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UNIT8

Weather Hazards (3)
SECTION 4. THUNDERSTORMS

1. General

The thunderstorm is a local storm which is produced by a cumulonimbus cloud and is always

accompanied by lightning and thunder. Thunderstorms are particularly dangerous for pilots

because they are almost always accompanied by strong gusts of wind, severe turbulence, and
icing. Heavy rainshowers normally accompany the thunderstorm, and hail is not uncommon.

Since thunderstorms are so dangerous to a pilot, he is frequently provided with information

as to the existence and location of thunderstorms. This section describes the structure,
general types, and hazards of thunderstorms so that the importance of the information

provided to pilots will be understood.

2. Formation

a. In order for a thunderstorm to form, the air must have sufficient water vapor, be

unstable, and (initially) be forced upward. The first updrafts can be caused by convection

currents from surface heating, sloping terrain, a front, converging winds, or any combination
of the above.

b. As the upward moving air expands and cools, it causes condensation of the water
vapor and the formation of a cumulus cloud. The process of condensation releases heat

which slows the cooling caused by expansion. If this saturated updraft becomes warmer than

the surrounding air, its lower density causes the updraft to increase the speed of its upward

movement as more and more water vapor is pulled into the cloud and condenses, the cloud
builds upward into a towering cumulus, and finally becomes a cumulonimbus cloud and

produces a thunderstorm.

c. When air is moist and unstable, thunderstorms may be caused by daytime heating
of the ground, low pressure areas, winds moving up a slope, or fronts. Even dry air masses

coming in contact with moist air can cause thunderstorms. Once started, thunderstorms

generally move with the winds aloft and may travel a considerable distance from their
source. Thunderstorms caused by a front can move well ahead of the front as a squall line.

Thunderstorms formed over mountains may move many miles out over the nearby flat lands.

3. Life-Cycle