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Lo; To explain the stages in the formation of a
To evaluate patterns affecting hurricane
Hurricane Damage
• The vast majority of hurricane deaths and
damage are caused by relatively
infrequent, yet powerful storms.
• The costliest natural disaster in U.S.
history, with damages in excess of $25
billion, was Hurricane Andrew in 1992.
Hurricane Formation
• Most hurricanes form between the latitudes
of 5° and 20° over all tropical oceans except
the South Atlantic and eastern South
• The North Pacific has the greatest number
of storms, averaging 20 per year.
• In the western Pacific, hurricanes are called
typhoons, and in the Indian Ocean, they
are referred to as cyclones.
Hurricane Occurrences and Geographic Distribution
Hurricane Generation
• A steep pressure gradient generates the
rapid, inward spiraling winds of a hurricane.
• As the warm, moist air approaches the core
of the storm, it turns upward and ascends in a
ring of cumulonimbus towers and forms a
doughnut-shaped wall called the eye wall.
• At the very center of the storm, called the
eye, the air gradually descends, precipitation
ceases, and winds subside.
In much the same way an
ice skater spins more
quickly as her arms are
tucked close into her body,
a hurricane also spins at a
faster pace near the
center than near the outer
Hurricane Generation
• A hurricane is a heat engine fueled by the
latent heat liberated when huge quantities of
water vapor condense.
• They develop most often in late summer
when ocean waters have reached
temperatures of 27°C (80°F) or higher and
are thus able to provide the necessary heat
and moisture to the air.
Stages of Development
• The initial stage of a tropical storm's life cycle,
called a tropical disturbance, is a disorganized
array of clouds that exhibits a weak pressure
gradient and little or no rotation.
• Tropical disturbances that produce many of the
strongest hurricanes that enter the western North
Atlantic and threaten North America often begin as
large undulations or ripples in the trade winds
known as easterly waves.
• Each year, only a few tropical disturbances
develop into full-fledged hurricanes that require
minimum wind speeds of 119 kilometers per
hour (74 mph).
• When a cyclone's strongest winds do not
exceed 61 kilometers per hour, it is called a
tropical depression.
• When winds are between 61 and 119
kilometers per hour, the cyclone is termed a
tropical storm.
Hurricane Diminishment
• Hurricanes diminish in intensity
whenever they:
• (1) move over ocean waters that cannot
supply warm, moist tropical air,
• (2) move onto land, or
• (3) reach a location where large-scale flow
aloft is unfavorable.
The Saffir Simpson Scale
•The Saffir-Simpson scale ranks the relative intensities
of hurricanes.
•The Saffir-Simpson Hurricane Scale is a 1-5 rating
based on the hurricane's present intensity.
•This is used to give an estimate of the potential
property damage and flooding expected along the coast
from a hurricane landfall.
•Wind speed is the determining factor in the scale, as
storm surge values are highly dependent on the slope
of the continental shelf in the landfall region.
•Note that all winds are using the U.S. 1-minute
Category One Hurricane:
Winds 74-95 mph (64-82 kt or 119-153 km/hr). Storm surge generally 4-5 ft
above normal.
Category Two Hurricane:
Winds 96-110 mph (83-95 kt or 154-177 km/hr). Storm surge generally 6-8
feet above normal.
Category Three Hurricane:
•Winds 111-130 mph (96-113 kt or 178-209 km/hr). Storm surge generally 912 ft above normal.
Category Four Hurricane:
Winds 131-155 mph (114-135 kt or 210-249 km/hr). Storm surge generally
13-18 ft above normal.
Category Five Hurricane:
Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge generally
greater than 18 ft above normal.
Damage Classes
• Damage caused by hurricanes can be divided
into three classes:
• (1) storm surge, which is most intense on the right
side of the eye where winds are blowing toward the
shore, occurs when a dome of water 65 to 80
kilometers (40 to 50 miles) wide sweeps across the
coast near the point where the eye makes landfall,
• (2) wind damage, and
• (3) inland freshwater flooding, which is caused by
torrential rains that accompany most hurricanes.
Storm Surge
Before a
After a
Key Terminology
Eye Wall
Tropical Disturbance
Tropical Depression
Saffir-Simpson Scale
Inland freshwater flooding
Hurricane Warning
Easterly Wave
Tropical Storm
Storm Surge
Hurricane Watch