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103
Text 4. Thunderstorm
A thunderstorm is a form of severe weather characterized by the presence of
lightning and its attendant thunder. It is often accompanied by copious rainfall, or, on
occasion, snowfall.
Thunderstorms form when significant condensation, resulting in the production of a
wide range of water droplets and ice crystals, occurs in an atmosphere that is unstable and
supports deep, rapid upward motion. This often occurs in the presence of three conditions:
sufficient moisture accumulated in the lower atmosphere, reflected by high dewpoint
temperatures; a significant fall in air temperature with increasing height, known as a steep
lapse rate; and a force such as mechanical convergence along a cold front that will focus the
lift.
Thunderstorms have had a lasting and powerful influence on mankind. Romans thought
them to be battles waged by Jupiter, who hurled lightning bolts forged by Vulcan.
Thunderstorms were associated with the Thunderbird, held by Native Americans to be a
servant of the Great Spirit. In more contemporary times, thunderstorms now have taken on
the role of a curiosity. Every spring, storm chasers head to the Great Plains to explore the
visual and scientific aspects of storms and tornadoes.
A given cell of a thunderstorm goes through three stages: the cumulus stage, the mature
stage, and the dissipation stage. This life cycle was identified in 1949 as the res ult of the
U.S. Weather Bureau's landmark Thunderstorm Project.
In the cumulus stage of a thunderstorm cell, masses of moisture are pushed upwards;
the moisture rapidly cools into liquid drops of water vapor, which appears as cumulus
clouds. Not only are the masses of water vapor warmer than the surrounding air, but water
vapor is less dense than dry air, and for both of these reasons the warm humid air will tend
to rise in an updraft due to the process of convection. This creates a low-pressure zone
beneath the forming thunderstorm. In a typical thunderstorm, some 5Ч10
8
kg of water vapor
are lifted and the amount of energy released when this condenses is about equal to the
energy used by a city (US-2002) of 100,000 over a month.
In the mature stage, the accumulated water vapor has become large, with the top layer
often spreading out into an anvil formation. The resulting cloud is called cumulonimbus.
The water vapor will coales ce into heavy droplets and ice particles , which will fa ll onto the
area below as rain. If temperatures in the upper atmosphere are cold enough, some of these
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