Tornadoes: a working definition

A tornado, also known as a twister, is a rapidly rotating column of air that extends vertically from the surface of the Earth to the base of a cumulonimbus or, less commonly, cumulus cloud. Tornadoes are often visible in the form of a condensation funnel originating from the cloud base, with a cloud of rotating debris and dust close to the ground. Most tornadoes have wind speeds less than 180 km/h (110 mph), are about 80 m (250 ft) across, and travel several kilometres before dissipating. The most extreme tornadoes can attain wind speeds of more than 480 km/h (300 mph), can be more than 3 km (2 mi) in diameter, and can stay on the ground for more than 100 km (62 mi).

Types of tornadoes include the multiple-vortex tornado, the landspout, and the waterspout. Waterspouts are characterised by a spiralling funnel-shaped wind current connecting to a large cumulus or cumulonimbus cloud, and are generally classified as non-supercellular tornadoes that develop over bodies of water. Similar phenomena in nature include the gustnado, dust devil, fire whirl, and steam devil — none of these are true tornadoes, although they share visual features.

Tornadoes most commonly occur in North America, particularly in the central and south-eastern United States. The country has by far the most tornadoes of any nation in the world. Tornadoes also occur in South Africa, much of Europe, parts of Australia and New Zealand, Bangladesh and adjacent eastern India, Japan, the Philippines, and south-eastern South America. They can be detected before or as they occur through pulse-Doppler radar by recognising velocity and reflectivity patterns such as hook echoes or debris balls, and through the work of trained storm spotters.

This pack is general reference. In any actual severe-weather event, follow instructions from local emergency services and the National Weather Service.