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).
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.
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. Recent measurements on Hurricane Bonnie (category 3) on 8/22/98 indicated that the highest peaks reached 59,000 feet (source: Tropical Rainfall Measuring Mission). Twice as high as Mount Everest, and about the altitude of the average airliner. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.
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 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.
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.
* 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.
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.
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.
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.
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!
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 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
Frequency of Hurricanes
In the final analysis, the only real defense against hurricanes is the informed readiness of your community, your family, and YOU.
All hurricanes are dangerous, but some are more so than others. The way storm surge, wind, and other factors combine determines the hurricanes destructive power. To make comparisons easier and to make the predicted hazards of approaching hurricane clearer to emergency forces-National Oceanic and Atmospheric Administration's hurricane forecasters use a disaster-potential scale which assigns storms to five categories. Category 1 is a minimum hurricane; category 5 is the worst case. The criteria for each category are shown below.
SAFFIR/SIMPSON HURRICANE SCALE
Storm surge is a great dome of water often 50 miles wide, that comes sweeping across the coastline near the area where the eye of the hurricane makes landfall. The surge, aided by the hammering effect of breaking waves, acts like a giant bulldozer sweeping everything in its path. The stronger the hurricane, the higher the storm surge will be. This is unquestionably the most dangerous part of a hurricane. Nine out of ten hurricane fatalities are caused by the storm surge. During the infamous Hurricane Camille in 1969, a 25-foot storm surge inundated Pass Christian in Mississippi. Lesser height are more usual but still extremely dangerous.
Many factors are involved in the formation and propagation of a storm surge such as the strength of the storm, bottom conditions where the surge comes ashore, and the point in the storm center in relation to the shore.
The floods and flash floods brought by the torrential rains of a hurricane are dangerous killers. Even though hurricanes weaken rapidly as they move inland, the remnants of the storm can bring 6 to 12 inches of rain or more to the area it crosses. The resulting floods have caused great damage and loss of life. Hurricane Diane of 1955 caused little damage as it moved into the continent; but long after its winds subsided it brought floods to Pennsylvania, New York, and New England that killed 200 persons and cost an estimated $700 million in damage. In 1972, Agnes fused with another storm system, flooding creek and river basins in the Northeast with more than a foot of rain in less than 12 hours, killing 117 people and causing almost $3 billion damage. Hurricane Beulah of 1967 brought major floods to southern Texas killing 10 persons and causing millions of dollars damage.
The winds of a hurricane by definition 74 miles an hour or more can be very dangerous. For some structures, wind force is sufficient to cause destruction. Mobile homes are particularly vulnerable to hurricane winds. Some hurricanes spawn tornadoes which contribute to incredible destruction. The greatest threat from a hurricane's winds is their cargo of debris, a deadly barrage of flying missiles such as lawn furniture, signs, roofing, and metal siding.
Here is a list of the many things to consider be fore, during and after a hurricane. Some of the safety rules will make things easier for you during a hurricane. All are important and could help save your life and the lives of others.
When a hurricane threatens your area, you will have to make the decision whether you should evacuate or whether you can ride out the storm in safety at home.
If local authorities recommend evacuation, you should leave! Their advice is based on knowledge of the strength of the storm and its potential for death and destruction.
Beyond individual and family actions during a hurricane emergency there is much to be done at the community level. Many communities on the Atlantic and Gulf coasts have made plans for action in the event a hurricane threatens, such as delineation of areas to be evacuated, shelter-designations, evacuation routes, and emergency operations of fire, police, and other public service units.
But many exposed coastal communities are not prepared for a hurricane, and others have waited for disaster's expensive lesson before taking corrective steps. To encourage community preparedness, NOAA's National Weather Service has invented a town, named Homeport, and made it a model of hurricane preparedness.
Copies of The Homeport story are available from Superintendent of Documents, U.S. Government Printing Office, Washington D.C., 20402. Stock number 0317-0046. Ask G.P.O. for current price.
By international agreement, tropical cyclone is the general term for all cyclone circulations originating over tropical waters, classified by form and intensity as follows:
Tropical disturbance: A moving area of thunder storms in the Tropics that
maintains its identity for 24-hours or more. A common phenomenon in the tropics.
Major hurricanes are relatively rare events at any location. Coastal residents from Brownsville Tex., to Eastport, Me., have a good chance of living many years without experiencing one. But none of our coastal areas are immune. "Not here! We haven't had a hurricane in years," could be the most dangerous words you'll ever hear. It's best to be prepared. This could be the year.
