1. Aviation Weather Hazards
Mark Sinclair
Department of Meteorology
Embry-Riddle Aeronautical University
Prescott, Arizona
Weather radar, observing equipment and balloon launching on roof
ERAU Academic Complex
Weather center

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

4. Weather related accidents

5. Nearly 87% or 7 out of 8 of these involved general aviation operationsGA
Commuter Ag
Air carrier

7. 19,562 total accidents
4,159 (21.3%) weather related
Main cause = wind

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 during
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

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 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

13. Fatal GA accidents

14. Causes of

15. 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

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

17. Turbulence in PIREPs
Turbulence Frequency
Turbulence Intensity

19. + Turbulence Can be thought of as random eddies within linear flow Hi!
I’m an eddy +

20. 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

21. 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

22. Low-level Turbulence (LLT)

23. 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

24. 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

25. 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

26. 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 kts on approach
Use power on approach and power on landing during gusty winds
Sudden lulls may put your airspeed below stall

27. 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

28. 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

29. 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

31. Towering cumulus over Prescott
Fall 2000
Photo by Joe Aldrich

32. 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

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 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

36. Deep turbulent friction layer Shallow non-turbulent friction layer
Friction layer during day
3,000 ft
Deep turbulent friction layer
Friction layer during night
Shallow non-turbulent friction layer
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

37. 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

38. 2. Mountain Wave Turbulence

40. 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

41. Air flow over mountains
Upwind
Downwind
Orographic cloud and possible IMC conditions on upwind side
Strongest wind speed and turbulence on downwind side, also warm and dry
Desired flight path
Actual flight path
Splat!
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 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

43. Rules of Thumb for Predicting Turbulence
Tropopause
Roll
Cloud
Lenticular
Cap 2 4 6 8 10 12 14 16 18 20
Miles
Troposphere
Stratosphere
Mountain Wave (> 25kt perpendicular component /stable air are key)
Rules of Thumb for Predicting Turbulence
Turbulent Layer 2
2kft above to 6kft below trop
Turbulent Layer 1 - SFC-~7kft above peaks
The 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 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 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)

45. Mountain wave terminology
Breaking waves
Wave clouds (altocumulus lenticularis)
Inversion
Fohn cloudwall
Hydraulic jump
rotor

46. 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

47. Breaking Wave Region
Vertically-propagating waves with sufficient amplitude may break in the troposphere or lower stratosphere.

48. Rotor cloud
cap cloud
Rotor cloud
Wind

49. 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.

50. 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

51. Satellite photo of lee waves over Scotland
Lee wave photos
Satellite photo of lee waves over Scotland

52. 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

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 310
Left outboard engine, ft of wing ripped off
10 sec duration, ft vertical excursions, 20 deg left/right rolls
Safe landing at Stapleton

55. Turbulence PIREPs

56. Web sites for turbulence information
Hit the turbulence button
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

57. B. Instrument Meteorological Conditions

58. Instrument Meteorological Conditions (Ceiling and visibility below specified minimum values)

59. 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.

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 IFR
Magenta LIFR
Blue 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, 2002
IFR 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

64. 1. Fog

65. 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)

66. 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."

67. 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.

68. Foggy Weather

69. 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

70. 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)

71. The COMET program

72. Radiation Fog Near Ground in Valley

73. Advection Fog over San Francisco

74. Fog Formation over San Francisco

75. Onshore Winds Advect Fog Inland

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

77. Upslope Fog Example

78. 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

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 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

81. 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

82. Identification of Current IFR Conditions
AWC - Aviation Weather Center
red dots IFR, magenta dots LIFR, blue dots MVFR
Also shows Icing and Turbulence reports

83. 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

84. 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.

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 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 NWS plans to develop graphical Area Forecast product in future.

87. 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

88. 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.

89. Forecasting LIFR is Difficult
LIFR=Low IFR
POD=Probability of Detection
It happened - was it forecast?
FAR=False Alarm Rate
It 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. 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

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

93. Thank you
Any questions?
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