Newspaper article The Christian Science Monitor

Global Weather Factors Set Hurricane Factory Spinning EYE ON THE STORM

Newspaper article The Christian Science Monitor

Global Weather Factors Set Hurricane Factory Spinning EYE ON THE STORM

Article excerpt

THERE'S good news about the North Atlantic hurricane season that runs from June through November. The outlook is for tropical cyclone activity generally to be a bit below normal.

Veteran hurricane forecaster William Gray of Colorado State University at Fort Collins says he expects nine tropical storms strong enough to be given names. Five of them should be hurricanes, including one intense hurricane. The long-term average is nine named storms, including six hurricanes, two of which are intense. Also, Dr. Gray expects 35 days when named tropical storms are active - including 15 hurricane days. The long-term average is 46 storm days, including 23 hurricane days.

There are caveats. Gray's forecasting record is good, with only two outright failures in the past 10 years. But he warns that his scheme could fail in any given year. More important, it doesn't tell when or where a hurricane may form or hit land. Also, the scheme doesn't work for the Pacific or Indian Oceans.

Gray and his colleagues have found several global factors that influence North Atlantic tropical-storm formation. These include El Nino - the presence of warm surface water in the equatorial Pacific and associated wind patterns that discourage North Atlantic hurricane activity. Other factors are atmospheric patterns that influence West African rainfall - drought, for example, indicates low tropical-storm activity - and the direction of stratospheric wind patterns.

Hurricanes are an intense form of tropical cyclone that occur in the North Atlantic, eastern and western North Pacific (where they are called typhoons), and in the Western South Pacific and Indian Ocean (where they are simply called cyclones). They are unknown in the eastern South Pacific and South Atlantic Oceans.

Tropical cyclones don't form when the sea surface is cooler than about 80 degrees F. They never form within 4 or 5 degrees latitude of the equator, where the influence of Earth's rotation is too weak to spin up their winds. Only about 13 percent of the storms form above 22 degrees latitude, about the latitude of Cuba.

Some 50 to 75 percent of the tropical cyclones that arise annually achieve the 74 m.p.h.-minimum wind speed to be classed as typhoons or hurricanes. Also, most Atlantic hurricanes occur during the so-called "intense" part of the hurricane season - August to October. Gray will update his forecast in early August.

WITHIN about 250 miles of a developing hurricane, inflowing air begins to converge strongly. Towering convective cumulus clouds, which form the main body of the storm, appear. …

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