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H u rricanes are severe tropical cyclones with pronounced ro t a ry circulation whose winds

consistently exceed 74 mph or 64 knots (1 knot = 1.15 miles/hour). One beneficial quali-

ty of hurricanes is serving as a source of rain to land in their path, although the rain is

often delivered in devastating quantities. A mature hurricane orchestrates more than a

million cubic miles of atmosphere, typically has an 8 to 12 day cycle, and averages about

300 miles in diameter. Winds may exceed 150 knots, and generate waves of 50 feet or

m o re over deep ocean.

H u rricane winds are produced, as all winds are, by diff e rences in atmospheric pre s s u re.

The hurr i c a n e ’s energy is derived from the latent heat of condensation. The maximum

s t rength that a storm can achieve is determined by the temperature diff e rence between

the sea surface and the top of the storm. Water below 80 degrees Fahrenheit does not

contribute much energy to a hurricane so all storms are doomed once they leave warm

t ropical waters. However, the remnants of some large hurricanes may travel for days over

cold ocean before dissipating.

H u rricanes form over tropical waters where the winds are light, the humidity is high, and

the surface water temperature is warm (typically 79 degrees F or greater) over a vast are a.

All hurricanes develop an eye. Within the eye, an average of 20 to 50 kilometers in diame-

t e r, clouds are broken and winds are light with very low surface air pre s s u re. Hurr i c a n e

s t rength is generally unrelated to overall size, however very strong hurricanes usually have

small eyes (less than 10 miles or 16 kilometers in diameter), which has the effect of concen-

trating the hurr i c a n e ’s energ y. Hurricane Gilbert ’s (1988) eye displayed the lowest sea-level

p re s s u re ever re c o rded in the We s t e rn Hemisphere with 26.22 inches (888 millibars) of

m e rc u ry — c o m p a red to the standard 29 inches of merc u ry in North America, and 30 inch-

es in the tropics. Surrounding the eye is the eye wall, a ring of intense thunderstorms that

whirl around the storm ’s center and extend upward to almost 15 km above sea level.

Although the high winds of hurricanes inflict a lot of damage, it is the waves and flooding

associated with the storm surge that cause the most destruction. High winds generate

waves over 10 meters (33 feet high), and an average hurricane brings six to twelve inches

of rain to the area it crosses. Storm surge claims nine of ten victims in a hurricane.

While a hurricane lives, the release of energy within its circulation is immense. The conden-

sation heat energy released by a hurricane in one day, converted to electricity, could supply

the United States’ electrical needs for about six months. Hurricane season in the Nort h e rn

H e m i s p h e re normally lasts from June through November, more hurricanes occur in the

Pacific than in the Atlantic Ocean.

The combined effect of high water and high winds that produce a rise in

ocean level that drenches low-lying coast.

The combination

of high tide and

s t o rm surge.

f i g u re 94.

HU R R I C A N E S

B

mean sea level 2' normal high tide

storm surge

17' storm tide 15' surge