Hurricanes are like many other occurrences in the physical geographic environ-
ment: there are many small hurricanes and a few large ones. For instance, in the
last century and a half only a third of the landfalling hurricanes in the mainland
United States have been major hurricanes (categories 3, 4, and 5). Yet, the vast
majority of damage and death are caused by the major hurricanes.
Hurricanes are giant heat engines that feed on heat in tropical oceans. They
“live” on latent heat release from ocean water at temperatures in excess of 26°C.
Energy is distributed upward in the troposphere thus helping to maintain the
Earth’s energy balance. Hurricanes are distinguished frommiddle latitude cyclo-
nesin that hurricanes are warm-core storms because of latent heat of condensa-
tion. They also weaken with altitude above the sea and must develop in
environments with weak upper winds.
Hurricanes have well-known structures. They are organized cyclones with cen-
ters of relative low pressure around which there is a large pressure gradient forcing
air to circulate from higher to lower pressure. The ferocity of the winds is depen-
dent on the pressure gradient so the fastest winds are found in the eyewall that sur-
rounds the placid air of the hurricane’s eye. Winds decrease outward from the
eyewall and are usually beneath hurricane strength a couple of hundred kilometers
out from the center. Winds spiral cyclonically inward toward the eyewall, in a
clockwise fashion in the Southern Hemisphere and in a counterclockwise fashion
in the Northern Hemisphere.
Apart from the eyewall, the most intense weather is arranged in spiral bands
feeding mass and energy toward the center. Typical hurricanes have cloud shields
that are about 500 km across (smaller than a middle latitude cyclone) and they track
at about 25 km per hour. Hurricanes usually originate from 5°through 20°north
and south of the equator. The do not originate over the tropical South Atlantic.
They commonly are embedded in winds on the equatorward peripheries of the
subtropical highs. Individual storms can take a wide variety of paths that can be
quite erratic because of the lack of upper air control associated with hurricanes.
Long-lasting storms tend to move to the west and then curve poleward as guided178 Hurricanes
Table 2. The Saffir-Simpson Scale
Category Wind speed (m/s) Wind speed (mph) Wind speed (knots)
Tropical depression <17 <39 <34
Tropical storm 17–33 39–73 34–63
1 34–42 74–95 64–82
2 43–49 96–110 83–95
3 50–58 111–130 96–113
4 59–69 131–155 114–135
5 >69 >155 >135