How Hurricanes Work

This photo is a composite of three days' views (Aug. 23, 24 and 25, 1992) of Hurricane Andrew as it slowly moved across south Florida from east to west.

Photo courtesy NASA

Lifecycle of a Hurricane

Given the destruction the storm unleashes, it's easy to think of a hurricane as a kind of monster. It may not be a living organism, but it does require sustenance in the form of warm, moist air. And if a tropical disturbance continues to find enough of this "food" and to encounter optimal wind and pressure conditions, it will just keep growing.

It can take anywhere from hours to days for a tropical disturbance to develop into a hurricane. But if the cycle of cyclonic activity continues and wind speeds increase, the tropical disturbance advances through three stages:

  1. Tropical depression: wind speeds of less than 38 mph
  2. Tropical storm: wind speeds of 39 to 73 mph
  3. Hurricane: wind speeds greater than 74 mph

Between 80 and 100 tropical storms develop each year around the world. Many of them die out before they can grow too strong, but around half of them eventually achieve hurricane status.

Hurricanes vary widely in physical size. Some storms are compact, with only a few bands of wind and rain trailing behind them. Other storms are looser -- the bands of wind and rain spread out over hundreds or thousands of miles. Hurricane Floyd, which hit the eastern United States in September 1999, was felt from the Caribbean islands to New England.

Once a hurricane has formed and intensified, the only remaining path for the atmospheric juggernaut is dissipation. Eventually, the storm will encounter conditions that deny it the warm, moist air it requires. When a hurricane moves onto cooler waters at a higher latitude, gradient pressure decreases, winds slow, and the entire storm is tamed, from a tropical cyclone to a weaker extratropical cyclone that peters out in days.

That important supply of warm, moist air also vanishes when the hurricane makes landfall. Condensation and the release of latent heat diminishes, and the friction of an uneven landscape decreases wind speeds. This causes winds to move more directly into the eye of the storm, eliminating the large pressure difference that fuels the storm's awesome power.