What Causes a Hurricane to Form?

Hurricanes form when rising warm, moist air displaces colder air high in the atmosphere. The cold air drops down on all sides of the warm spot, swirling slightly as it falls, then becomes warm and moist itself, repeating the process. Over time, the swirling grows into a hurricane.

Hurricanes rarely form unless the warm water they need is abundant, at least 80 degrees Fahrenheit and surrounded by still air or calm wind blowing in a consistent direction. For this reason, they always form over warm ocean waters in tropical areas. The warming and rising process leaves a low pressure area. When the low pressure area hits a certain size and pressure level, meteorologists refer to it as a "tropical depression," the first stage toward becoming a hurricane. In order for the hurricane to form, however, the pressure must drop still more as warm air rises faster, displacing more cool air and creating more spin.

As the depression continues toward hurricane, an eye is formed. This is a very low pressure area in the hurricane that is far enough away from the cooler, dropping air to be unaffected. A large, well-formed eye indicates that a storm has plenty of energy for destruction. As the storm moves and the eye is disrupted, usually when it moves onshore and loses its warm water supply, the storm grows weaker, eventually dwindling into a tropical depression once more.