Stages of a Hurricane

A TROPICAL DISTURBANCE is the first stage of development of a hurricane. It consists of a mass of thunderstorms that have only a slight wind circulation. The tropical disturbance becomes a tropical depression when the winds increase to more than 20 knots or 23 miles per hour.

A TROPICAL DEPRESSION forms when a group of thunderstorms comes together under the right atmospheric conditions for a certain length of time. Winds near the center of the tropical depression are constantly between 20 and 34 knots (23 - 39 mph). Lowered pressure is indicated with at least one closed isobar on a surface pressure chart. Also, the organized circulation of wind in the center of the thunderstorms is detected.

This is a satellite picture of a tropical depression.
Tropical depressions look like individual thunderstorms that are grouped together.

A TROPICAL STORM forms when the maximum sustained winds have intensified to between 35-64 knots (39-73 mph). It becomes better organized and begins to look like a hurricane with a circular shape. At this point, the storm is given a name. Most of the problems from tropical storms come from heavy rainfall.

This is a satellite picture of tropical storm Charlie in 1998 over Texas. This storm dumped between 5-10 inches of rain over many southern Texas cities.

A HURRICANE finally forms when surface pressures continue to drop and when sustained wind speeds reach 64 knots (74 mph). There is also a definite rotation about the eye.

Hurricanes have definite parts that are easily recognized in this satellite animation.

Imax Movie-Fly Into a Hurricane

Listen to Max Mayfield who is Deputy Director of the National Hurricane Center in Miami, Florida as he gives a play-by-play account of the stages of a hurricane.

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Images on this page from the University of Illinois WW2010 Project.