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Hurricanes
GEOL 1350: Introduction To Meteorology
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•  In the tropics, diurnal and seasonal changes in
temperature are small compared with high
latitudes.
•  The daily heating of the surface and high
humidity favor the development of cumulus
clouds and afternoon thunderstorms.
•  Most of these are individual thunderstorms that
are not severe. Sometimes, however, they grow
together into loosely organized systems called
non-squall clusters. On other occasions, the
thunderstorms will align into a row of vigorous
convective cells or a squall line.
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•  Winds in the tropics generally blow from the east,
northeast, or southeast - the trade winds.
•  Streamlines that depict the wind flow are drawn in the
tropical region. It is useful because they show where
surface air converges and diverges.
•  Occasionally, the streamlines will be disturbed by a
weak trough of low pressure
called a tropical wave, or
easterly wave, because it
tends to move from east to
west.
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•  Hurricane is an intense storm of tropical origin, with
sustained winds exceeding 64 knots (74 mi/hr), which
forms over the warm northern Atlantic and eastern North
Pacific oceans.
•  In the western North Pacific, it is called typhoon, in India
a cyclone, and in Australia a tropical cyclone. By
international agreement, tropical cyclone is the general
term for all hurricane-type storms that originate over
tropical waters.
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•  Diameter - 500 km
•  Eye - 40 km; Area of broken
clouds at the center; Winds
are light
•  Surface Air Pressure - 955 mb
•  Surface winds increase in
speed as they blow
counterclockwise and inward
toward this center (NH).
•  Adjacent to the eye is the
eyewall, a ring of intense
thunderstorms that whirl
around the storm’s center and
may extend upward to almost
18 km.
•  Within the eyewall, we find the
heaviest precipitation and the
strongest winds.
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•  Near the surface, moist tropical air flows in toward the hurricane
center. Adjacent to the eye, this air rises and condenses into huge
cumulonimbus clouds that produce heavy rainfall (10 inches per
hour).
•  Near the top of the clouds, the relatively dry air, having lost much of
its moisture, begins to flow outward away from the center.
•  This diverging air aloft actually produces a clockwise flow of air
several hundred kilometers from the eye.
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•  As the outflow reaches the storm’s periphery, it begins to sink and
warm, inducing clear skies.
•  In the vigorous convective clouds of the eyewall, the air warms due
to the release of latent heat. This warming produces slightly higher
pressures aloft, which initiate downward air motion within the eye.
•  As the air sinks, it warms by compression. This process helps to
account for the warm air and the absence of thunderstorms in the
center of the storm.
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•  The cutaway view shows concentric bands of heavy rain (red areas
inside the clouds) encircling the eye. Notice that the heaviest rain
occurs in the eyewall.
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•  Where and how do hurricanes form?
•  Tropical waters where the winds are light
•  Humidity is high in a deep layer extending up
through the troposphere
•  Surface water temperature is warm, typically
26.5°C (80°F) or greater over a vast area
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Hurricane-Ready Water in the Atlantic
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•  Where and how do hurricanes form?
•  Hurricanes usually form over the tropical and
subtropical North Atlantic and North Pacific
oceans.
•  Hurricane season normally runs from June
through November.
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•  Energy for a hurricane comes from the direct
transfer of sensible heat and latent heat from the
warm ocean surface.
•  For a hurricane to form, a cluster of
thunderstorms must become organized around
a central area of surface low pressure.
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•  One theory proposed that a hurricane forms in
the following manner:
1. Trade wind inversion is weak and that
thunderstorms start to organize along the ITCZ,
or along a tropical wave.
2. In the deep moist conditionally unstable
environment, a huge amount of latent heat is
released inside the clouds during condensation.
3. The process warms the air aloft, causing the
temperature near the cluster of thunderstorms
to be much higher than the air temperature at
the same level farther away.
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4. This situation causes a horizontal pressure
gradient aloft that induces the air aloft to move
outward, away from the region of higher
pressure in the anvils of the cumulonimbus
clouds.
5. This diverging air aloft, coupled with warming of
the vertical air column, causes the surface
pressure to drop and a small area of surface low
pressure to form.
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6. Air now begins to spin counterclockwise and in
toward the region of surface low pressure.
7. As the air moves inward, its speed increases.
8. As the air moves over the warm water, small
swirling eddies transfer heat energy from the
ocean surface into the overlying air. The warmer
the water and the greater the wind speed, the
greater the transfer of sensible and latent heat
into the air above.
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9. As the air sweeps in toward the center of low
pressure, the rate of heat transfer increases
because the wind speed increases.
10. Similarly, the higher wind speed causes
greater evaporation rates, and the overlying air
becomes nearly saturated.
11. The turbulent eddies then transfer the warm
moist air upward, where the water vapor
condenses to fuel new thunderstorms.
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•  Hurricane is similar to that of a heat engine.
•  In a hurricane, heat is taken in near the warm ocean
surface, converted to kinetic energy, and lost at its top
through radiational cooling.
