A gust front is the leading edge of cool air rushing down and out from a
thunderstorm. There are two main
reasons why the air flows out of some thunderstoms so rapidly.
The primary reason is the presence of relatively
dry (low humidity) air in the lower atmosphere. This dry air causes some of the rain falling through it
to evaporate, which cools the air. Since cool air sinks (just as warm air rises), this causes a down-rush of air that spreads out
at the ground. The edge of this rapidly spreading cool pool of air is the gust front.
The second reason
is that the falling precipitation produces a drag on the air, forcing it downward. If the wind following the gust front
is intense and damaging, the windstorm is known as a downburst.
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