Tropical cyclones (hurricanes, typhoons) form and grow over
warm ocean water, drawing their energy from latent heat. Latent heat is the
energy released when water vapor in rising hot, humid air condenses into clouds
and rain. As warmed air rises, more air flows into the area where the air is
rising, creating wind. The Earth’s rotation causes the wind to follow a curved
path over the ocean (the Coriolis effect), which helps give tropical cyclones
their circular appearance.
Hurricanes and tropical cyclones form, maintain their
strength, and grow only when they are over ocean water that is approximately
27°C (80°F). Such warmth causes large amounts of water to evaporate, making the
air very humid. This warm water requirement accounts for the existence of
tropical cyclone seasons, which occur generally during a hemisphere’s summer
and autumn. Because water is slow to warm up and cool down, oceans do not
become warm enough for tropical cyclones to occur in the spring.
Oceans can become warm enough in the summer for hurricanes
to develop, and the oceans also retain summer heat through the fall. As a
result, the hurricane season in the Atlantic Basin, which comprises the
Atlantic Ocean, Caribbean Sea, and the Gulf of Mexico, runs from June 1 through
November 30. At least 25 out-of-season storms, however, have occurred from 1887
through 2003, and 9 of these strengthened into hurricanes for at least a few
hours.
Hurricanes weaken and die out when cut off from warm, humid
air as they move over cooler water or land but can remain dangerous as they
weaken. Hurricanes and other tropical cyclones begin as disorganized clusters
of showers and thunderstorms. When one of these clusters becomes organized with
its winds making a complete circle around a center, it is called a tropical
depression.
When a depression’s sustained winds reach 63 km/h (39 mph)
or more, it becomes a tropical storm and is given a name. By definition, a
tropical storm becomes a hurricane when winds reach 119 km/h (74 mph) or more.
For a tropical depression to grow into a hurricane, winds
from just above the surface of the ocean to more than 12,000 m (40,000 ft) in
altitude must be blowing from roughly the same direction and at the same speed.
Winds that blow in opposite directions create wind shear—different wind speeds
or direction at upper and lower altitudes—that can prevent a storm from
growing.
(Encarta encyclopedia)