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The Fujita Scale

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How Tornadoes Form
What causes tornadoes?

How do tornadoes form?

Why do some storms produce tornadoes and other do not?

These are common questions asked by weather watchers of all ages about tornadogenesis. With millions of dollars spent on weather research, these questions are still not easily answered.

Basics of Tornado Formation

Tornadoes are produced when two differing air masses meet. When cooler polar air masses meet warm and moist tropical air masses, the potential for severe weather is created. In tornado alley, air masses to the west are typically continental air masses meaning there is little moisture in the air. This warm, dry air meets the warm, moist air in the Central Plains creating a dryline. It is a well-known fact that tornadoes and severe thunderstorms often form along drylines.

Most tornadoes form during supercell thunderstorms from an intensely rotating updraft. It is believed that differences in vertical wind shear are contributors to the rotation of a tornado. The larger scale rotation inside the severe thunderstorm is known as a mesocyclone and a tornado is one extension of that mesocyclone. An excellent flash animation of tornado formation is available from USA Today.

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