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Hurricane dynamics

Like all tropical cyclones, a hurricanes need the warm water of the tropics, which feeds the storms with energy.

In a mature hurricane, wind picks up warmth and moisture from the ocean, circling inward ever faster from outer cloud bands to the inner eyewall, where winds are the strongest and where it finally rises rapidly and is pushed out the top.

Here's a more detailed look at the process:

Most hurricanes that hit the United States begin either in the Caribbean or the Atlantic. Many of the worst start as seedlings coming off the coast of Africa.

In the beginning, a disturbance forms in the atmosphere, developing into an area of low atmospheric pressure. Winds begin to move into the center of the storm seedling from surrounding areas of higher air pressure. Warm water heats the air, and it rises as it nears the center.

The ocean feeds warmth and moisture into the storm, providing energy that causes the warm air in the center to rise faster. It condenses high in the atmosphere, creating thunderstorms. The tropical depression develops (if conditions are favorable) into a tropical storm, then finally into a hurricane, which is not unlike a giant swirling mass of thunderstorms.

As rising air in the storm's center condenses, it produces heat, forcing it to rise even faster. The air is pushed out the top -- much like smoke out the chimney of a fire -- and more air has to rush in at the surface to take its place. This kicks the ocean up more and, well, you can see that the storm essentially feeds on itself.

The ocean's role

While many factors control the number of hurricanes in a given season, forecaster Bill Gray has said the long-term trend is ruled by a conveyor belt of sorts that moves Atlantic ocean water northward from near the Caribbean to an area east of Greenland.

There, the current sinks deep, moves southward and flows into the southern Atlantic Ocean and beyond. It's the warm, northward-moving surface water -- fuel for any hurricane -- that helps produce strongest storms.

Recent measurements (in 1999) show that the water in the conveyor belt contains more salt than normal. This increases the water's density, researchers explain, causing it to sink to greater depths. This in turn increases the flow of deep water southward, which forces an increase in the flow of warm, tropical surface water northward. Thus, the northern Atlantic becomes warmer than normal, providing the heat needed to turn hurricanes into powerful killers. ez

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