Atmospheric conditions typical during tornado formation.
How a Tornado Forms
Most tornadoes form in a part of a
supercell thunderstorm called a
mesocyclone. The mesocyclone draws energy into
the storm so it can last for hours. Scientists aren't sure why, but some
can create tornadoes. Mesocyclones can be detected by conventional
radar as a hook echo (
example--28K JPEG). In the
mesocyclone, air is drawn into the storm. Scientists believe a vertical
wind sheer (wind that changes direction with height) causes the tornado to
begin spinning. Most tornadoes spin
cyclonically but a few spin
anticyclonically.
Most tornadoes in the United States form in a section of the Great Plains
know as
Tornado
Alley. Strong fronts develop between cold polar air and warm
tropical air and when the atmosphere is
unstable tornadoes can
form.
Tornadoes form throught the year but most occur in May. Though, the most
damage is usually caused in April which means that the more dangerous
tornadoes form then. The more north you go, the later the main tornado
season becomes. The atmosphere in the norther plains is cooler and more
stable earlier in the year; it takes longer for the sunlight to heat it
up.
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