Presentation on theme: "Aviation Weather Hazards"— Presentation transcript:
1 Aviation Weather Hazards Mark SinclairDepartment of MeteorologyEmbry-Riddle Aeronautical UniversityPrescott, ArizonaWeather radar, observing equipment and balloon launching on roofERAU Academic ComplexWeather center
2 Talk Overview Survey of weather related accidents Turbulence Low-level turbulence and surface windThermal turbulenceMicroburstsMountain wave turbulenceIMC conditions
3 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
10 GA weather-related fatalities – a study by D.C. Pearson (NWS) Looked at NTSB data from 2,312 GA fatal accidents in the US duringWeather a factor in 697 or 30% of all GA fatalitiesA similar study by AOPA showed an average of 35% but decliningWeather a bigger factor in FATAL accidents than for non-fatal
11 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
12 Flight Safety and Weather Clearly, the responsibility for flight safety is YOU, the pilotYou need to brief (up to 41% don’t)Clear sky and light wind now does not mean it will be that wayOne hour from now50 miles from here1,000 ft AGL
15 Aviation Weather Hazards Surface wind is the major listed hazard in in ALL weather related GA accidentsContinued flight into IMC conditions (reduced visibility and/or low ceilings) the leading cause of FATAL GA accidents
16 A. Turbulence “Bumpiness” in flight Four types Measured as Low-level turbulence (LLT)Turbulence near thunderstorms (TNT)Clear-air turbulence above 15,000 ft (CAT)Mountain wave turbulence (MWT)Measured asLight, moderate or severeG-load, air speed fluctuations, vertical gust
17 Turbulence in PIREPsTurbulence FrequencyTurbulence Intensity
19 + Turbulence Can be thought of as random eddies within linear flow Hi! I’m an eddy+
20 TurbulenceLinear wind and eddy components add to gusts and lulls, up and down drafts that are felt as turbulence+15 kt wind5 kt eddy10 kt lull20 kt gustdowndraftupdraft
21 Low-level Turbulence (LLT) Occurs in the boundary layerSurface layer of the atmosphere in which the effect of surface friction is feltTypically 3,000 ft deep, but varies a lotFriction is largest at surface, so wind increases with height in friction layerVertical wind shear turbulenceImportant for landing and takeoffsResults in pitch, yaw and roll
23 Factors that make low-level turbulence (LLT) stronger Unstable air – encourages turbulenceAir is unstable when the surface is heatedAir is most unstable during the afternoonCumulus clouds or gusty surface winds generally indicate an unstable atmosphereStrong windMore energy for turbulent eddiesRough terrainWhen LLT is stronger than usual, the turbulent layer is deeper than usual
24 Low-level turbulence (LLT) MechanicalCreated by topographic obstacles like mountains, and by buildings and treesIncreases with increasing flow speed and increasing surface heating (afternoon)ThermalOccurs when air is heated from below, as on a summer afternoonIncreases with surface heating
25 Mechanical Turbulence Created by topographic obstacles in flowIncreases in both depth and intensity with increasing wind strength and decreasing stability. Worst in afternoonExtends above 3000 ft for gusts more than 50 ktStrongest just downwind of obstaclesOver flat terrain, mechanical turbulence intensity is usually strongest just above surface and decreases with height
26 Mechanical Turbulence (cont.) Over flat terrainMaximum surface wind gusts are typically 40% stronger than the sustained windModerate or greater turbulence for surface wind > 30 ktWhen sustained surface wind exceeds 20 kt, expect air speed fluctuations of kts on approachUse power on approach and power on landing during gusty windsSudden lulls may put your airspeed below stall
27 Thermal turbulenceProduced by thermals (rising bubbles of warm air) during day in unstable airmassCommon on sunny days with light windStronger above sun-facing slopes in pmTurbulence intensity typically increases with height from surface and is strongest 3-6,000 ft above the surface
28 Thermal turbulence (cont.) Generally light to moderateCommonly reported CONT LGT-MODUsually occurs in light wind situations, but can combine with mechanical turbulence on windy daysOften capped by inversionTop of haze layer (may be Sc cloud)~3,000 ft, but up to 20,000 ft over desert in summerSmoother flight above the inversion
29 Deep summer convective boundary layer causes thermal turbulence (more stable air above)up to 20,000’ MSLthermalthermaldust devilHot, dry, unstable air
