Tornadoes

A tornado is a rapidly-spinning column of air which is in contact with both a cumulonimbus (or, in rare cases, cumulus) cloud base and the surface of the earth. It can vary in size from only a few metres to over 1,000 metres in diameter and its track may be from a few hundred metres to more than 100 kilometres. It can last from only a few minutes to an hour or more.

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.

Inside the air column, there is a very powerful updraught which causes very low atmospheric pressure at the surface. Immediately outside the column, winds are rotating (anticlockwise in the northern hemisphere, clockwise in the southern) at speeds that can be higher than 90 metres per second (200 miles per hour).

Tornadoes have been observed on every continent except Antarctica; however, a significant percentage of the world's tornadoes occur in the USA. 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.