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Hurricanes
This chapter discusses:
1. Tropical cyclone and hurricane development,
structure, and movement
2. Hurricane damages, warning systems, and naming
conventions
Tropical Weather & Waves
Figure 16.1
Tropical winds
typically blow
from the east,
and when they
encounter a
slow moving
trough of low
pressure, called
a tropical
wave, the
winds initially
converge and
lift to produce
showers and
thunderstorms.
Structure of a Hurricane
Tropical cyclones are
the international
name of hurricanes,
which typically form
from an organized
mass of storms which
formed along a
tropical wave.
In this image of
Hurricane Elena, the
central area of broken
clouds is the eye,
surrounded by an eye
wall cloud and spiral
rain bands, with a
total diameter nearing
Figure 16.2
Hurricane Wind Profile
Figure 16.3
The low pressure core of the hurricane is surrounded by several
thunderstorms, each with updraft and downdraft cycles.
The wind and moisture cycle is repeated as:
surface moist air converges in a counterclockwise pattern at the eye,
rises to create high pressure aloft, condenses, precipitates, dries,
diverges outward in a clockwise pattern, sinks, and warms.
3-D Radar Image of Hurricane
Figure 16.4
Several key features of a hurricane are shown in this radar
composite image, including overshooting clouds, the area of
strongest echoes (heaviest rain), and the eyewall.
Formation by Organized Convection
Figure 16.5A
Figure 16.5B
One theory explains that hurricane formation requires cold air above an
organized mass of thunderstorms, where the release of latent heat warms
the upper troposphere, creates high pressure aloft, which pushes air
outward and causes a low to deepen at the surface.
Air moving toward this low intensifies the cycle.
Formation by Heat Engine
Another theory of hurricane development proposes that
a heat engine cycle, fueled by warm moist input air and
the release of heat when it converts to cool dry air.
Differences in the input and output temperatures
determine the amount of work on the ocean and winds
that is performed.
Hurricane Stages of Development
The initial
components of a
hurricane may
form as a tropical
disturbance, grow
into a tropical
depression when
winds exceed 20
knots, become a
tropical storm
when winds
exceed 35 knots,
and finally then
qualify as a
hurricane when
winds exceed 64
knots.
Figure 16.6
Hurricane Movement
Global patterns of
tropical cyclone
formation and
movement have
been recorded on
this figure, which
notes regional names
for these systems.
Figure 16.7
Travel speeds for the
hurricane my range
from 10 to 50 knots,
but they may also
stall over a region
and cause
destructive flooding.
Erratic Paths of Hurricanes
Figure 16.8
Historical charts of hurricane location may reveal
erratic, and hard to predict, patterns of movement.
As this figure shows, hurricanes may occasionally
double back.
Further, when 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 3 storms per year
move inland and bring damaging winds and rain.
North Atlantic Hurricanes
Composite infrared
imagery of
Hurricane Georges
reveals the pattern
of a seasonal threat
for Central and
North America
coastlines.
Tropical cyclones at
the same latitude
survive longer in
the Atlantic than
Pacific Ocean
because of warmer
Atlantic Ocean
waters.
Figure 16.9
Hurricane Damage & Warning
Figure 16.11
Figure 16.10
Hurricanes have their highest wind speeds on the side where
storm pushing winds amplify cyclonic, or counterclockwise,
rotational winds. In coastal areas, flooding is aggravated by the
hurricane low pressure triggering higher tides and Ekman
transport piling up water.
Hurricane Watch & Warning
Figure 16.5A
The National Hurricane Center in Florida issues a hurricane watch 24 to
48 hours before a threatening storm arrives, and if it appears that the
storm will strike within 24 hours, a hurricane warning is issued.
While some consider the warning area too large, causing unneeded
evacuation, such evacuations have saved many lives.
Hurricane Hugo, with peak winds near 174 knots, caused tremendous
damage.
Hurricane Saffir-Simpson Winds
Figure 16.13A
Figure 16.14
In 1989 Hugo caused nearly $7 billion in damages in the U.S.,
killing 49 in the Caribbean and United States.
Current classification of hurricanes is based on their wind speed,
however, and not on human or property damage.
Hurricanes range from category 1 to 5, with winds of 64 to more
than 135 knots.
Hurricane Names and Cost
Category 5 Hurricane
Andrew (1992) was
the costliest US storm,
but it ranks as less
intense than 1935 and
1969 hurricanes.
Hurricane names are
chosen from an
alphabetical list of
male and female
names for the Atlantic
and Pacific, some of
which are retired if
the storm was
especially damaging.
Figure 16.14
Hurricane Naming Scheme
Retired Names
Hurricane Paloma
• Reached category 4 strength (145 mph)
• Struck the Cayman Islands and Cuba
• Retired because:
– Third most powerful November hurricane on
record in the Atlantic Basin
– $154 million in damages to Cayman Islands,
$300 million in Cuba
• Replaced with Paulette
Hurricane Luis
• Reached category 4 strength (140 mph)
• Was one of four simultaneous systems in the Atlantic
(Humberto, Iris, Karen)
• Retired because:
– Hit most of the Leeward Islands as a category 4
– caused catastrophic damage, especially in Antigua,
Barbuda, St. Barthelemy, St Martin and Anguilla (~$3
billion)
– Spawned a 98’ wave that hit the QE2 (largest wave
ever recorded, though Ivan may have created a 130’
wave)
• Replaced with Lorenzo
How names are Retired
• Each country participating in the WMO
counsel can request a name be retired if the
storm affected them
• Usually requested if it does significant
damage or is related to high fatalities
• Full counsel votes on whether the storm
name is retired
How new names are added
• Any retired name must be replaced with the
same gender name
• Any retired name must be replaced with
same “nationality” name
• If name was Spanish, the Spanish countries
pick three new names and the full counsel
votes on the replacement name
• U.S. will ask for Sandy to be retired and
propose replacement names
Hurricane Andrew Devastation in Homestead, Florida
August 24, 1992
Figure 16.15
Likelihood for Landfall
Between 1900 and
1999, only two
category 5
hurricanes have
made landfall
along the Gulf or
Atlantic.
Numerous
category 1, and
less damaging
storms, that do
make landfall may
not cause much
damage, but bring
needed rainfall.
Figure 16.16