31 Towering cumulus over Prescott Fall 2000Photo by Joe Aldrich
32 Dry microbursts from high based thunderstorms When precipitation falls through unsaturated air, evaporative cooling may produce dry microburstsResult in very hazardous shear conditionsVisual clue: fallstreaks or virga (fall streaks that don’t reach the ground)Flight path of plane45 kt downburst45 kt headwind45 kt tailwind
33 Downburst (Phoenix, AZ) July 2003—Photo by Phillip Zygmunt
34 Downburst (Prescott Valley, AZ) 1999—Photo by Jacob Neider
35 The nocturnal boundary layer Clear nights, moderate flowShallow friction layerGreatly reduced turbulenceLack of mixing possibility of strong vertical shearSurface air decoupled from gradient flow in free air above friction layerSurface flow often unrelated to pressure pattern (and flow above friction layer)May have super-gradient flow and turbulence at top of inversion
36 Deep turbulent friction layer Shallow non-turbulent friction layer Friction layer during day3,000 ftDeep turbulent friction layerFriction layer during nightShallow non-turbulent friction layerStrong turbulence during day means a deep layer is stirredMixing means 3,000 ft wind better mixed down to surfaceStronger turbulence, reduced vertical wind shearReduced turbulence means only a shallow layer is mixedSuppressed downward mixing means surface wind falls to near zero at nightStronger vertical shear
37 Diurnal variation of surface wind Wind at 3,000 ft AGLWind speed (kt)102030Midnight6am6pmnoonSurface windSurface wind is stronger and more turbulent during afternoon
40 In mountainous terrain ... Watch for strong downdrafts on lee sideClimb above well above highest peaks before crossing mountain or exiting valleyIntensity of turbulence increases with wind speed and steepness of terrainHighest wind speed directly above crest of ridge and on downwind sideMaximum turbulence near and downwind of mountain
41 Air flow over mountains UpwindDownwindOrographic cloud and possible IMC conditions on upwind sideStrongest wind speed and turbulence on downwind side, also warm and dryDesired flight pathActual flight pathSplat!Mountain
42 Mountain wave turbulence (MWT) Produces the most violent turbulence (other than TS)Occurs in two regions to the lee of mountains:Near the ground andNear the tropopauseTurbulence at and below mountain top level is associated with rotorsTurbulence near tropopause associated with breaking waves in the high shear regions just above and below trop
43 Rules of Thumb for Predicting Turbulence TropopauseRollCloudLenticularCap2468101214161820MilesTroposphereStratosphereMountain Wave (> 25kt perpendicular component /stable air are key)Rules of Thumb for Predicting TurbulenceTurbulent Layer 22kft above to 6kft below tropTurbulent Layer 1 - SFC-~7kft above peaksThe Roll/Rotor cloud forms on the leeside of the mountains with its base near mountain top level.The tops normally will extend to twice the height of the highest peak and can merge with the lenticular clouds above.The Roll/Rotor cloud are extremely turbulent and severe to extreme should be expected both in and below the clouds’ up and downdrafts within the clouds reach 5,000ft per minute on its windward and leeward edge.The Roll/Rotor cloud itself is stationary, constantly forming on the trailing edge (updrafts) and dissipating on the leading edge (downdrafts).The Roll/Rotor cloud can form immediately to the lee of the mountains or up to 10 miles downstream of the mountains.
44 MWT (cont)Severity increases with increasing wind speed at mountain crestFor mountain top winds between 25 and 50 kt, expect mod turb at all levels between the surface and 5,000 ft above the tropFor mountain top winds > 50 kt, expect severe turb miles downstream of mountain at and below rotor level, and within 5,000 ft of the tropopauseSevere turb in boundary layer. May be violent downslope windsDust may indicate rotor cloud (picture)
46 Mountain WavesMountain waves become more pronounced as height increases and may extend into the stratosphereSome pilots have reported mountain waves at 60,000 feet.Vertical airflow component of a standing wave may exceed 8,000 feet per minuteVertical shear may cause mountain waves to break, creating stronger turbulenceOften happens below jet streak or near front
47 Breaking Wave RegionVertically-propagating waves with sufficient amplitude may break in the troposphere or lower stratosphere.
