How puffy clouds became a monster hurricane

It started as a thunderstorm over West Africa about two weeks ago. But an unusual kneading of natural extremes - from a larger-than-usual pool of warm water to a helmet of high pressure that opened a clear path across the Atlantic - coalesced to turn those puffs into one of the most powerful storms of the century.

The precise details of why hurricanes form - and why some become monsters - are still poorly understood. Indeed, favorable hurricane conditions often appear, yet only a tiny fraction of tropical thunderstorms turn into hurricanes - let alone enormous hurricanes like Floyd.

But right from the start, when Floyd drifted into the Atlantic and began to spin, there was something different about the storm that made scientists take note.

"People noticed it and said, 'Gee, that's a big disturbance,' " recalls National Weather Service meteorologist Frank Wells, referring to the pocket of low-pressure, unstable air that likely fed the hurricane's size.

The nascent Floyd, then merely a storm, soaked up energy from a large pool of warm, relatively deep water in the Atlantic Ocean. According to current theory, hurricanes only form in ocean water that is 79 degrees F. or warmer, and Floyd came from waters significantly warmer than that threshold.

"A hurricane is a heat engine," explains Gary Barnes, a meteorologist at the University of Hawaii who has studied tropical storms in the Atlantic and the Pacific. "To make it work, you take energy from a hot source and move it to a cold source. You are taking energy from the ocean and moving it up towards the cold upper atmosphere. So you need a warm ocean."

Floyd got another boost from a big high-pressure dome parked over the Atlantic. This dome acted like a shield, sheltering the storm from other weather fronts and giving it lots of room to grow as easterly trade winds pushed it toward the Caribbean.

"Think of it as a chimney," says Mr. Wells, who works in Guam. "The heat from the fire goes up the chimney and goes out aloft. If you put a lid on the chimney, the fire goes out."

At the same time the wind shear - differentiation between wind direction at different levels of the atmosphere - was minimal. High wind shear tends to snuff out hurricanes quickly by blowing the top off and destroying the column of rising hot air in that fuels the storm.

Only a handful of Category 5 hurricanes - the strongest hurricanes with sustained winds of more than 156 m.p.h. - have formed in the Atlantic during the past several decades. Only two have made landfall in the US this century.

Furthermore, there is significant variation within hurricanes themselves. While 1992's hurricane Andrew was a Category 4 storm like Floyd, it was much more compact. Its hurricane-force winds extended out from its eye far less than Floyd's, which reach out 120 miles - a very long reach for a powerful hurricane.

And two seemingly identical storms with the same wind speed and atmospheric pressure might project dramatically different winds a few miles out from their centers. "It's like the difference between a truck and a sports car. Both have 300 horsepower but they are very different critters," says Mr. Barnes. "Two hurricanes could look pretty much the same but one could have winds of 15 knots 20 miles out, while the other could have winds of 50 knots 20 miles out."

In the end, scientists say, making a monster hurricane is a difficult natural recipe to concoct and an even more difficult thing to predict.

"Even out here in the Western Pacific where we get more tropical cyclones than anywhere else in the world, you don't have them every day, even when all the conditions seem to be right," says Wells. "The model keeps generating them, but they don't occur. That's a head-scratcher."

(c) Copyright 1999. The Christian Science Publishing Society

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