How a Hurricane Forms

Q: Who is in charge of naming hurricanes? A: The names are chosen by the World Meteorological Organization.

Hurricanes form in tropical regions where there is warm water (at least 80 degrees Fahrenh­eit / 27 degrees Celsius), moist air and converging equatorial winds. Most Atlantic hu­rricanes begin off the west coast of Africa, starting as thunderstorms that move out over the warm, tropical ocean waters. A thunderstorm reaches hurricane status in three stages:

• Tropical depression - swirling clouds and rain with wind speeds of less than 38 mph (61.15 kph / 33 kt)
• Tropical storm - wind speeds of 39 to 73 mph (54.7 to 117.5 kph / 34 to 63 kt)
• Hurricane - wind speeds greater than 74 mph (119 kph / 64 kt)

It can take anywhere from hours to several days for a thunderstorm to develop into a hurricane. Although the whole process of hurricane formation is not entirely understood, three events must happen for hurricanes to form:

• A continuing evaporation-condensation cycle of warm, humid ocean air
• Patterns of wind characterized by converging winds at the surface and strong, uniform-speed winds at higher altitudes
• A difference in air pressure (pressure gradient) between the surface and high altitude

Warm, moist air from the ocean surface begins to rise rapidly. As this warm air rises, its water vapor condenses to form storm clouds and droplets of rain. The condensation releases heat called latent heat of condensation. This latent heat warms the cool air aloft, thereby causing it to rise. This rising air is replaced by more warm, humid air from the ocean below. This cycle continues, drawing more warm, moist air into the developing storm and continuously moving heat from the surface to the atmosphere. This exchange of heat from the surface creates a pattern of wind that circulates around a center. This circulation is similar to that of water going down a drain.

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

"Converging winds" are winds moving in different directions that run into each other. Converging winds at the surface collide and push warm, moist air upward. This rising air reinforces the air that is already rising from the surface, so the circulation and wind speeds of the storm increase. In the meantime, strong winds blowing at uniform speeds at higher altitudes (up to 30,000 ft / 9,000 m) help to remove the rising hot air from the storm's center, maintaining a continual movement of warm air from the surface and keeping the storm organized. If the high-altitude winds do not blow at the same speed at all levels -- if wind shears are present -- the storm loses organization and weakens.

High-pressure air in the upper atmosphere (above 30,000 ft / 9,000 m) over the storm's center also removes heat from the rising air, further driving the air cycle and the hurricane's growth. As high-pressure air is sucked into the low-pressure center of the storm, wind speeds increase.

In the next section, we'll take a closer look at the parts of a hurricane.