General Information About Hurricanes
Hurricanes- there are no other storm like them on earth. Hurricanes are
products of the tropical ocean and atmosphere. Powered by heat from the warm
sea water, 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, the 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 the Gulf of
Mexico. Many of these remain over the ocean. However, about five hurricanes
strike the United States coastline every three years. Of these five, two will
be major hurricanes (category 3 or greater).
A hurricane is a type of tropical cyclone - the general term for all
circulating weather systems over tropical waters. Tropical cyclones are
classified as follows:
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Tropical Depression - An organized system of clouds and thunderstorms with a
defined circulation and maximum sustained winds of 38 mph or less.
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Tropical Storm - An organized system of strong thunderstorms with a defined
circulation and maximum sustained winds of 39 to 73 mph.
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Hurricane - An intense tropical weather system with a well defined circulation
and maximum sustained winds of 74 mph or higher. In the western Pacific,
hurricanes are called "typhoons" and similar storms in the Indian Ocean are
called "cyclones."
Timely warnings have greatly reduced the number of hurricane fatalities in the
United States. In spite of this, property damage continues to mount due to the
explosion of growth along our coastline. There is little we can do about the
hurricanes themselves. However, the National Oceanic and Atmospheric
Administration's (NOAA) 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 to warn the public and help them become better prepared
for these storms.
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 begins June 1 and ends November
30. 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 Structure
The process by which a disturbance forms and subsequently strengthens into a
hurricane depends on at least three conditions: warm waters and moisture as
mentioned above, and 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.
Storm Fury:
Storm Surge
Storm surge is a large dome of water, often 50 to 100 miles wide that seeps
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 water, the higher the storm 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 storm surge and the
normal astronomical tide.
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. Even well built
buildings, if openings in the building envelope, such as doors and windows are
not protected may suffer wind damage. Wind blown debris may cause a window to
break or a door to fail and allow high winds to enter the home. The interior
forces of the wind then work together with the outside forces (creating uplift
on roofs) to cause a failure of parts of the building. Winds often stay above
hurricane strength well inland. Hurricane Hugo in 1989, battered Charlotte,
North Carolina (almost 200 miles inland) with gusts of 100 mph, downing trees
and power lines.
Heavy Rains and Flooding
Widespread torrential rains often in excess of 6 inches can produce deadly and
destructive flooding. This is the major threat to areas well inland.
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Tropical Storm Claudette (1979) brought 45 inches of rain to an area near
Alvin, Texas, contributing to more than $600 million in damage.
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Long after the winds of Hurricane Diane (1955) subsided, the storm brought
flood to Pennsylvania, New York, and New England that contributed to nearly 200
deaths and $4.2 billion in damage.
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Hurricane Georges (1998) caused major flooding
from Mississippi through the Florida panhandle.
Tornadoes
Hurricanes also produce tornadoes, which add to the hurricane's destructive
power. These tornadoes most often occur in the thunderstorms embedded in rain
bands well away from the center of the hurricane. However, they can also occur
near the eyewall. Microbursts, columns of fast moving air, are also produced
during hurricanes, and are blamed for much of the destruction from Hurricane
Andrew (1992) in south Dade County, FL.