hurricanes...
The Greatest Storms on Earth
A PREPAREDNESS GUIDE
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Weather Service
American Red Cross
March 1994
Hurricanes
There are no other storms like hurricanes on earth.
Views of hurricanes from satellites located
thousands of miles above the earth show how
unique these powerful, tightly coiled weather
systems are.
What is a hurricane?
A hurricane is a type of tropical cyclone-the general term for
all circulating weather systems (counterclockwise in the Northern
Hemisphere) over tropical waters. Tropical cyclones are
classified as follows:
1. Tropical Depression - An organized system of clouds
and thunderstorms with a defined circulation and maximum
sustained winds of 38 mph (33 knots) or less.
2.Tropical Storm - An organized system of strong
thunderstorms with a defined circulation and maximum
sustained winds of 39 to 73 mph (34-63 knots).
3. Hurricane - An intense tropical weather system with a
well defined circulation and maximum sustained winds of
74 mph (64 knots) or higher. In the western Pacific,
hurricanes are called "typhoons," and similar storms in the
Indian Ocean are called "cyclones."
Hurricanes are products of the tropical ocean and atmosphere.
Powered by heat from the sea, they are steered by the easterly
trade winds and the temperate westerlies as well as by their own
ferocious energy. Around their core, winds grow with great
velocity, generating violent seas. Moving ashore, they sweep the
ocean inward while spawning tornadoes and producing torrential
rains and floods. Each year on average, ten tropical storms (of
which six become hurricanes) develop over the Atlantic Ocean,
Caribbean Sea, or Gulf of Mexico. Many of these remain over the
ocean. However, about five hurricanes strike the United States
coastline every 3 years. Of these five, two will be major
hurricanes (category 3 or greater on the Saffir-Simpson Hurricane
Scale).
Saffir-Simpson Hurricane Scale
Scale Sustained Damage Examples (States Affected)
Number Winds
Category) (MPH)
1 74-95 Minimal Florence (1988) (LA)
Charley 1988 (NC)
2 96-110 Moderate Kate 1985 (FL Panhandle)
Bob 1991 (RI)
3 111-130 Extensive Alicia 1983 (N TX)
Emily 1993(NC Outer Banks)
4 131-155 Extreme Andrew 1992 (S FL)
Hugo 1989 (SC)
5 >155 Catastrophic Camille (1969 (WMS)
Labor Day Hurricane 1935
(Keys)
Timely warnings have greatly diminished hurricane fatalities in
the United States. In spite of this, property damage continues to
mount. There is little we can do about the hurricanes themselves.
However, NOAA's National Hurricane Center and National Weather
Service field offices team up with other Federal, State, and
local agencies; rescue and relief organizations; the private
sector;and the news media in a huge warning and preparedness
effort.
Storm Structure
The process by which a disturbance forms and subsequently
strengthens into a hurricane depends on at least
three conditions. Warm waters and moisture are mentioned above.
The third condition is a wind pattern near the ocean surface that
spirals air inward. Bands of thunderstorms form, allowing the air
to warm further and rise higher into the atmosphere. If the
winds at these higher levels are relatively light, this structure
can remain intact and allow for additional strengthening.
The center, or eye, of a hurricane is relatively calm. The most
violent activity takes place in the area immediately around the
eye, called the eyewall. At the top of the eyewall
(about 50,000 feet), most of the air is propelled outward,
increasing the air's upward motion. Some of the air,
however, moves inward and sinks into the eye, creating a
cloud-free area.
Breeding Grounds
In the eastern Pacific, hurricanes begin forming by mid-May,
while in the Atlantic, Caribbean, and Gulf of Mexico,
hurricane development starts in June. For the United States, the
peak hurricane threat exists from mid-August to late October
although the official hurricane season extends through
November. Over other parts of the world, such as the western
Pacific, hurricanes can occur year-round.
Developing hurricanes gather heat and energy through contact with
warm ocean waters. The addition of moisture by evaporation from
the sea surface powers them like giant heat engines.
Storm Fury
Storm Surge
Storm surge is a large dome of water often 50 to 100 miles wide
that sweeps across the coastline near where a hurricane makes
landfall. The surge of high water topped by waves is devastating.
The stronger the hurricane and the shallower the offshore water,
the higher the surge will be. Along the immediate coast, storm
surge is the greatest threat to life and property.
Storm Tide
If the storm surge arrives at the same time as the high tide, the
water height will be even greater. The storm tide is the
combination of the storm surge and the normal astronomical
tide.
Storm Tide Facts
* over 6,000 people were killed in the Galveston Hurricane of
1900-most by the storm tide.
* Hurricane Camille in 1969 produced a 25-foot storm tide in
Mississippi.
* Hurricane Hugo in 1989 generated a 20-foot storm tide in
South Carolina.
