Monday, December 20 1999

How a hurricane forms
What everyone wants to know - how a hurricane forms

HERALD STAFF REPORT



In the eastern Pacific, hurricanes begin forming by mid-May, while in the Atlantic Ocean, Caribbean Sea and Gulf of Mexico, hurricane development starts in June.

For the United States, the peak hurricane threat exists from mid-August to late October, although the official hurricane season extends through November. Over other parts of the world, such as the western Pacific, hurricanes can occur year-round.

Developing hurricanes gather heat and energy through contact with warm ocean waters. The addition of moisture by evaporation powers the storms like giant heat engines.

In addition to warm water and moisture, the wind pattern near the ocean surface that spirals water inward provides the force for a hurricane to develop. Bands of thunderstorms from these winds allow the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this storm will grow and strengthen.

The center, or eye, of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall.

At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.




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