| Thunderstorms form when an air mass becomes so unstable
that it overturns
(convects) violently. "Unstable" means that the air in the lowest
layers is unusually warm and humid, or that the upper layers are unusually cool, or oftentimes, both.
Rising near-surface air in an unstable air mass expands and cools, and finds itself still warmer than
it's environment, which causes it to rise even farther. If enough
water vapor is present, some of
this vapor condenses into a cloud, releasing heat, which
makes the air parcel even warmer, forcing it to rise yet again. In thunderstorms, this process
continues to feed on water vapor in the lower atmosphere, pumping air warmed by condensation as
high as 40,000 to 60,000 feet (8 to 12 miles). Thunderstorms are more common in the afternoon
over land, when daytime heating of the land by the sun causes the lower part of the
troposphere
to become unstable. Or, some thunderstorms can form from the upper atmosphere becoming unusually
cool, due to the approach of an upper air disturbance. In this case storms can form at any time
of day, even when there hasn't been daytime heating of the land. One absolute requirement, however,
is there has to be sufficient water vapor to feed the storm. This is the fuel for the thunderstorm.
As the storm uses up this fuel, it is converted to rainfall. Eventually, the storm stabilizes the
atmosphere, through using up the excess water vapor, cooling the lower troposphere,
and warming the upper troposphere. A discussion of what causes lightning can be found
here.
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