How Do Hurricanes Form?

diagram of hurricane formation

Hurricanes form through an exchange of warm, humid air and cold, unstable air between the upper and lower atmosphere. (Diagram from NASA)

Hurricanes begin when areas of low atmospheric pressure move off Africa and into the Atlantic, where they grow and intensify in the moisture-laden air above the warm tropical ocean. Air moves toward these atmospheric lows from all directions and curves to the right under the influence of the Coriolis effect, thereby initiating rotation in the converging windfields. When these hot, moist air masses meet, they rise up into the atmosphere above the low pressure area, potentially establishing a self-reinforcing feedback system that produces weather systems known to meteorologists as tropical disturbances, tropical depressions, tropical storms, and hurricanes.

Fortunately, fewer than 10 percent of disturbances grow into hurricanes. Development of a full-fledged hurricane requires a rare combination of atmospheric events. First, the tropical disturbance must produce converging air masses. Second, the converging air must rise – but not in an area where there are either strong winds or descending air masses aloft. Hurricane development requires both an organized pattern of convection that is destroyed by upper atmosphere winds, as well as unstable air masses in the upper atmosphere that can carry rising surface air away from the upper end of the developing storm.

If these three phenomena occur together, a self-sustaining circulation develops in which moist surface air rises and its moisture condenses, releasing latent heat that warms the upper atmosphere. The heated atmosphere creates lift that extends the low pressure area upward and further reduces its already low pressure. As winds in the upper atmosphere carry moist air away from this growing cylinder of low pressure, dry warm air from above can enter the center of the cylinder, ultimately reaching the sea surface and forming the cloud-free area known as the eye of the hurricane.

A system of this type will continue to intensify as long as the upper-level outflow of air exceeds low-level inflow. The relationship between inflow and outflow is controlled by the heat content of the ocean water and the latent heat contained in the moisture in the rising air. In other words, once formed, hurricane circulation will continue as long as the storm is over warm water, has access to moist air, and doesn't drift into areas where upper-level winds can tear it apart.

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