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Thunderstorms

Introduction:

  • Thunderstorms are weather phenomena usually accompanied by heavy rainfall as well as strong winds, hail, and tornadoes

Thunderstorms:

  • In the winter months, snowfall can occasionally take place in a thunderstorm
    • Such is often termed thunder-snow
  • Alert Examples:
    • ATC: "[Callsign] EXTREME precipitation between ten o'clock and two o'clock, one five miles. Precipitation area is two five miles in diameter."
    • AFSS/FSS: [Callsign] EXTREME precipitation two zero miles west of Atlanta V-O-R, two five miles wide, moving east at two zero knots, tops flight level three niner zero."

Characteristics:

  • Form in squall lines
  • Differing air masses cause turbulence, worst where updrafts and downdrafts meet
    • Most severe turbulence in top 1/3rd of storm
  • A shear zone can be created between the surrounding air and the cooler air of the downdraft
  • Lightning causes air to rapidly expand
  • Lightning is associated with thunderstorms
  • Hail can be significant and thrown miles from a storm

Thunderstorm Formation:

  • Thunderstorm formation require three conditions:
    1. Unstable conditions
    2. Lifting force:
      • Unequal warming of the surface of the Earth
      • Orographic lifting due to topographic obstruction of air flow
      • Dynamic lifting because of the presence of a frontal zone
    3. High moisture levels

Life Cycle:

  1. Cumulus Stage:

    • Lifting cools air to dew point, condensing water vapor into water droplets or ice crystals
    • High winds moving up and down and may extend above thunderstorm
  2. Mature Stage:

    • Beginning of precipitation
    • Turbulent conditions
    • Up to 6,000' FPM winds
  3. Dissipating Stage:

    • Characterized by downdrafts
    • Energy is released and storm weakens

Cells:

  • Single-Cell Storms:
    • Form when the atmosphere is unstable, but there is little or no wind shear
    • These storms are short lived and last for less than an hour after becoming strong enough to produce lightning
  • Multi-Cell Storms:
    • Groups of cells in different stages of development, which have merged into a larger system
    • The cloud becomes divided into updraft and downdraft regions, separated by a gust front
    • The gust front may extend for several miles ahead of the storm, bringing with it an increase in wind speed and atmospheric pressure, a decrease in temperature, and a shift in wind direction
  • Squall Line or Multi-Cell Line Storms:
    • Formed as an organized line or lines of multi-cell storms frequently with a gust
    • They often arise from convective updrafts in or near mountain ranges and linear weather boundaries, usually strong cold fronts or troughs of low pressure
    • The squall line is propelled by its own outflow, which reinforces continuous development of updrafts along the leading edge
    • Squall lines tend to be hundreds of miles long
    • These lines can move swiftly and in some parts of the line, bow echoes can form, bringing with it high winds, dangerous lightning, and possibly tornadoes
    • Heavy rain, hail, and damaging winds, such as derechos, can occur in a squall line
  • Super Cell Storms:
    • Large, severe quasi-steady-state storms, which form when the wind speed and direction vary with height ("wind shear") separates downdrafts from updrafts and contain a strong, rotating updraft
    • These storms normally have such powerful updrafts that the top of the cloud (or anvil) can reach miles into the air and can be 15 miles wide
    • Produce destructive tornadoes
    • Most tornadoes occur from this kind of thunderstorms
    • Air Mass Thunderstorms:
      • Short lived
      • Dissipation "kills" the storm
    • Steady State Thunderstorms:
      • Last awhile

Turbulence:

  • While there is some evidence that maximum turbulence exists at the middle level of a thunderstorm, recent studies show little variation of turbulence intensity with altitude
  • There is no useful correlation between the external visual appearance of thunderstorms and the severity or amount of turbulence or hail within them
  • The visible thunderstorm cloud is only a portion of a turbulent system whose updrafts and downdrafts often extend far beyond the visible storm cloud
  • Severe turbulence can be expected up to 20 miles from severe thunderstorms
  • This distance decreases to about 10 miles in less severe storms

Weather Radar:

  • Weather radar, airborne or ground based, will normally reflect the areas of moderate to heavy precipitation (radar does not detect turbulence)
  • The frequency and severity of turbulence generally increases with the radar reflectivity which is closely associated with the areas of highest liquid water content of the storm
  • NO FLIGHT PATH THROUGH AN AREA OF STRONG OR VERY STRONG RADAR ECHOES SEPARATED BY 20-30 MILES OR LESS MAY BE CONSIDERED FREE OF SEVERE TURBULENCE
  • Turbulence beneath a thunderstorm should not be minimized
  • This is especially true when the relative humidity is low in any layer between the surface and 15,000'
  • Then the lower altitudes may be characterized by strong out flowing winds and severe turbulence
  • The probability of lightning strikes occurring to aircraft is greatest when operating at altitudes where temperatures are between minus 5°C and plus 5&degC;
  • Lightning can strike aircraft flying in the clear in the vicinity of a thunderstorm
  • METAR reports do not include a descriptor for severe thunderstorms
  • However, by understanding severe thunderstorm criteria, i.e., 50 knot winds or 3/4 inch hail, the information is available in the report to know that one is occurring
  • Current weather radar systems are able to objectively determine precipitation intensity
  • These precipitation intensity areas are described as "light," "moderate," "heavy," and "extreme."

