Cause of Tornadoes

The conditions, of the tornado, happen when various temperatures and humidity come together to form thunderclouds. There are warmer wet winds that move northward in the warmer seasons. When these warmer winds meet colder dryer winds that move southward, they create a dry line. The dry air piles on the moist air. The warmer winds attempt to rise, but the cooler air blocks them. When this takes place the warm air rotates horizontally between the two winds. Simultaneously, the sun warms the earth, causing more air to try and rise. The warm wind then becomes strong enough to move up through the cooler layer.

(library.thinkquest.org/C003603/english/tornadoes/causesoftornadoes.shtml)