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How Do Tornadoes Form?

If someone mentions the word tornado, people conjure up a picture of a roaring twister crashing through a field in the Midwest. You may have heard of Tornado Alley, the most active tornado zone in the U.S. But how do tornadoes form?

Basically, when hot air collides with cold air, the cold air shifts above the hot air. Hot air rises through the cold air (since hot air naturally rises), creating an updraft. This will produce a typical thunderstorm.

Example of how a tornado forms
If there is a lot of shear, then this hot and cold air colliding creates a spin. It starts out horizontal though. While somewhat unpredictable, they can become vertically spinning and produce a tornado.