Penetration:

  • Avoidance:
    • Circumnavigate, avoid thunderstorms and any area they may be found
    • Over the top (at least 1000' per 10 knots of wind)
    • Under
    • Penetrate lower 1/3rd
  • Penetrate perpendicular to the storm
  • HALT
    • Heat:
      • Pitot heat on
    • Airspeed/Attitude:
      • Do not attempt to chase every variation of altitude and speed, they are often unreliable
      • Slow to maneuvering speed or the aircrafts max turbulence speed immediately, to prevent an over-stress
      • Do not change course at this point:
        • Aircraft loose stability in a turn and it is easier to become disoriented
        • Fly the attitude indicator and the aircraft will do the rest
        • Avoid the upper 2/3 of a mature cell (turbulence and hail) and freezing level +/- 2000' (lightning)
    • Light:
      • Turn all cockpit lights to bright to include floodlights
    • Tight:
      • Lower the seat to the bottom to prevent striking head against the canopy and to reduce the blinding effect of lightning
      • Do not look outside the cockpit

Thunderstorms:

  • Turbulence, hail, rain, snow, lightning, sustained updrafts and downdrafts, icing conditions-all are present in thunderstorms
    • While there is some evidence that maximum turbulence exists at the middle level of a thunderstorm, recent studies show little variation of turbulence intensity with altitude
  • There is no useful correlation between the external visual appearance of thunderstorms and the severity or amount of turbulence or hail within them
    • The visible thunderstorm cloud is only a portion of a turbulent system whose updrafts and downdrafts often extend far beyond the visible storm cloud
    • Severe turbulence can be expected up to 20 miles from severe thunderstorms
    • This distance decreases to about 10 miles in less severe storms
  • Weather radar, airborne or ground based, will normally reflect the areas of moderate to heavy precipitation (radar does not detect turbulence)
    • The frequency and severity of turbulence generally increases with the radar reflectivity which is closely associated with the areas of highest liquid water content of the storm
    • NO FLIGHT PATH THROUGH AN AREA OF STRONG OR VERY STRONG RADAR ECHOES SEPARATED BY 20-30 MILES OR LESS MAY BE CONSIDERED FREE OF SEVERE TURBULENCE
  • Turbulence beneath a thunderstorm should not be minimized
    • This is especially true when the relative humidity is low in any layer between the surface and 15,000'
    • Then the lower altitudes may be characterized by strong out flowing winds and severe turbulence
  • The probability of lightning strikes occurring to aircraft is greatest when operating at altitudes where temperatures are between minus 5 degrees Celsius and plus 5 degrees Celsius
    • Lightning can strike aircraft flying in the clear in the vicinity of a thunderstorm
  • METAR reports do not include a descriptor for severe thunderstorms. However, by understanding severe thunderstorm criteria, i.e., 50 knot winds or 3/4 inch hail, the information is available in the report to know that one is occurring
  • Current weather radar systems are able to objectively determine precipitation intensity. These precipitation intensity areas are described as "light," "moderate," "heavy," and "extreme"
    • Reference the Pilot/Controller Glossary: Precipitation Radar Weather Descriptions
  • Example:
    • Alert provided by an ATC facility to an aircraft:
      • (aircraft identification) EXTREME precipitation between ten o’clock and two o’clock, one five miles. Precipitation area is two five miles in diameter
    • Alert provided by an FSS:
      • (aircraft identification) EXTREME precipitation two zero miles west of Atlanta V−O−R, two five miles wide, moving east at two zero knots, tops flight level three niner zero

Thunderstorm Flying:

