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Facts of the Matter

BY RICHARD BRILL

Sunday, November 17, 2002



Tornado is
weather
at its worst


Last week, more than 70 tornadoes cut swaths of death and destruction stretching from Louisiana to Pennsylvania, a testament to the fact that tornadoes are the most violent and destructive of all weather phenomena.

Tornadoes produce wind speeds that can reach 300 mph in the most extreme cases, capable of lifting cars, buses and livestock like toys, ripping homes and other buildings to shreds and turning broken glass and other objects into lethal projectiles. Simply defined, a tornado is a vortex of air that rises into a cloud.

Tornadoes are known just about everywhere, but in the United States they are most common in the plains between the Rocky mountains and the Appalachians. They are especially prone to occur in a band west of the Mississippi from Texas into Minnesota known as "tornado alley." They occur most often in April, May and June but can occur any time of the year. In an average year around 1,000 tornadoes are reported nationwide, resulting in 80 deaths, 1,500 injuries and billions of dollars of damage.

Although certain atmospheric conditions are known to spawn tornadoes, the exact conditions are not completely understood. The presence of those conditions can lead to forecasts that tornadoes are likely to occur, but it is not possible to tell exactly when and where they will develop, how strong they will be or precisely what path they will follow.

Tornadoes are almost always associated with thunderstorms, and all thunderstorms are characterized by rising air called updrafts. Most tornadoes in the United States form when warm, moist air from the Gulf of Mexico meets cold, dry air from Canada. But this happens all of the time without generating tornadoes as cold fronts sweep down from the north and across the continent.

Many thunderstorms form along the interface between cold and warm air known as a front. Some of the ensuing storms are quite violent, yet only rarely does a thunderstorm produce a tornado.

For a tornado to form there must be source of instability such as heat or moisture, and a large-scale rotation of the rising air. Rising air cools, and air is said to be unstable if it remains warmer than the surrounding air as it rises. Unstable air will continue to rise once it starts moving upward, releasing large amounts of heat as water vapor condenses from the air to form clouds.

The instability of most tornadoes that form on the Great Plains comes from the high moisture content of warm air that has flowed onto the continent over the Gulf of Mexico. The warm, moist air is uplifted by cold air tunneling under it along the cold front. If there is cold air and an upper-level disturbance above the rising air, they add to the instability.

Upper-level disturbances are pockets of rapidly rotating air that ripple along the jet stream. They can spawn storms but are not typically associated with any surface weather features.

Tornadoes are not common in Hawaii, but we have had a few. Between 1950 and 1995 a total of 28 tornadoes were reported, making Hawaii 48th in the nation for frequency of tornadoes. Most of the time, descending air in the atmosphere above us keeps a lid on updrafts and prevents thunderstorms from developing.




We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu



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