Hurricanes develop over warm tropical oceans. The primary source of energy for a hurricane comes from the release of latent heat when warm air rises in the core and cold air sinks. This causes a conversion of heat and potential energy to kinetic energy (Miller 1967). The hurricane feeds on the warm waters of the tropical oceans and is driven by easterly flow of air. Without a strong westerly drift (<20 knots), a hurricane will intensify (NOAA 1993).
A young hurricane has winds that extend for thousands of square miles and is most intense at the lower levels where the winds follow a wide pattern like a whirlpool in which the winds accelerate the closer they get to the eye (NOAA 1993). These low-level circulation patterns are described in three distinct regions: 1. The outer portion which extends from the exterior to the interior regions of maximum winds. 2. The interior region surrounding the eye. This area has the most concentrated winds and is a defining characteristic of a mature hurricane. 3. The innermost area, the eye, in which the wind speed diminishes rapidly to almost calm. Hurricanes also extend vertically to close to the tropopause (14,000 to over 15,000 meters; above 40,000 ft). However, at the uppermost layers, the winds decrease, change direction, frequently become anti-cyclonic (Miller 1967), and sometimes help dissipate some of the energy of the hurricane (NOAA 1993).
The devastating winds associated with hurricanes develop when the atmospheric pressure or density changes drastically. The barometric pressure can drop from 30 to around 26 inches of mercury (1,016 to below 888 millibars) and cause a pressure force that induces a ninefold increase in pressure with a threefold increase in windspeed. Therefore, a 25 mph wind causes 1.6 lb. of pressure, but a 75 mph wind causes 450 lb. and a 125 mph wind causes 1,250 lb. of pressure. This force is enough to cause structural failure in many buildings and blow down and/or break large trees (NOAA 1993).
As a hurricane encounters colder waters (<80o F), the warm core cools and the storm weakens. Once it is over land, the storm breaks up quickly because it is cut off from its main source of energy, and frictional drag weakens and disrupts the circulation patterns. However, torrential rains may continue for an extended period even after the winds die. In fact, about 1/4 of the total annual rainfall of the southeastern US is attributed to diminishing hurricanes (NOAA 1993). Furthermore, this is not always the end of a hurricane. They can re-evolve as extratropical cyclones at higher latitudes or combine with temperate-zone disturbances/winds and form small tornadoes around the perimeter of a hurricane. Usually storms that move up the Atlantic coast and strike New England are in this transitional stage, and large continental lows can be energized by the remnants of these tropical storms (NOAA 1993).