Page 276 - NS-2 Textbook

METEOROLOGY 271

LIFE OF A HURRICANE This area is relatively calm, with light winds and clear or
moderately clear skies and a little drizzle.
The birth of a hurricane often occurs in that region near
An Atlantic hurricane, then, starts as a tropical low,
the equator where trade winds meet to form the Inter-
grows into a storm, and eventually matures into a hurri-
tropical Convergence Zone (ICZ). Tropical cyclones,
cane when its winds exceed 75 mph. While it is moving
however, never occur right on the equator because they
. along its track, it is growing in intensity. By the time it be-
require the twisting Coriolis forces of Earth's rotation to
gins to curve to the northeast, it comes over cooler waters
start them spinning, which are not present there.
and into cooler air. Cooling reduces its internal action,
A hurricane is born in a hot, Inoist air nlass over the
until it finally dissipates into an extra tropical low, usu-
ocean. The rotating low at the ICZ pushes air toward its
ally ending as a gale or storm over the North Atlantic or
center, forcing the hot moist air to lift. The lifting causes
North Sea.
the moisture to condense. As this moisture condenses, it
The elements of wind, temperature, pressure, hu-
heats the rotating au' morel causing it to rise even more
midity, and rain vary little in the different quadrants of a
swiftly. As more moist tropical sea air sweeps in to re-
tropical cyclone. Winds increase from the outer limits to
place this rising air, more condensation takes place, and
the edge of the eye. The temperature rises and the hu-
the cycle intensifies. That is why hurricanes are so vio-
midity falls at the center. Precipitation is in the form of
lent-because of the tremendous energy released by the
showers at the outer limits. It becomes heavier toward
continuous condensation and an inexhaustible source of
the center, and is heaviest in the right front quadrant.
moisture. All the while, the Coriolis effect keeps the air
Hurricanes are usually associated with great wind-
turning more and more rapidly, tmtiI it is a giant wheel
caused tides called storm surges that inundate the land
of swirling winds.
areas they approach, and cause more damage than do the
Although their tracks cannot be predicted individu-
wind and rain of the storm itself. The doldrums, with
ally, in general in the Northern Hemisphere hurricanes
their baffling winds and frequent rain squalls and thun-
move westward from their point of origin, then curve to
derstorms, is the breeding place of most tropical cy-
the northeast. In the Southem Hemisphere they start out
clones.
westward and then curve southeastward. They vary in
diameter from 60 to 1,000 mlles. They have moderate HURRICANES, TYPHOONS, AND THEIR TRACKS
winds at their outer edges l increasing tovvard the center,
where velocities higher than 175 knots (200 mph) have Hurricanes occur most frequently in September and Oc-
been recorded. At the center is an area called the eye of tober, but they can happen anytime from June to Decem-
the storm, which averages about 14 mlles in diameter. ber. A typical hurricane that originates in the doldrums
TOP VIEW

wind
low
TOP VIEW pressure,
no wind,
few clouds

Anatomy of a hurricane. Upper Jeft: Hurricane winds rise in a counterclockwise spiral around an eye of calm air. Lower left: A cross section
shows the shape of the hurricane cloud layers. Upper right: The whole hurricane system moves toward the southeast coast of the United States.
Lower right: Seen from above, the hurricane has a circular shape with a central eye.