arise in the tropical latitudes (between 10oand 25oN) in summer and autumn. The warm
seas create a large humid air mass. The warm air rises and forms a low pressure cell, known
as atropical depression. Thunderstorms materialize around the tropical depression.
If the temperature within the cell reaches or exceeds 28oC (82oF) the air begins to rotate
around the low pressure. The rotation is counterclockwise in the Northern Hemisphere and
clockwise in the Southern Hemisphere. As the air rises, water vapor condenses, releasing
energy from latent heat. If winds frequently shift directions in the upper atmosphere, the
storm cannot grow upward. If wind shear is low, the storm builds into a hurricane within
two to three days.
Figure 16.32: A cross-sectional view of a hurricane. ( 6 )Hurricanes are roughly 600 km (350 miles) across and 15 km (50,000 feet) high. Winds reach
at least 118 km (74 miles) per hour. The exception is the relatively calm eye of the storm,
which is about 13 to 16 km (8 to 10 miles) in diameter. The eye is calm because it is where
air is rising upward.
Rainfall can be as high as 2.5 cm (1”) per hour, resulting in about 20 billion metric tons of
water released daily in a hurricane. The release of latent heat generates enormous amounts
of energy, about 2,000 billion kilowatt hours per day. This amount of energy is nearly the
total annual electrical power consumption of the United States. Hurricanes can also generate
tornadoes.
Hurricanes are assigned to categories based on their wind speed. An estimate can be made
as to the damage that will be caused based on the category of storm. The categories are
listed on the Saffir-Simpson hurricane scale (Table16.4).