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.