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104
droplets may actually form into masses of ice and fall as hail. While updrafts are still
present, the falling rain creates downdrafts as well. The pres ence of both updrafts and
downdrafts during this stage can cause considerable internal turbulence in the storm system,
which sometimes manifests as strong winds, severe lightning, and even tornadoes.
Finally, in the dissipation stage, updraft conditions no longer exis t, and the s torm is
characterized largely by weak downdrafts. Because most of the moisture has precipitated
out as rain or ice (precipitation) there is no longer sufficient mois ture in the lower air to
sustain the cycle.
Thunderstorms are often classified into a spectrum based on their cell structure: single
cells , multice llular s torms , and supercells . The type of s torm depends on the ins tability and
relative wind conditions at different layers of the atmosphere (shear).
The single-cell (unicell) thunderstorm is the singular three-stage situation as described
above, usually lasting about 30 minutes from the start of significant precipitation. A severe
unicell storm is often referred to as a pulse thunderstorm. The life cycle of the storm usually
takes place within a one-hour span.
In a multicell thunderstorm, several thunderstorm cells merge into a larger system. The
cloud becomes divided into updraft and downdraft regions separated by a gust front. The
gust front may extend for several miles ahead of the storm, bringing with it increases in
wind speed and atmospheric pressure, decreases in temperature, and shifts in wind direction.
The storm itself will have different portions sequentially going through the various
thunderstorm stages. In many cases the immature cells develop along a line known as a
flanking line, res ulting in what is known as a line multicell.
The supercell is the most dangerous type of thunderstorm, as it typically contains
violent gusts of wind, large hail, and damaging tornadoes. It is caus ed by s trong instability
or strong helicity in the flow feeding the storm. The supercell possesses a mesocyclone, the
results of which are strong vertical shear, differences in wind speed at different layers and
separate updraft and downdraft regions, with the effect being that the storm will both last
longer and continue to grow larger and more dangerous.
In a thunderstorm, the Earth's Coriolis effect can twist the huge vertical motions of air
and moisture into horizontal rotation - a vortex - which can result in tornadoes in larger
thunderstorms.
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