Tornadoes are formed when warm air combines into storm clouds causing an updraft that mixes with a burst of colder air, which leads to rotation within the cloud. The down draft of cooler air causes the tornado to form, pulling more warm air from the ground. As the updraft strengthens, it mixes to create a spot of low pressure that pulls at the cloud's vortex and forms a funnel cloud.
The funnel cloud then becomes a tornado as it builds, usually within a few minutes, causing damage on the ground as it is continuously fed by more warm air. Tornadoes vary in shape and size and can be hundreds of miles wide. A tornado can last anywhere from a few seconds to over an hour and can travel dozens of miles during its life cycle.
Tornadoes are also called twisters. They are some of the most violent storms on earth, tearing paths of destruction seemingly at random. Tornadoes often kill large numbers of people. About 800 tornadoes occur in the United States annually, and they can form with little advance warning.
Tornadoes that form over water are referred to as waterspouts by the U.S. National Weather Service. They most often occur in the Florida Keys and parts of the Adriatic Sea and can form in both fair and poor weather conditions.