A tornado is a violently rotating column of air which is in contact with both a cumulonimbus (or, in rare cases, cumulus) cloud base and the surface of the earth. Tornadoes can come in many shapes, but are typically in the form of a visible condensation funnel, with the narrow end touching the earth. Often, a cloud of debris encircles the lower portion of the funnel.

Tornadoes can be the most destructive storms on earth. Most have winds of 112 mph (180 km/h) or less, are approximately 250 feet (75 meters) across, and travel a mile (1.6 km) or more before dissipating. However, some tornadoes can have winds of more than 300 mph (500 km/h), be more than 2 miles (3 km) across, and stay on the ground for dozens of miles (more than 100 kilometers). Williams, Jack. "Doppler radar measures 318 mph wind in tornado." USA Today. May 17, 2005."Hallam Nebraska Tornado." National Weather Service Weather Forecast Office, Omaha/Valley, NE. November 2, 2005. Edwards, Roger. "The Online Tornado FAQ." Storm Prediction Center. April 4, 2006.

They have been observed on every continent except Antarctica; however, a significant percentage of the world's tornadoes occur in the United States. Perkins, Sid. "Tornado Alley, USA." Science News. May 11, 2002. This is mostly due to the unique geography of the country, which allows the conditions which breed strong, long-lived storms to occur many times a year. Other areas which often experience tornadoes are south-central Canada, northwestern Europe, east-central South America, South Africa, and south-central Asia. "Tornado." Encyclopædia Britannica. 2006.