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 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).[1][2][3]
Although tornadoes have been observed on every continent except Antarctica,most occur in the United States.[4] They also commonly occur in southern Canada,south-central and eastern Asia,east-central South America,Southern Africa,northwestern and southeast Europe,Italy,western and southeastern Australia,and New Zealand.[5]