Aviation Weather Hazards LT Clayton Martin NAS Patuxent River Air Operations

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

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 accidents

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 as Light, moderate or severe G-load, air speed fluctuations, vertical gust

Turbulence Can be thought of as random eddies within linear flow +

Turbulence Linear wind and eddy components add to gusts and lulls, up and down drafts that are felt as turbulence + 15 kt wind 5 kt eddy 10 kt lull 20 kt gust downdraft updraft

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

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 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 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

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

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

Diurnal variation of surface wind Wind at 3,000 ft AGL Wind speed (kt) 10 20 30 Midnight 6am 6pm noon Surface wind Surface wind is stronger and more turbulent during afternoon

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 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

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 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

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

Turbulence PIREPs

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

Instrument Meteorological Conditions

VFR Minimums

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.

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.

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

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 Advection 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 Precipitation 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)

Types of Fog - Upslope Fog Air is lifted by moving up to higher ground.

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 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?

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 10-40 40-50 10-40 < 10 40-50 40-50 10-40 > 50 < 10 10-40 10-40 40-50 < 10 10-40 40-50 10-40

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

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

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

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