Hail - P-3030087

AVIATION WEATHER

CHAPTER FOUR

Hail

As a rule, the larger the storm, the more likely it is to produce hail. Hail has been encountered as

high as 45,000 feet in completely clear air and may be carried up to 30 miles downwind from the

storm core. Aircrews should anticipate possible hail with any thunderstorm, especially beneath

the anvil of a large thunderstorm. Hailstones larger than 1/2 to 3/4 of an inch (Figure 4-4) can

cause significant aircraft damage in a few seconds. Give yourself a clearance of at least 20 miles

around a thunderstorm.

Figure 4-4 Hailstones

Lightning and Electrostatic Discharge

Lightning occurs at all levels in a thunderstorm. The majority of lightning bolts never strike the

ground, but occur between clouds or within the same cloud. Lightning also occurs in the clear

air around the tops, sides, and bottoms of storms. Aircrews flying several miles from a

thunderstorm can still be struck by the proverbial "bolt out of the blue." Lightning strikes can

also occur in the anvil of a well-developed or dissipated thunderstorm. Additionally, lightning

strikes in the anvil have occurred up to three hours after the thunderstorm has dissipated.

An electrostatic discharge (ESD) is similar to a lightning strike, but caused by the aircraft itself.

The larger and faster the aircraft, the more particles it impacts, generating a greater static

electricity charge on the airframe. The electrical field of the aircraft may interact with the cloud

and an ESD may then occur. Aircraft have reported damage from ESD occurring in cirrus

clouds downwind of previous thunderstorm activity, in cumulus clouds around a thunderstorm's

periphery, and even in stratiform clouds and light rain or showers. This release of static

electricity is frequently called Saint Elmo's fire.

Thunderstorms

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