According to Wikipedia, "a tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud." They form from the clouds that cause thunderstorms turning to supercell storms from rotating air that causes the clouds to form a funnel shape that drops to the ground and moves at violent wind speeds. They mainly occur in spring and summer months. This is why so many tornado breakouts are occuring now. With climate change it can worsen and be more destructive in comings years. Tornadoes are very unpredictable and disasterous. I just hope they continue to work on ways to get a stronger prediction to save people's lives.
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Here's a Wikipedia article.