How & Where Hurricanes Form

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.

Like all tropical cyclones, a hurricane needs the warm water of the tropics, which feeds a storm with energy, in order to form. The atmosphere must be laden with moisture.

Complete Coverage
Seasonal forecasts plus the science, facts and numbers behind nature's greatest storms.
Go >>>




The yellow and orange areas in this image from the summer of 2003 show warm water where hurricanes form. Africa is at the right, South America at bottom-left, and and the U.S. East Coast is in the upper-left.

Image: NASA


Saffir-Simpson Scale

Category

Winds (mph)

Storm Surge (feet)

1

74-95

4-5

2

96-110

6-8

3

111-130

9-12

4

131-155

13-18

5

155 +

18 +

LiveScience / SOURCE: NHC



Hurricanes from Above

The water must be at least 80 degrees Fahrenheit (26.5 Celsius) down to at least 150 feet (50 meters), scientists estimate.

Storm seeds

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 developing storm, providing energy that causes the warm air in the center to rise faster. It condenses high in the atmosphere, creating thunderstorms.

If conditions are favorable, a tropical depression develops 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.

Formation can be throttled or thwarted by, among other things, strong winds aloft that shear off a storm's chimney.

Counterclockwise, always

All low pressure systems, including hurricanes, rotate counterclockwise in the Northern Hemisphere.

Here's why:

When an area of low atmospheric pressure forms, wind begins to flow toward the center of it, near the surface. All the while, Earth is rotating under the atmosphere. In the Northern Hemisphere, Earth's spin causes an apparent deflection of the wind to the right, as seen from above.

This Coriolis effect, as it is called, forces a counterclockwise rotation for all storms in the Northern Hemisphere.

Storms south of the equator rotate clockwise.

You can visualize the effect by imagining an old-fashioned record album spinning on a turntable. Suppose you draw a chalk line from the edge of the record to the center while it is spinning -- the line will be curved even though your motion is straight.

Stop the storm

When hurricanes move over cooler northern waters, as while racing toward the U.S. Northeast, they loose strength as the fuel supply cools.

Storms also tend to pick up speed as they head north, and can reach forward speeds of 55 mph by the time they near Long Island. This forward speed is added to the internal wind speed on the right side of a storm, and it can take a Category 1 storm and effectively make it a Category 4 in a narrow region.

Hurricanes also run out of steam over land. Some people think it's due to friction. That's a myth.

The truth is a hurricane loses its fuel, the warm water, when it moves over land. In hours, a Category 4 or 5 storm can weaken to tropical storm status.

But the moisture picked up while offshore can continue to pour down copiously for days.

A hurricane over land is robbed not just of heat but of moisture source. That cuts down on the ability to produce thunderstorms near the center. This reduces convection, which also serves to weaken the storm.

Studies have found that curious things happen at landfall that are related to the friction myth, however.

Upon landfall, the top sustained winds -- those that last a minute or longer -- can drop due to a dampening effect caused by rough terrain -- bushes, trees and houses. But a corresponding increase in brief gusts sort of makes up for this, danger-wise. The gusts are enhanced by turbulence caused by the rough terrain, bringing faster winds to the surface in bursts that last a few seconds.