THUNDERSTORM WEATHER HAZARDS  

Some of the basic information on thunder-and storms was discussed previously in unit 5, lesson 4 of AG 2 - Volume 1. Although reviewing that material is not absolutely necessary, you may find it helpful to review it before going on. 

Obviously, you should tell pilots to avoid thunderstorms if possible. However, with 44,000 thunderstorms occurring daily over Earths surface, almost every pilot will end up flying in or near one eventually.

Hazardous turbulence is present in all thunder-storms, and in a severe thunderstorm it can damage an airframe and cause serious injury to passengers and crew. The strongest turbulence within the cloud occurs with shear between up-drafts and downdrafts. Outside the cloud, shear turbulence has been encountered several thousand feet above and 20 miles laterally from a severe storm. Severe turbulence can be encountered in the anvil 15 to 30 miles downwind. Remember, storm clouds are only the visible portion of a turbulent system, whose updrafts and downdrafts often extend outside the storm proper.

A low-level turbulent area is the shear zone between the cold air downdraft and the surround-ing air. This often occurs up to 10 or 15 miles ahead of a mature storm and is called the gust a true estimate of the actual wind above the surface.  

Often a "roll cloud" on the leading edge of a storm marks the eddies to this shear. The roll cloud is most prevalent with cold-front or squall-line thunderstorms and indicates an extremely turbulent zone.

Hail is regarded as one of the worst hazards of thunderstorm flying. It usually occurs during the mature stage of cells having a strong updraft. As a rule, the larger the storm, the more likely it is to have hail. Hail has been encountered as high as 45,000 feet in completely clear air and may be carried up to 10 miles downwind from the storm core. Hail can occur anywhere in a thunder-storm, but it is usually found beneath the anvil of a large cumulonimbus.

Hailstones larger than 1/2 to 3/4 inch can cause significant aircraft damage in a few seconds.

Icing  

Icing should be expected where ambient (free-air) temperatures are at or below freezing. In general, icing is associated with temperatures from 0C to 40C. The most severe icing occurs from 0C to 10C, while the heaviest icing conditions usually occur just above the freezing level. Since the freezing level is also the zone where heavy rain-fall and turbulence most frequently occur, this particular altitude appears to be the most hazardous.

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