JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
JX105 - THUNDERSTORMS
OVERVIEW
Thunderstorms contain many of the most severe weather hazards. They are often accompanied
by strong wind gusts, severe turbulence, lightning, heavy rain showers, severe icing, and possibly
hail and tornadoes. As a result, thunderstorms should be avoided if possible.
About 44,000 thunderstorms occur daily over the earth and pilots can expect to encounter one
occasionally. In some tropical regions, thunderstorms occur year-round. In the mid-latitudes,
they develop most frequently in spring, summer, and fall. This chapter presents hazards a pilot
must consider when flying in the vicinity of, or actually entering, a thunderstorm. Being familiar
with these factors will help you better understand what is going on both inside and outside the
cockpit. Knowledge of thunderstorm characteristics and the application of tested procedures will
help aircrews operate more safely near thunderstorms.
REFERENCES
AFH 11-203, Weather for Aircrews, Volume 1, Chapters 1, 3, and 4.
THUNDERSTORM DEVELOPMENT
The basic requirements for thunderstorm formation are moisture, unstable air, and some type of
lifting action. Lifted air does not always result in thunderstorm activity. Air may be lifted to a
point where the moisture condenses and clouds form, but these clouds may not grow
significantly unless the air parcel reaches a point where it will continue to rise freely (recall the
LFC from Chapter 2). The higher the moisture content, the easier the LFC is reached. One of the
four lifting methods (from Chapter 2) is necessary to force warmer air from its lower level to the
LFC, which is the trigger to starting the cumulus cloud through the thunderstorm life cycle. Once
moist air is lifted in an unstable environment, the rapidly rising unstable air quickly forms
towering cumulus and eventual cumulonimbus clouds. The degree of vertical cloud growth often
indicates the potential severity of the thunderstorm.
THUNDERSTORM WEATHER HAZARDS
Thunderstorms are accompanied by some or all of the following hazards: extreme turbulence,
hail, microbursts, severe icing, lightning, and tornadoes.
Turbulence
Severe turbulence is present in all thunderstorms. One of the major characteristics of every
thunderstorm is updrafts and downdrafts that can occur near each other creating strong, vertical
shear and turbulence. This turbulence can extend over 5000 feet above the cloud tops and down
to the ground beneath the cloud base. It can damage an airframe and cause serious injury to
passengers and crew.
The first gust or gust front of an approaching thunderstorm is another form of turbulence that can
cause a rapid and drastic change in the surface wind (Figure 5-1). An attempt to take off or land
with an approaching thunderstorm nearby could have disastrous results. Gust fronts can travel 5
to 20 miles from the thunderstorm.
Version 3.2/Dec 08 5-1