49 Lee WavesLee 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.
50 Lee waves (cont.)Lee waves are usually smooth, however, turbulence occurs in them near the tropopauseAvoid lenticular cloud with ragged or convective edgesWatch for smooth (but rapid) altitude changesLee wave clouds in NZ
51 Satellite photo of lee waves over Scotland Lee wave photosSatellite photo of lee waves over Scotland
52 Flow over/around mountains Strongest flow near top and on downwind sideFor stable air and/or lighter winds, air will tend to go around rather than over mountainFor less stable air and strong winds, air will go over mountain
53 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
54 Example: Extreme MWT encounter DC8 cargo plane over Evergreen, CO 9 Dec encountered extreme CAT at FL 310Left outboard engine, ft of wing ripped off10 sec duration, ft vertical excursions, 20 deg left/right rollsSafe landing at Stapleton
56 Web sites for turbulence information Hit the turbulence buttonLots of aviation links to real time weather infoLook down to turbulence sectionThese are tools to help pilots better visualize aviation weather hazards.Not intended as a substitute for a weather briefing from a Flight Service Station
59 IFR/MVFR/VFRVFR- 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 capabilitiesIFR – Instrument Flight Rules – Pilot has special training and equipment to fly in clouds.LIFR – Low IFR.
60 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
61 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.
62 D. C. Pearson, 2002IFR conditions are a factor in over half of the General Aviation weather related accidents
63 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
65 Fog Fog = low cloud with base < 50 ft AGL Generally reported when vis <5 miles and there is no precipitation reducing visibilityFormed by condensation of water vapor on condensation nucleiLonger-lived when layer of cloud aboveNeedA cooling mechanismMoistureEither lower T (cool) or raise DP (add moisture)
66 MistMist (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."
67 FogCan 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.
69 Fog types Radiation fog Advection fog Air near ground cools by radiation to saturationAlso called ground fogNeeds clear night, light breeze < 5 kts and high surface relative humidity at nightfallAdvection fogOccurs when warm moist air moves over colder bodies of water (sea fog), or over cold landNeeds winds up to about 15 ktOccurs mostly near coasts, day or nightCalifornia coast (+ other upwelling regions)Near Gulf coast in winter in southerly flow
70 Fog types (cont.) Upslope fog Precipitation fog Occurs on windward side of mountainsMoist air moves upslope and coolsPrecipitation fogOccurs with surface inversion during rainOccurs over land areas in winterRaindrops fall to cold ground and saturate the air there firstThree thermodynamic typesWarm fog (temp > 0°C)Supercooled fog (-30°C < temp < 0°C)Ice fog (temp < -30°C)
78 Types of Fog - Precipitation Fog Rain falling into layer of cold airEvaporation below cloud base raises the dew-point and lowers the temperatureTypically occurs in winter when there is a surface inversionThe precipitation itself can also lower visibility to below IFR criteria in heavy snow or rain conditions
79 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?
80 Climatology of IMCIn west, highest frequency of IFR conditions occur inPacific northwest - lots of cyclones & fronts> 40% in winterCalifornia coast - coastal upwelling & fogLA basin - smogElswhere in west < 10% IFR conditionsHigher frequency in east, particularly in midwest and southIn IL, IN, OH, PA, > 50% frequency in winterAlso > 40% along Gulf coast in winter
81 Climatology of IMC, winter 10-4040-5010-40< 1040-5040-5010-40> 50< 1010-4010-4040-50< 1010-4040-5010-40
82 Identification of Current IFR Conditions AWC - Aviation Weather Centerred dots IFR, magenta dots LIFR, blue dots MVFRAlso shows Icing and Turbulence reports
83 Other Sources of Current IFR Conditions AWC Standard Brief – Satellite with AFC AWC - Standard BriefADDS (Aviation Digital Data Service – run by AWC) Metar regional plots are color coded for IFR conditions ADDS – METARsADDS Interactive Java tool using sky cover ADDS - METARs Java ToolNCAR-RAP Surface Observations (similar to ADDS site) RAP Real-Time Weather
84 IFR Forecast ProductsTerminal 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 conditionsTAF has standard format so can be decoded and displayed as graphics or plain text.
85 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.
86 Area ForecastsText 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 NWS plans to develop graphical Area Forecast product in future.
87 AIRMETAIRMET regularly issued for IFR or Mountain Obscuration conditions covering at least 50% of an area.6 hour forecast with 6 hour outlookText product with graphical products generated from decoding of “from” lines.Available at ADDS - AIRMETs
88 Model GuidanceNCEP 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.
89 Forecasting LIFR is Difficult LIFR=Low IFRPOD=Probability of DetectionIt happened - was it forecast?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.
90 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
91 SummaryIssues to do with low-level wind are the main weather hazard facing GAProbably includes cross winds, low-level turbulence, mountain effects and shearContinued flight into IMC conditions the main cause of GA fatalitiesGet a weather brief from your FSS
92 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
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