Hurricane Structure

A hurricane, one of nature's most destructive forces, is a closed cyclone of tropical origins with sustained winds of 74 mph or greater. These massive storms (also called typhoons and cyclones) develop and feed off of the heat and moisture released from the warm tropical waters at lower latitudes.

Before developing into a hurricane, a tropical system must go through a rather complex development process. Often times, the birth of a tropical cyclone begins with an increase in thunderstorm activity within a westward moving tropical wave. As the thunderstorms continue to grow, surface pressures within the area of disturbed weather begin to drop, and eventually a surface low pressure may form. Once this low pressure develops a well-defined, closed circulation and winds increase within it, it is deemed a tropical depression by the Tropical Prediction Center.

As the development process continues, thunderstorm activity and overall cyclonic organization increases and improves. The system absorbs increasing amounts of heat and moisture from the ocean surface. When the system is found to have sustained winds of 37 mph or higher, it is categorized as a tropical storm and is given a name.

If the conditions are right, (most importantly sea surface temperatures at or above 80°F, and favorable winds aloft) the tropical storm will continue to develop and its overall structure will improve. If sustained winds near the center reach 74 mph, the system is upgraded to a hurricane. As long as the system remains over warm water and upper level winds remain light (strong winds high in the atmosphere will retard the development of thunderstorms, thus impeding the structure of the hurricane), the system can continue to grow. The Saffir-Simpson Scale was developed to help categorize these storms as they mature.

One of the most well-recognized and well-known parts of a hurricane is the “eye.” The eye of the hurricane is the center of circulation, where sinking air produces relatively light winds, very little precipitation and sometimes even a glimpse of sunshine. The eye of the hurricane can range from only a few miles in diameter to well over 20 miles across. In the most well developed storms, the eye resembles a near perfect circle on satellite imagery.

Immediately surrounding the eye is the dangerous “eyewall.” The eyewall is the region in a hurricane containing the storm's most powerful winds and heaviest rains. These intense conditions are associated with the greatest convective activity within the cyclone.

Further away from the eye and the eyewall is the region known as the “CDO” or “Central Dense Overcast.” The central dense overcast is a region of thunderstorm activity that extends outward from the center of the storm in all directions. Depending on the size, strength and structure of a hurricane, the central dense overcast can contain hurricane conditions. However, often times, the weather is more squally in nature, with only the most intense bands of thunderstorms producing hurricane conditions.

Another tell-tale sign of a well-developed hurricane is a cocentric “outflow” pattern. Even though these monster storms produce some of the lowest barometric pressures on the surface of the Earth, they actually contain a region of high pressure above them, which enables them to survive. The area of high pressure aloft creates light winds which allow the thunderstorm activity to grow higher in the atmosphere and the rising air to evacuate the system. This anticyclonic spin aloft will carry high cirrus clouds in a clockwise fashion out and away from the center, creating the outflow region.