The chief characteristics of these violent tropical storms are low central pressures and high wind velocities. A tropical hurricane is a nearly circular vortex averating 500 to 600 km in diameter. It extends about 12,000 meters above the ocean surface.
The hurricane lasts for many days and, in certain cases, for more than a week. The central pressure in a well- developed hurricane may be 50 to 60 millibars lower than the pressure at its outer edge.
The lowest pressure ever recorded in the United States was 892.31 millibars, which was measured during a hurricane in September, 1935.
According to Trewartha, there is a spiraling inflow of air at lower levels, a rapid upward movement at intermediate levels, and a spiraling outward flow aloft. It is the steep pressure gradient which causes the rapid, spiraling winds of a tropical storm.
From the central low pressure core of the cyclone, winds converging from all directions are of whirled upward. As a result of the lifting of air, condensation starts producing cumulonimbus or clouds which give the inner structure of the storm a peculiar shape. Spiraling bands of cumulonimbus clouds surround the core of the hurricane.
The top of the hurricane is marked by divergent air flow which carries the ascending air away from the storm centre. This phenomenon is essential to maintain the inward flow at the surface.