by Francis Jock

Created on: September 29, 2007   Last Updated: February 05, 2009

The best way to describe how hurricanes form and where hurricanes occur begins with a look at the relationship between air temperature, atmospheric pressure, water vapor and wind. Hurricane researchers believe that two important factors are atmospheric pressure and high altitude air temperatures.

According to the Hurricane Research Center, a sudden drop in atmospheric pressure at the surface of the sea and a one to three degree increase in air temperature at 20,000 to 40,000 feet occurring around 24 hours before the storm develops are the primary catalysts in early stages of development of this powerful phenomenon. A third important factor contributing to be storm's development is the interaction of low and high altitude winds that circulate in a counter-clockwise direction around the center of a low-pressure area.

After decades of research, hurricanes are now being computer-modeled as gigantic heat engines, powered by temperature changes at the sea surface and driven by powerful high-altitude winds. In this model, rising air currents form a definable column of wind and rain that vents the warm surface heat energy upwards into the upper atmosphere.

The heat engine model exhibits the properties of a chimney with hot air convection currents that pick up moisture from the ocean's surface, propelling the vapor to high altitudes where the intense heat from the summer sun further heats them. At the same time, the moisture condenses, releasing more heat into the atmosphere, causing further increases in temperature within a defined area. This, in its own turn, enhances the rising air currents and upward motion of the water vapor. If there is sufficient upward motion and localized wind shears at both low and high altitudes, a low-pressure center develops at the surface level. Surface winds blowing into the low-pressure center generate a vortex about the center of the disturbance and results in the creation of a column of rising air, the so-called chimney effect. In the center of the chimney is the calm eye of the storm, which average around 14 miles in diameter. Because of the chimney effect, rotating winds rising from the surface to the altitudes between 20,000 and 60,000 feet high will steadily increase in velocity.

Upon reaching a surface wind velocity of 75 mph, these powerful summer storms become classified as hurricanes by the National Hurricane Center. Hurricanes are classified according to the Saffir-Simpson Hurricane Scale, which assesses the velocity of a storm's

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