Heavy Rains/Floods
Widespread torrential rains often in excess of
6 inches can produce deadly and destructive
floods. This is the major threat to areas well inland.
* Tropical Storm Claudette (1979) brought
45 inches of rain to an area near Alvin, Texas,
contributing to more than $600 million* in
damage.
* Long after the winds of Hurricane Diane (1955)
subsided, the storm brought floods to
Pennsylvania, New York, and New England
that contributed to nearly 200 deaths and
$4.2 billion* in damage.
* Hurricane Agnes (1972) fused with another
storm system, producing floods in the Northeast
United States which contributed to 122 deaths
and $6.4 billion* in damage.
* Adjusted to 1990 dollars.
Winds
Hurricane-force winds, 74 mph or more, can
destroy poorly constructed buildings and mobile
homes. Debris, such as signs, roofing material,
siding, and small items left outside, become flying
missiles in hurricanes. Winds often stay above
hurricane strength well inland. Hurricane
Hugo (1989) battered Charlotte, North Carolina
(which is about 175 miles inland), with gusts to
near 100 mph, dawning trees and power lines and
causing massive disruption.
Tornadoes
Hurricanes also produce tornadoes, which add to
the hurricane's destructive power. These
tornadoes most often occur in thunderstorms
embedded in rain bands well away from the center
of the hurricane. However, they can also occur
near the eyewall.
Areas At Risk
Coastal Areas and Barrier Islands
All Atlantic and Gulf coastal areas are subject to hurricanes
or tropical storms. Although rarely struck by hurricanes, parts
of the Southwest United States and Pacific Coast suffer
heavy rains and floods each year from the remnants of hurricanes
spawned off Mexico. Islands, such as Hawaii, Guam,
American Samoa, and Puerto Rico, are also subject to
hurricanes. During 1993, Guam was battered by five
typhoons. Hurricane Iniki struck the island of Kauai, Hawaii,
on September 11, 1992, resulting in $1.8 billion damage.
Due to the limited number of evacuation routes, barrier islands
are especially vulnerable to hurricanes. People on barrier
islands and in vulnerable coastal areas may be asked by local
officials to evacuate well in advance of a hurricane landfall. If
you are asked to evacuate, do so IMMEDIATELY!
Inland Areas
Hurricanes affect inland areas with high winds,floods, and
tornadoes. Listen carefully to local authorities to determine
what threats you can expect and take the necessary precautions to
protect yourself, your family, and your property.
Camille - August 14-22, 1969: 27 inches of rain in Virginia
caused severe flash flooding.
Agnes - June 14-22, 1972: Devastating floods from North
Carolina to New York produced many record-breaking river
crests. The storm generated 15 tornadoes in Florida and 2 in
Georgia.
Hugo - September 10-22, 1989: Wind gusts reached
nearly 100 mph as far inland as Charlotte, North Carolina.
Hugo sustained hurricane-strength winds until shortly after it
passed west of Charlotte.
Andrew - August 16-28, 1992: Damage in the United
States is estimated at $25 billion, making Andrew the most
expensive hurricane in United States history. Wind gusts in
south Florida were estimated to be at least 175 mph.
The United States Hurricane Problem
Population Growth
The United States has a significant hurricane problem. Our
shorelines attract large numbers of people. From Maine to Texas,
our coastline is filled with new homes, condominium towers, and
cities built on sand waiting for the next storm to threaten its
residents and their dreams.
There are now some 45 million permanent residents along the
hurricane-prone coastline, and the population is still growing.
The most rapid growth has been in the sunbelt from Texas through
the Carolinas. Florida, where hurricanes are most frequent, leads
the Nation in new residents. In addition to the permanent
residents, the holiday, weekend, and vacation populations swell
in some coastal areas 10- to 100-fold.
A large portion of the coastal areas with high population
densities are subject to the inundation from the hurricane's
storm surge that historically has caused the greatest loss of
life and extreme property damage.
Perception of Risk
Over the past several years, the warning system has provided
adequate time for people on the barrier islands
and the immediate coastline to move inland when hurricanes have
threatened. However, it is becoming more
difficult to evacuate people from the barrier islands and other
coastal areas because roads have not kept pace
with the rapid population growth. The problem is further
compounded by the fact that 80 to 9O percent of the
population now living in hurricane-prone areas have never
experienced the core of a "major" hurricane. Many of
these people have been through weaker storms. The result is a
false impression of a hurricane's damage
potential. This often leads to complacency and delayed actions
which could result in the loss of many lives.
Frequency of Hurricanes
During the 70's and 80's, major hurricanes striking the United
States were less frequent than the previous three decades. With
the tremendous increase in population along the high-
risk areas of our shorelines, we may not fare as well in the
future. This will be especially true when hurricane activity
inevitably returns to the frequencies experienced during the 40's
through the 60's.
In the final analysis, the only real defense against hurricanes
is the informed readiness of your community,your family, and YOU.
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