Chapter 11: Weather Theory - FAA

AnvilHailMOVEMENT OF STORMFigure 11-29. Movement and turbulence of a maturing thunderstorm.of lightning and precipitation. Thus, any cloud connected toa severe thunderstorm carries a threat of violence.TurbulencePotentially hazardous turbulence is present in allthunderstorms, and a severe thunderstorm can destroy anaircraft. Strongest turbulence within the cloud occurs withshear between updrafts and downdrafts. Outside the cloud,shear turbulence has been encountered several thousand feetabove and 20 miles laterally from a severe storm. A low-levelturbulent area is the shear zone associated with the gust front.Often, a “roll cloud” on the leading edge of a storm marks thetop of the eddies in this shear and it signifies an extremelyturbulent zone. Gust fronts often move far ahead (up to 15miles) of associated precipitation. The gust front causes arapid and sometimes drastic change in surface wind ahead ofan approaching storm. Advisory Circular (AC) 00-50A, LowLevel Wind Shear, explains in detail the hazards associatedwith gust fronts. Figure 1 in the AC shows a schematic trusssection of a thunderstorm with areas outside the cloud whereturbulence may be encountered.IcingUpdrafts in a thunderstorm support abundant liquid waterwith relatively large droplet sizes. When carried abovethe freezing level, the water becomes supercooled. Whentemperature in the upward current cools to about –15 °C,much of the remaining water vapor sublimates as ice crystals.Above this level, at lower temperatures, the amount ofsupercooled water decreases.Supercooled water freezes on impact with an aircraft. Clearicing can occur at any altitude above the freezing level, but athigh levels, icing from smaller droplets may be rime or mixedrime and clear ice. The abundance of large, supercooledwater droplets makes clear icing very rapid between 0 °C and–15 °C and encounters can be frequent in a cluster of cells.Thunderstorm icing can be extremely hazardous.Thunderstorms are not the only area where pilots couldencounter icing conditions. Pilots should be alert for icinganytime the temperature approaches 0 °C and visible moistureis present.HailHail competes with turbulence as the greatest thunderstormhazard to aircraft. Supercooled drops above the freezing levelbegin to freeze. Once a drop has frozen, other drops latch onand freeze to it, so the hailstone grows—sometimes into ahuge ice ball. Large hail occurs with severe thunderstormswith strong updrafts that have built to great heights.Eventually, the hailstones fall, possibly some distance fromthe storm core. Hail may be encountered in clear air severalmiles from thunderstorm clouds.11-24