A tornado is a violently rotating column of air which is in contact with both a cumulonimbus cloud or, in rare cases, a cumulus cloud base and the surface of the earth. Tornadoes come in many sizes but are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris.

Most tornadoes have wind speeds of 110 mph (177 km/h) or less, are about 250 feet (75 m) across, and travel a few miles (several kilometers) before disappears. Some reach wind speeds of more than 300 mph (480 km/h), stretch more than a mile (1.6 km) across, and stay on the ground for dozens of miles (more than 100 km).

Although tornadoes have been observed on every continent except Antarctica, most occur in the United States. They also commonly occur in southern Canada, southcentral and eastern Asia, east-central South America, Southern Africa, north-western and central Europe, Italy, western and south-eastern Australia, and New Zealand.