JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
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 10 to 20 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 ½ to ¾ of an inch (Figure 5-3) can cause
significant aircraft damage in only a few seconds. Give yourself a clearance of at least 20 miles
around a thunderstorm.
Figure 5-3 — 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 3 hours after the thunderstorm has dissipated.
An electrostatic discharge (ESD) is similar to a lightning strike, but it is 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 electrostatic discharge may then occur. Aircraft have reported damage from electrostatic
discharges 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.
Aircraft Lightning or ESD Encounters
Lightning strikes and electrostatic discharges are the most reported weather related aviation
incidents. All types of aircraft are susceptible to lightning strikes and electrostatic discharges.
Aircraft have been struck by lightning or experienced electrostatic discharges at altitudes ranging
from the surface to at least 43,000 feet.
Version 3.2/Dec 08 5-3