by Lea Miller

Created on: May 28, 2010

The meteorologist appears on your local news station with a concerned look on his face. “We are closely monitoring this area of low pressure for development, and will keep you informed as conditions change,” he says. Each year millions of people living in areas susceptible to hurricanes wait anxiously to see if the storm will get stronger or if it will make a turn toward their hometown.

But how does a hurricane come to be and what stages does it pass through as it develops?

Hurricanes are formed from simple thunderstorms, but they can only grow and become stronger with the right conditions in the ocean and the atmosphere. The official hurricane season in the Atlantic is June 1 through November 30, but hurricanes can and do occur outside these dates when conditions are right.

First, the ocean water must be warmer than 80 degrees Fahrenheit, and this temperature must exist throughout the water column down to a depth of about 150 feet. The heat and moisture from this ocean water is the hurricane’s source of energy.

Second, wind shear is an important factor. Wind shear is a change in speed and direction over a short distance. When the wind shear is weak, the storm grows vertically, and heat lost from condensation is released directly above the storm, encouraging development. When the wind shear is strong, the storm is less centered and the heat lost from condensation is spread over a much wider area, discouraging development.

Third, rotation is necessary. Rotation is started when the air within the storm is pulled inward toward the center. In conjunction with the Coriolis effect, the rotation causes the storm to begin a counterclockwise movement (in the Northern Hemisphere).

There are five measured stages of development as the storm system gradually becomes more organized and stronger.

The first stage of storm development is a Tropical Wave. This is a low pressure trough of persisting winds blowing in a westerly direction, lacking a closed circulation, with wind speeds less than 25 mph. Atlantic storms often form off the coast of Africa, leaving them thousands of miles for development as they move westward.

As warm, moist air over the ocean rises in the low pressure area, cold air from above replaces it. This produces the second stage, a Tropical Disturbance, which is a concentrated area of thunderstorms from 100 to 300 miles in diameter. It is accompanied by heavy rain and winds. At this point, the storm is still quite disorganized.

The third

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