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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