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We need three basic ingredients to make a thunderstorm. The
basic fuel is moisture (water vapor) in the lowest levels of the atmosphere.
The air above the lowest levels has to cool off rapidly with height, so that
2-3 miles above the ground, it is very cold. Finally, we need something in the
atmosphere to push that moist air from near the ground up to where the air around
it is cold. This "something" could
be a cold front or the boundary between where the cold air from one thunderstorm
meets the air outside of the storm (called an outflow boundary) or anything else
that forces the air at the ground together. When that happens the moist air
is pushed up. What happens to the moist air as it rises? It cools off and
after a while, some of the water vapor turns into liquid drops (that we see as
clouds). That
warms up the rest of the air so that it doesn't cool off as fast
as it would if the air was dry. When the air gets to the part of the
atmosphere where it is very cold, it will be warmer and less dense than the
air around it. Since it is less dense, it will start to rise faster without being
pushed, just like a balloon filled with helium does. Then more water vapor turns
into liquid in
the mass of air and the air warms up more and rises even faster until all of
water vapor is gone and the mass eventually reaches a part of the atmosphere
where
it isn't warmer than the environment (typically
5-10 miles). |
