Tropical Storms and Hurricanes

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Chapter 12 Tropical Storms and Hurricanes
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Extremely strong tropical storms go by a number
of different names, depending on where they
occur. Over the Atlantic and the eastern Pacific
they are known as hurricanes. Those over the
extreme western Pacific are called typhoons
those over the Indian Ocean and Australia are
cyclones.
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Middle- and upper-level air along the eastern
side of anticyclones sinks as it approaches the
west coasts of adjacent continents. Because the
air does not descend all the way to the surface,
a subsidence inversion called the trade wind
inversion forms above the surface. The air below
the inversion, called the marine layer, is cool
and relatively moist.
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Hurricanes have sustained wind speeds of 120
km/hr (74 mph) and are typically about 600
kilometers (350 mi) wide. Sea level pressure near
the center of a typical hurricane is around 950
mb, but pressures as low as 870 mb have
been observed for extremely powerful hurricanes.
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Hurricanes obtain most of their energy from the
latent heat released by condensation and are
most common where a deep layer of warm water
fuels them. August and September are the prime
hurricane months in the Northern Hemisphere,
while January to March is the main season in the
Southern Hemisphere.
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Hurricanes around the globe.
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The hurricane eye is a region of relatively clear
skies, slowly descending air, and light
winds. Along the margin of the eye lies the eye
wall, the zone of most intense storm activity
with the strongest winds, thickest cloud cover,
and most intense precipitation of the entire
hurricane.
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A cross section of a typical hurricane.
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Tropical disturbances are disorganized groups of
thunderstorms having weak pressure gradients
and little or no rotation. Most tropical
disturbances that enter the western Atlantic and
become hurricanes originate in easterly waves,
large undulations or ripples in the normal trade
wind pattern.
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Easterly waves have surface convergence and cloud
cover east of the axis and divergence to the
west.
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When a tropical disturbance develops to the point
where there is at least one closed isobar on a
weather map, the disturbance is classified as a
tropical depression. If the depression
intensifies further and maintains wind speeds
above 60 km/hr, it becomes a tropical storm. A
further increase in sustained wind speeds to 120
km/hr creates a true hurricane.
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A hypothetical hurricane moves northward at 50
km/hr. Along the right-hand side, the 200 km/hr
winds are in the same direction as the movement
of the storm, so there is a net-wind speed of 250
km/hr. On the left side, the net winds are
southward at 150 km/hr.
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Hurricanes form only where the ocean has a
deep surface layer with temperatures above 27 C
(81 F). The need for warm water precludes
hurricane formation poleward of about 20 degrees
because sea surface temperatures are usually too
low there.
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Hurricane formation also depends on the Coriolis
force, which must be strong enough to prevent
filling of the central low pressure. The absence
of a Coriolis effect at the equator
prohibits hurricane formation between 0 and 5
latitude.
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Hurricanes and tropical storms have a tendency to
move north or northeast out of the tropics along
the southeast coast of North America and often
move in wildly erratic ways. After making
landfall, a tropical storm may die out completely
within a few days.
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Storm surge is a rise in water level induced by
the hurricane. Strong winds blowing toward a
coast force surface waters landward and thereby
elevate sea level. Low atmospheric pressure in a
hurricane also contributes to the storm
surge. For every millibar of pressure decrease,
the water level rises 1 cm.
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The 1900 Galveston hurricane was the deadliest in
U.S. history.
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When forecasters predict that an approaching
hurricane will reach land in more than 24 hours,
they issue a hurricane watch. If it is expected
to make landfall over the United States within 24
hours, they issue a hurricane warning.
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The Saffir-Simpson scale classifies
hurricanes into five categories, with increasing
numbers corresponding to lower central
pressures, greater wind speeds, and larger storm
surges.
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The next chapter examines weather forecasting
and analysis.