The word tornado is probably derived from the Spanish tronada ("thunderstorm"). Tornadoes are also popularly called twisters or cyclones and are characterized by rapidly rotating columns of air hanging from cumulonimbus clouds. They are generally observed as tube- or funnel-shaped clouds. At ground level they usually leave a path of destruction only about 50 m (170 ft) wide and travel an average of only about 8 to 24 km (5 to 15 mi). Contact with the ground is often of an intermittent nature — lasting for a period of usually less than a couple of minutes in any particular area — because the funnel tends to skip along.

Tornadoes generally exhibit a certain characteristic cycle of behavior between formation and final disappearance. The first sign of a tornado may be a strong whirlwind of dust from the ground surface, often in conjunction with the appearance of a short funnel growing from the storm cloud above it. The funnel then becomes more organized and descends further from the cloud, sometimes touching the ground. (The winds forming the funnel generally move counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, but exceptions are observed.) The funnel as a whole commonly moves forward slowly but can travel at speeds greater than 30 m (100 ft) per second. The tornado eventually becomes fragmented and dissipates.

Causes and Classification
Tornadoes are the result of great instability in the atmosphere and are often associated with severe thunderstorms. The full details of the formation of tornadoes are not known. The existence of a strong updraft, such as that generated by a severe thunderstorm, and the conservation of angular (rotational) momentum, however, are fundamental considerations. The falling of rain or hail probably drags air from aloft, and the resultant inrush of air tightens the rotational motion.

The tornado cyclone is an area of low pressure about 8 to 24 km (5 to 15 mi) in diameter, with wind speeds of approximately 240 km/h (150 mph) or less. At the cyclone's center is the tornado proper, the funnel that becomes darker as it picks up surface matter. The funnel exhibits exceedingly high winds and low pressures. Such winds can pick up and hurl objects with terrible force and cause tremendous damage to structures insufficiently well built to resist them. Wind speeds of 800 km/h (500 mph) or more have been inferred.

In the United States, tornadoes are most often associated with conditions in advance of cold fronts, and weather forecasts include tornado alerts when these conditions arise. Tornadoes can occur, however, ahead of warm fronts or even behind cold fronts. Tornadoes also occur frequently in association with hurricanes. A tornado that begins on land and then crosses water may be called a waterspout. That term, however, is applied more commonly to a less intense form of rotational activity that originates over a body of water and is not necessarily associated with storms. The water in the spout comes from condensation, not from the water below.

Tornadoes are now classified on the Fujita-Pearson scale, which links maximum wind speed, path length, and path width. A 0,0,0 tornado would have maximum wind speeds of below 117 km/h (73 mph), a path length of less than 1.6 km (1 mi), and a path width of no greater than 16 m (53 ft). A 5,5,5 tornado would have values of 420 to 512 km/h (261 to 318 mph), 161 to 507 km (100 to 315 mi), and 1.6 to 5.0 km (1.0 to 3.1 mi), respectively. The Fujita scale for damaging winds uses only the first digit of the Fujita-Pearson scale.

The greatest incidence of tornadoes is generally assumed to be in North America, and especially in the Mississippi Valley. On an equal-area basis, however, other countries, such as Italy, New Zealand, and the United Kingdom, exceed or at least challenge the incidence rate of the United States. In actual numbers observed, Australia, with about 20 annually, ranks second to the United States. The United States is notable for the incidence of severe tornadoes of scale 4 or 5. Tornadoes occurring in the tropics are usually extremely weak and often begin as waterspouts. The Stockholm and Saint Petersburg areas appear to be the northernmost regions that experience tornadoes.

Within the United States, Texas records the greatest number because of its size, usually about 15 to 20 percent of the nation's annual total of about 1,000. On an equal-area basis, however, Texas ranks ninth, far behind Oklahoma, Kansas, and Massachusetts. A rather steady increase in the annual total has been observed, probably as a result of the improving reporting system. The seasonal maximum occurs in spring and early summer, although tornadoes have been reported in all months. The height of activity in early spring is in the southern United States. Later it occurs in more northerly regions and, in July, in western Canada. Tornadoes occur most frequently during the middle and late afternoon. There is a large interannual variation, as well.

by John F. Griffiths

Bibliography: Battan, L. J., The Nature of Violent Storms (1961; repr. 1981); Bluestein, H. B., Tornado Alley: Monster Storms of the Great Plains (1999); Faidley, Warren, Storm Chaser: In Pursuit of Untamed Skies (1996); Fuller, J. G., Tornado Watch No. Two Eleven (1987); Navlikin, D. V., ed., Hurricanes, Storms and Tornadoes (1983); Stanford, J. L., Tornado, 2d ed. (1987); Weems, J. E., The Tornado (1977; repr. 1991).