•  The maximum strength a hurricane can achieve is
proportional to the difference in air temperature between
the tropopause and the surface, and to the potential for
evaporation from the sea surface.
•  The warmer the ocean surface, the lower the minimum
pressure of the storm, and the higher its wind.
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•  If the hurricane remains over warm water, it may survive
for a long time.
•  Hurricanes weaken rapidly when they travel over colder
water and lose their heat source.
•  Studies show that if the water beneath the eyewall of the
storm cools by 2.5 C, the storm’s energy source is cut
off, and the storm will dissipate.
•  Even a small drop in water temperature beneath the
eyewall will noticeably weaken the storm.
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•  A hurricane can also weaken if the layer of warm water
beneath the storm is shallow.
•  Hurricane also dissipate rapidly when they move over a
large landmass. They not only lose their energy source
but friction with the land surface causes surface winds
to decrease and blow more directly into the storm, an
effect that causes the hurricane’s central pressure to
rise.
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•  Hurricanes go through a set of stages from birth
to death
•  Initially, a tropical disturbance shows up as a
mass of thunderstorms with only slight wind
circulation.
•  The tropical disturbance becomes a tropical
depression when the winds increase to between
20-34 knots.
•  When the winds are between 35-64 knots, the
tropical depression becomes a tropical storm.
•  The tropical storm is classified as a hurricane
only when its wind exceed 64 knots (74 miles
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per hour).
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Regions where tropical storms form, the names given to storms,
and the typical paths they take (red arrows)
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Regions where tropical storms form, the names given to storms,
and the typical paths they take (red arrows)
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* When hurricanes removed from the ocean and without a moisture
source to supply energy, they may still continue an inland journey.
* In the North Atlantic, on average three storms per year move
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Inland and bringing damaging winds and rain.
Max wind
75 knots
Max wind
125 knots
•  Winds on the eastern side is much larger than that in the western
side.
•  In coastal areas, flooding is aggravated by the hurricane low
pressure triggering higher tides and Ekman transport piling up
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water.
•  National Hurricane Center issues a hurricane watch 24-48 hours
before a threatening storm arrives. It will issue a hurricane warming
within 24 hours if the storm appears to strike an area.
•  Hurricane Hugo (Sep 21, 1989) made landfall as a category 4
hurricane near Charleston, South Carolina. The high winds and
storm surge, which ranged between 2.5-6 m, hurled a thundering
wall of water against the shore. This knocked out power, flooded
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streets, and caused widespread destruction to coastal
communities.
•  Hurricane Ike (Sep 2008) caused a
lot of damages in the coastal area.
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Ike started as a tropical disturbance near Africa
at the end of August. On September 1, 2008, it
became a tropical storm west of the Cape Verde
islands. By the early morning hours of
September 4, Ike was a Category 4 hurricane,
with maximum sustained winds of 145 mph
(230 km/h) and a pressure of 935 mbar
(27.61 inHg). That made it the most intense
Atlantic storm of 2008. Ike passed over the
Turks and Caicos Islands as Category 4, with
winds 135 mph (217 km/h) on September 7.
Moving west along Cuba, it made 2 landfalls as
a Category 4 hurricane on September 7 and a
Category 1 hurricane on September 9. Ike made
its final landfall over Galveston, Texas as a
strong Category 2 hurricane, with Category 5
equivalent storm surge, on September 13, 2008
at 2:10 a.m. CDT.
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•  Saffir-Simpson scale was developed to estimate the
possible damage a hurricane’s sustained winds and
storm surge could do to a coastal area.
•  The scale numbers (range from 1 to 5) are based on
actual conditions at some time during the life of the
storm.
•  Category 1 (wind > 64 knots) hurricanes are the
weakest
•  Category 5 (wind > 135 knots) are the most intense
•  Category 3 (wind > 96 knots) and above are
considered intense
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•  Between 1900 and 1999, only two category 5 hurricanes have
made landfall along the coastline of the United States.
•  Numerous category 1, and less damaging storms, that do make
landfall may not cause much damage, but bring needed rainfall.
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[http://www.jpl.nasa.gov/video/index.cfm?id=928]
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SUMMARY
1.  Hurricanes are tropical cyclones with winds that exceed 64 knots
(74 mi/hr) and blow counterclockwise about their center in the
Northern Hemisphere.
2.  A hurricane consists of a mass of organized thunderstorms that
spiral in toward the extreme low pressure of the storm’s eye.
3.  The heaviest rain, and the highest winds occur outside the eye, in
the region known as the eyewall. In the eye itself, the air is warm,
winds are light, and skies may be broken or overcast.
4.  Hurricanes are born over warm tropical waters where the air is
humid, surface winds converge, and thunderstorms become
organized in a region of weak upper level winds.
5.  The energy source for hurricane comes primarily from the warm
tropical oceans and from the release of latent heat.
6.  The Saffir-Simpson hurricane scale was developed to estimate the
potential destruction that a hurricane can cause.
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