The whirling tropical cyclones that on occasion have wind speeds reaching
320 kilometers per hour are known as hurricanes - the most powerful storms on earth.
Out at sea they can generate 15-meter waves capable of inflicting destruction hundreds of
kilometers from their source. Should a hurricane smash into land, gale force winds
coupled with extensive flooding can impose great loss of life and millions of dollars in

These storms form in all tropical waters (except those of the South Atlantic)
between the latitudes of 5 degrees and 20 degrees and are known in each locale by a
unique name. In the western Pacific they are called Typhoons. In Australia they are called
Willy-Willys. In the Indian Ocean they are called Cyclones.

The North Pacific has the greatest number of these storms, averaging 20 per year.
Fortunately for those living in the coastal regions of the southern and eastern United
States, fewer than five hurricanes, on the average, develop each year in the warm sector of
the North Atlantic.

Although hurricanes are most noted for their destruction, some parts of the world,
especially eastern Asia, rely on them for much of their precipitation.

Although numerous tropical storms develop each year, only a few reach hurricane
status, which by international agreement requires wind speeds in excess of 119 kilometers
per hour and a rotary circulation. Hurricanes average 600 kilometers in diameter, and
often extend 12,000 meters above the ocean surface. From the outer edge of the
hurricane to the center the barometric pressure has on occasion dropped 60 millibars, from
1010 to 950 millibars. The lowest pressure ever recorded in the United States was 892.31

The steep pressure generates the rapid, inward spiraling winds of a hurricane. As
the inward rush nears the core of the storm, it is whirled upward. Upon ascending, the air
condenses, generating the cumulonimbus clouds that constitute the doughnut-shaped inner
structure of the hurricane. Surrounding this core are the bands of cumulonimbus clouds
that trail away in a spiral fashion. Near the top of the hurricane the airflow is outward,
carrying the rising air away from the storm center, thereby providing room for more
inward flow at the surface.

At the center of the storm is the eye. Averaging 20 kilometers in diameter, this
area of calm and scattered cloud cover is unique to the hurricane.

Separating period of torrential rains, the eye, has a blue sky and sunlight against a
background of circling clouds that are 12 kilometers high. The air within the eye slowly
descends and heats by compression, making it the warmest part of the storm.

A hurricane can be described as a heat engine that is fueled by the energy formed
during the condensation of water vapor from the air. The release of heat warms the air
and provides buoyancy for its upward flight. (The condensed water vapor becomes clouds
and rain). The result is to reduce the pressure near the surface that encourages a more
rapid inward flow of air. A large quantity of warm, moisture-laden air is required and a
continual supply is needed to keep it going.

A location only a few hundred kilometers from a hurricane (just one day's striking
distance away) may experience clear skies and virtually no wind. As a typical storm
approaches, wind speeds increase, first to an intensity of 60 kilometers per hour and
eventually to over 160 kilometers per hour at the eye wall of the storm. With the
increased wind speed comes torrential rain, which drops from 15 to 30 centimeters of
water as it passes overhead. Flooding usually inflicts more destruction than wind.

The greatest flooding in coastal areas is caused by the rising sea level that
accompanies the low pressure of the storm (called the storm surge). It may raise the
ocean a meter, due to the low pressure at the center of the storm, and the resulting surge
of water pulled along by the storm may reach several meters above sea level as the center
of the storm passes onto low-lying land areas, causing highly destructive flooding.

The practice of naming hurricanes has been common since the early 1800's in
Australia and the Caribbean. Currently, the United States applies mixed male and female
names to hurricanes according to annual lists of proposed hurricane names given by the
World Meteorological Organization. Each year a unique set of names begin with 'A' and
progress through the alphabet except for the letters Q, U, X, Y, and Z.

Hurricanes are classified into 5 categories:

Category 1- Damage primarily to