Most tornadoes have wind speeds between 40 mph (64 km/h) and 110 mph (177 km/h), are approximately 250 feet (75 m) across, and travel a few miles (several kilometers) before dissipating. Some attain 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, south-central and eastern Asia, east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand.

Definitions

Tornado

A tornado is defined by the Glossary of Meteorology as "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud..." In practice, for a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base. Scientists have not yet created a complete definition of the word; for example, there is disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes.

Condensation funnel

A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (see Bernoulli's principle) and rapid rotation (due to cyclostrophic balance) usually causes water vapor in the air to become visible as a condensation funnel. The tornado is the vortex of wind, not the condensation cloud.

A funnel cloud is a visible condensation funnel with no associated strong winds at the surface. Not all funnel clouds evolve into a tornado. However, many tornadoes are preceded by a funnel cloud. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to discern the difference between a funnel cloud and a tornado from a distance.

Tornado family

Occasionally, a single storm will produce more than one tornado, either simultaneously or in succession. Multiple tornadoes produced by the same storm are referred to as a tornado family.