-5010-40

10-40

< 10

< 10

< 10

10-40

10-40

10-40

10-40

10-40

Climatology of IMC, winter

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

POD=Probability of DetectionIt happened - was it forecast?

LIFR=Low IFR

FAR=False Alarm RateIt 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

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