What conditions are necessary for the formation of a tornado?

Tornadoes form in unusually violent thunderstorms when there is sufficient (1) instability and (2) wind shear present in the lower atmosphere. Instability refers to unusually warm and humid conditions in the lower atmosphere, and possibly cooler than usual conditions in the upper atmosphere.

In order for a tornado to form, the air must include warm moisture near the surface and relatively cold, dry air above.

Please correct me if I'm wrong :)

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