  • Above all, remember: never regard any thunderstorm "lightly" even when radar observers report the echoes are of light intensity; Avoiding thunderstorms is the best policy
  • Do's and Don'ts of thunderstorm avoidance:
    1. Don't land or takeoff in the face of an approaching thunderstorm. A sudden gust front of low level turbulence could cause loss of control
    2. Don't attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and wind shear under the storm could be hazardous
    3. Don't attempt to fly under the anvil of a thunderstorm. There is a potential for severe and extreme clear air turbulence
    4. Don't fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded usually can be visually circumnavigated
    5. Don't trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm
    6. Don't assume that ATC will offer radar navigation guidance or deviations around thunderstorms
    7. Don't use data­linked weather next generation weather radar (NEXRAD) mosaic imagery as the sole means for negotiating a path through a thunderstorm area (tactical maneuvering)
    8. Do remember that the data­linked NEXRAD mosaic imagery shows where the weather was, not where the weather is. The weather conditions may be 15 to 20 minutes older than the age indicated on the display
    9. Do listen to chatter on the ATC frequency for Pilot Weather Reports (PIREP) and other aircraft requesting to deviate or divert
    10. Do ask ATC for radar navigation guidance or to approve deviations around thunderstorms, if needed
    11. Do use data­linked weather NEXRAD mosaic imagery (for example, Flight Information Service­Broadcast (FIS­B)) for route selection to avoid thunderstorms entirely (strategic maneuvering)
    12. Do advise ATC, when switched to another controller, that you are deviating for thunderstorms before accepting to rejoin the original route
    13. Do ensure that after an authorized weather deviation, before accepting to rejoin the original route, that the route of flight is clear of thunderstorms
    14. Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo. This is especially true under the anvil of a large cumulonimbus
    15. Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage
    16. Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm
    17. Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is visually sighted or determined by radar
    18. Do give a PIREP for the flight conditions
    19. Do divert and wait out the thunderstorms on the ground if unable to navigate around an area of thunderstorms
    20. Do contact Flight Service for assistance in avoiding thunderstorms. Flight Service specialists have NEXRAD mosaic radar imagery and NEXRAD single site radar with unique features such as base and composite reflectivity, echo tops, and VAD wind profiles
  • Do's and Don'ts if you cannot avoid thunderstorms:
    1. Tighten your safety belt, put on your shoulder harness (if installed), if and secure all loose objects
    2. Plan and hold the course to take the aircraft through the storm in a minimum time
    3. To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of minus 15°C
    4. Verify that pitot heat is on and turn on carburetor heat or jet engine anti-ice. Icing can be rapid at any altitude and cause almost instantaneous power failure and/or loss of airspeed indication
    5. Establish power settings for turbulence penetration airspeed recommended in the aircraft manual
    6. Turn up cockpit lights to highest intensity to lessen temporary blindness from lightning
    7. If using automatic pilot, disengage Altitude Hold Mode and Speed Hold Mode. The automatic altitude and speed controls will increase maneuvers of the aircraft thus increasing structural stress
    8. If using airborne radar, tilt the antenna up and down occasionally. This will permit you to detect other thunderstorm activity at altitudes other than the one being flown
  • Do's and Don'ts of thunderstorm penetration:
    1. Do keep your eyes on your instruments. Looking outside the cockpit can increase danger of temporary blindness from lightning
    2. Don't change power settings; maintain settings for the recommended turbulence penetration airspeed
    3. Don't attempt to maintain constant altitude; let the aircraft "ride the waves."
    4. Don't turn back once you are in the thunderstorm. A straight course through the storm most likely will get you out of the hazards most quickly. In addition, turning maneuvers increase stress on the aircraft
Lightning Strikes Tree
Figure 1: Lightning Strikes Trees

Lightning

  • Can cause temporary or permanent loss of vision
  • Can puncture aircraft skin
  • Can damage electronic equipment and compass
  • Probability of strikes to aircraft:
    • Probability is highest near the top of thunderstorms (near the anvil region), decreasing rapidly with altitude
    • Highly electrified clouds can be advected many miles from the parent storm
    • Areas of low precipitation and/or low turbulence indicates high probability of lightning; conversely in areas of high precipitation and/or turbulence, the probability of lightning is low
    • Storms with high rates of natural lightning indicate a low probability of aircraft strikes
    • There is a greater probability of lightning strikes to aircraft during storm's decaying stages
    • The highest probability for direct lightning strikes to aircraft are in those parts of the storm where ambient temperature is lower than -40° Celsius (pressure altitude of 38,000' to 40,000')
    • Most lightning strikes to aircraft are triggered by the aircraft itself
    • When flying in or around thunderstorms, the probability of lightning strikes exist at all altitudes
      • Therefore, the only proven way to avoid lightning strikes is to avoid thunderstorms by a wide margin when possible

Case Studies:

  • NTSB Identification: CEN16LA163: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The flight instructor’s improper decision to depart on a local instructional flight in rapidly deteriorating weather conditions, including thunderstorms and hail, which resulted in subsequent impact with terrain during a go-around

References: