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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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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA