At the top of the stratosphere, upper level winds blow the cloud top sideways to make the
anvil shape that characterizes a cloud as a thunderhead (Figure16.22).
Figure 16.22: Winds at the top of the stratosphere blow the top of a cumulonimbus cloud
sideways to create the classic anvil-shape of a thunderhead. ( 1 )
Clouds form when water vapor condenses. Remember that when water changes state from a
gas to a liquid, it releases latent heat. Latent heat makes the air in the cloud warmer than
the air outside the cloud and supplies the cloud with a lot of energy. Water droplets and ice
travel through the cloud in updrafts. When these droplets get heavy enough, they fall. This
starts a downdraft, and soon there is a convection cell within the cloud. The cloud grows
into a cumulonimbus giant. Droplets traveling through the convection cell grow. Eventually,
they become large enough to fall to the ground. At this time, the thunderstorm is mature,
it produces gusty winds, lightning, heavy precipitation and hail (Figure16.23).
Once downdrafts have begun, the thunderstorm can no longer continue growing. The down-
drafts cool the air at the base of the cloud, so the air is no longer warm enough to rise. As
a result, convection shuts down. Without convection, water vapor does not condense, no
latent heat is released, and the thunderhead runs out of energy. A thunderstorm usually
ends only 15 to 30 minutes after it began, but other thunderstorms may start in the same
area.
Severe thunderstorms grow larger because the downdrafts are so intense, they flow to the
ground. This sends warm air from the ground upward into the storm. The warm air feeds
the convection cells in the cloud and gives them more energy. Rain and hail grow huge before
gravity pulls them to Earth. Hail that is 1.9 cm (0.75 inch) in diameter is not uncommon.
Severe thunderstorms can last for hours and can cause a lot of damage due to high winds,
flooding, intense hail, and tornadoes.
Thunderstorms can form individually or in squall lines, which can run along a cold front
for hundreds of kilometers. Individual storms within the line may reach an altitude of more