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A tornado is formed when warm air that is holding a lot of moisture comes in contact with air that is dryer and cooler.
Warm air rises, so there is a sharp updraft in some thunderstorms. When a tornado forms, the warm moist air meeting with cooler air sometimes causes a "spin." When there is visible rotation, a wall cloud is formed.
A tornado is not always formed with this rotation, but about 30% of the time a tornado will be produced.