According to the Glossary of Meteorology (AMS; 2000), a tornado is “a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud. In order for a vortex to be classified as a tornado it must be in contact with the ground and at the cloud base.”
Approximately 1,000 tornadoes occur in the United States every year. The majority of tornadoes occur in Tornado Alley, an area in the central United States that stretches north from Texas to Iowa. The Southern US, however, has experienced tornado volumes in recent years that rivals Tornado Alley. Aside from the mountainous coastal regions of the western US, tornadoes can occur almost anywhere. Violent tornadoes have crossed almost every major river east of the Rocky Mountains and have struck major downtown areas.
Tornadoes are often accompanied by damaging hail, frequent lightning, heavy rain, severe non-tornadic winds, and flash floods.