Hurricanes, Physical Geography of

Hurricane Winds

Strong winds are the defining characteristic of a hurricane. Wind is caused by the change in air pressure from one location to another. In the eye of a hurricane, the air pressure, which is the weight of a column of air from the surface of the Earth to the top of the atmosphere, is quite low compared with the air pressure outside the hurricane. This pressure difference causes the air to move inward, toward the center of the hurricane.

By a combination of friction, as the air rubs on the ocean below, and the spin of the Earth, as it rotates on its axis, the air spirals inward in a counterclockwise direction toward the region of lowest pressure. The vertical component of the Earth's spin is too weak to support a spiral within about 5° of latitude from the equator, so hurricanes do not develop close to the equator.

To a first approximation, the pressure difference between the eye and the air outside the hurricane determines the speed of the wind. Since the pressure outside the hurricane is roughly uniform, a hurricane's central pressure is another measure of a hurricane's intensity. The lower the central pressure, the more intense is the hurricane. Pressures inside the most intense hurricanes are among the lowest that occur anywhere on the Earth's surface at sea level.

Hurricanes vary widely in intensity, as measured by their fastest-moving winds. Hurricane intensities are grouped into five categories (Saffir-Simpson scale), with the weakest, Category 1, winds blowing at most 42 m/s and the strongest, Category 5, winds exceeding speeds of 69 m/s.

Three Category 5 hurricanes hit the United States during the 20th century, including the Florida Keys Hurricane in 1935, Hurricane Camille in 1969, and Hurricane Andrew in 1992.

...