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Thunderstorms are simply storms that produce lightning and thunder. But thunderstorms can also produce gusty winds, hail, and heavy rain. Thunderstorms form when warm humid air rises in an unstable environment. There are several mechanisms that can cause this air to rise. These mechanisms are as follows: lifting that occurs along a cold front, orographic lifting, and unequal heating of the Earth's surface. Since Memphis' geography is relatively flat, when Memphis experiences thunderstorms, they are most likely either frontal or "air mass thunderstorms" (produced due to unequal heating).
In the late afternoon, especially in summer, Memphis experiences
a large number of thunderstorms due to unequal heating of the surface.
As the sun heats up the city an air mass of warm, humid air is formed and
begins to rise. Often this rising air forms the first of three stages of
thunderstorm development and eventually produces an air mass thunderstorm.
The stages of a thunderstorm are pictured below in the following order:
1) Cumulus Stage, 2) Mature Stage, 3) Dissipating Stage.
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The first stage of thunderstorm development is the Cumulus
Stage. As air rises, due to uneven heating of the Earth, the air begins
to form clouds. At first the clouds may be fair weather cumulus clouds.
As the air continues to rise and becomes more humid, the clouds begins
to grow vertically. In order to become a cumulonimbus tower (a 
thunderstorm
cloud) the air mass must have a continuous supply of moisture. The release
of latent heat allows the warm air mass to rise higher and higher. This
stage is dominated by this continuation of rising warm air called an updraft.
Eventually, the cloud passes beyond the freezing point and produces precipitation.
As more and more precipitation is formed within the cloud, it becomes too
heavy for the cloud to support it. The precipitation begins to fall and
produces a downdraft.
Stage two is the Mature Stage. The Mature Stage begins when the downdraft leaves the base of the cloud. These downdrafts produce cool gusts of air at the surface. During the Mature Stage, downdrafts and updrafts exist side by side. When the cloud grows enough to reach the top of the unstable region, the updraft spreads out laterally. This produces the anvil top cloud that are characteristic of thunderstorms. The Mature Stage is the stage in which gusty winds, hail, and heavy precipitation occur.
The final stage in thunderstorm development is the Dissipating
Stage. In this stage, downdrafts predominate over updrafts. Once the downdraft
begins, the air that is moving out at the surface encourages an influx
of cool dry air. The effect of the cooling of the falling precipitation
mixed with the colder air aloft mark the end of the thunderstorm.
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 Questions/Comments? |
Windows to the Universe, (2000). University Corporation for Atmospheric Research, as Retrieved from the World Wide Web, (July 20, 2003).
Photos
Gore, Pamela (2000). cloud photographs, Earth
& Space Science for Middle School Teachers, as Retrieved from the
World Wide Web, July 20, 2003).
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