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Small swirls spin hurricane's top winds.

Flying extremely low over areas damaged by Hurricane Andrew, a meteorologist has discovered evidence of a previously unknown whirlwind pattern that develops during hurricanes and creates the most dangerous conditions during those storms. T. Theodore Fujita of the University of Chicago reports that these small vortices produced winds of up to 200 miles per hour that cut a narrow path of severe destruction during Hurricane Andrew.

Fujita, who has spent decades studying tornadoes, identified the new phenomenon by examining high-resolution photographs he snapped while flying only 400 feet above areas damaged by the storm in August 1992. In the past, he says, most meteorologists have attributed the worst hurricane damage to strong gusts. But the damage patterns in the photographs did not follow the straight parallel lines that gusts would produce, Fujita says. Instead, the worst damage patterns exhibit a curved trace, evidently produced by swirling vortices carried along by the hurricane's normal winds, he says.

Fuiita proposes that the eye wall of the hurricane-the region of strongest winds surrounding the calm "eye"-spawns small, swirling eddies that measure only 500 feet in diameter. If these eddies drift into the eye, they are harmless. But if they pass under the fast-growing clouds outside the eye, the cloud convection pulls air upward, stretching the vortex skyward and causing it to tighten. Like spinning skaters who pull their arms inward, the tightening vortex swirls faster, reaching speeds of up to 80 miles per hour during Hurricane Andrew. Because these spinning features were carried along by the 120-mile-per-hour winds of the hurricane, they produced combined winds of 200 miles per hour, Fujita says.

The discovery of these so-called spin-up vortices has implications for building critical structures, such as nuclear power plants, in hurricane-prone regions, Fujita says. But it would be impossible to construct most buildings to withstand such high winds, which strike only limited regions during a hurricane. Instead, says Fujita, the discovery of spin-up vortices adds more incentive for residents to evacuate when officials call for such action.
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Title Annotation:small, swirling eddies create most dangerous conditions
Publication:Science News
Article Type:Brief Article
Date:Feb 6, 1993
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