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Whether it's a slender, graceful tube or a massive, roaring wall of cloud, the tornado is among the most fascinating and frightening of atmospheric phenomena. On average, about 1,000 tornadoes touch down in the United States each year, by far the highest frequency reported for any nation in the world. However, U.S. death and injury tolls have dropped considerably in recent decades due to better warnings. Scientists at NCAR and elsewhere are trying to unravel the causes of these evanescent whirlwinds.

Not all tornadoes are alike. At left is an example of the tornadoes often called "landspouts," which tend to be relatively weak. This one was photographed near Denver on 30 June 1987. Below is a powerful tornado that struck in the Texas Panhandle on 2 June 1995. Such violent twisters originate from supercell thunderstorms and occur most often in the central and eastern United States.
Photo by Harald Richter.

Virtually all tornadoes develop out of thunderstorms. Dust devils, the rotating columns of dust you might see on a hot, sunny day, are only distant cousins of tornadoes. Hurricanes--far larger and more complex systems that develop over the tropical ocean--are another distant relative. All of these are marked by rising warm air and rotation (usually counterclockwise). Strong thunderstorms tend to form on a boundary between air masses, such as a cold front, that pushes surface air upward. Moisture in the warm air adds to the air's potential buoyancy.

As springtime unfolds, the clashing of warm and cold air results in frequent tornado activity in the Gulf Coast states in March, with the zone moving northward and westward to the upper Midwest by June and July. Tornadoes are rare west of the Continental Divide, where moisture for strong storms is usually lacking. However, every state in the union, including Alaska and Hawaii, has reported tornadoes. On the High Plains of eastern Colorado, the most frequent type of tornado is a ''landspout''--a relatively weak vortex somewhat like the waterspouts that occur at sea.

It takes more than rising air in a thunderstorm to create a tornado. Somehow, the air must be given a spin. Thunderstorms often develop weak rotation as strong winds aloft, sometimes racing eastward at 160 kilometers (100 miles) per hour or more, impart a spin to the column of rising air. On one side of a severe storm, you can sometimes see clouds moving in a circular fashion. A "wall cloud" may hang from a larger rain-free cloud base. Large hail and heavy rain may occur near the wall cloud, and winds can be blowing upward at 160 kilometers per hour.

What finally produces a tornado? By some mechanism not fully understood, the circulation in the storm's lower levels may tighten into a narrow cylinder, elongate, and speed up, much as figure skaters spin faster by pulling in their arms. Computers can simulate the large-scale rotation of a thunderstorm, and a newer NCAR computer model examines the motions in an area as small as a half kilometer (a quarter of a mile) across, providing insight on the preexisting circulations that can tighten into a tornado. Doppler radar, able to sense the winds in the vicinity of a tornado, is another valuable tool. One portable Doppler radar trained on tornadoes in Oklahoma has measured winds of over 450 kilometers (280 miles) per hour--the fastest winds ever measured near the earth's surface.

Researchers at NCAR and elsewhere collect many kinds of information on tornadoes by going into the field. On a storm "chase," scientists take instruments and cameras into the countryside and attempt to document tornadoes as thoroughly as possible. Wall clouds and other visual precursors of tornadoes discovered by chasers are now used by law enforcement and civil defense agencies as clues to issue warnings. The most damaging twisters are often spotted well before they strike populated areas, enabling residents to take cover. The homes of some 20,000 people lay in the path of the 1979 Wichita Falls, Texas, tornado, but extensive warnings kept the death toll to fewer than 50. Simple construction techniques, such as bolting roofs to walls, can also reduce tornado damage and injury.

The outward appearance of tornadoes can vary tremendously, even for the same storm viewed at different times or from different angles. A complete funnel need not be visible from cloud to ground for a tornado to be capable of damage. Large tornadoes may resemble a cloud on the ground rather than a narrow funnel. Especially in the eastern United States, rain may wrap around a tornado's circulation and cut off visibility.

Tornado Safety

The cardinal rule for tornado safety is: get as close to the ground as you can. If inside a building that lacks a basement, find a small, centrally located room such as a bathroom; avoid auditoriums and other places with large free-span ceilings that may collapse. Cover your head and get under a mattress or blanket to help protect yourself from flying debris. If you are in an automobile, drive at a right angle to the tornado's direction of approach; if time is short, abandon the car and find a ditch or other low-lying area. Automobiles and mobile homes are where most tornado fatalities occur. Keep in mind that tornadoes can travel at 110 kilometers (70 miles) per hour or faster and can change direction without warning. They usually--but not always--move from west to east or from southwest to northeast.

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Edited by Rene Munoz, munoz@ucar.edu

Prepared for the Web by Jacque Marshall
Last revised: Mon Apr 10 14:14:36 MDT 2000