Hurricanes are tropical cyclones in which winds reach constant speeds of 74 miles per hour or more, and blow in a large spiral around a relatively calm center he eye of the hurricane. Every year, these violent storms bring destruction to coastlines and islands in their erratic path.
Stated very simply, hurricanes are giant whirl- winds in which air moves in a large tightening spiral around a center of extreme low pressure, reaching maximum velocity in a circular band extending outward 20 or 30 miles from the rim of the eye. This circulation is counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere. Near the center, hurricane winds may gust to more than 200 miles per hour. The entire storm dominates the ocean surface and lower atmosphere over tens of thousands of square miles.
The eye, like the spiral structure of the storm, is unique to hurricanes. Here, winds are light and skies are clear or partly cloudy. But this calm is deceptive, bordered as it is by maximum force winds and torrential rains. Many persons have been killed or injured when the calm eye lured them out of shelter, only to be caught in the maximum winds at the far side of the eye, where the wind blows from a direction opposite to that in the leading half of the storm.
Hurricane winds do much damage, but drowning is the greatest cause of hurricane deaths. As the storm approaches and moves across the coast line, it brings huge waves and storm tides which may reach 25 feet or more above normal. The rise may come rapidly, flooding coastal lowlands. Waves and currents erode beaches and barrier islands, undermine waterfront structures, and wash out highway and railroad beds. The torrential rains that accompany the hurricane pro duce sudden flooding as the storm moves inland. As its winds diminish, rainfall floods constitute the hurricane's greatest threat.
The hurricanes that strike the eastern United States are born in the tropical and subtropical North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Most occur in August, September, and October, but the six-month period from June 1 to November 30 is considered the Atlantic hurricane season.
The principal regions of tropical cyclone origin vary during the season. Most early (May and June) storms originate in the Gulf of Mexico and western Caribbean. In July and August, the areas of most frequent origin shift eastward, and by September are located over the larger area from the Bahamas southeastward to the Lesser Antilles, and thence eastward to south of the Cape Verde Islands, near the west coast of Africa. After mid-September, the principal areas of origin shift back to the western Caribbean and Gulf of Mexico.
On average, six Atlantic hurricanes occur per year. However, there are significant deviations from this average. In 1916 and 1950, 11 hurricanes were observed, and no hurricanes were observed in 1907 and 1914. During 1893, 1950, and 1961 seasons, four hurricanes were observed in progress at the same time.
Some hurricanes (usually weaker than their Atlantic counterparts) may strike Southern California and bring torrential rains to the southwest U.S.
Hurricanes begin as relatively small tropical cyclones which drift gradually to the west-north west (in the Northern Hemisphere), imbedded in the westward-blowing, tradewinds of the tropics. Under certain conditions these disturbances in crease in size, speed, and intensity until they become full-fledged hurricanes.
The storms move forward very slowly in the tropics, and may remain almost stationary for short periods of time. The initial forward speed is usually 15 miles per hour or less. Then, as the hurricane moves farther from the Equator, its forward speed tends to increase; at middle latitudes it may exceed 50 miles per hour in extreme cases.
The great storms are driven by the heat released by condensing water vapor, and by external mechanical forces. Once cut off from the warm ocean, the storm begins to die, starved for water and heat energy, and dragged apart by friction as it moves over the land.
These advisories and warnings are "headlined" in marine forecasts. (Details are included elsewhere in this brochure.) Small Craft Advisories can be issued up to 12 hours and warnings up to 24 hours prior to onset of adverse conditions.
Few people are affected more by weather than the mariner. An unexpected change in winds, seas, or visibility can reduce the efficiency of marine operations and threaten the very safety of a vessel and its crew. The National Weather Service (NWS), a part of the National Oceanic and Atmospheric Administration (NOAA), provides marine warnings and forecasts to serve all who sail for livelihood or recreation. This pamphlet describes those marine services available from the NWS and other agencies.
The warning and forecast program is the core of the NWS's responsibility to mariners. Warnings and forecasts provide the mariner with information for planning and decision making to protect life and property.
WIND SEA CONDITIONS
THIS CHART IS BASED ON CRITERIA USED BY THE WORLD METEOROLOGICAL ORGANIZATION. MARINERS SHOULD REALIZE THAT THESE VALUES ARE REACHED AFTER WINDS HAVE BLOWN STEADILY OVER A LARGE AREA FOR AN EXTENDED PERIOD OF TIME. ALSO, THE VALUES GIVEN ARE AVERAGE OBSERVED WAVE HEIGHTS, NOT THE HIGHEST THAT MAY BE SEEN FOR A GIVEN WIND SPEED.
Link to Ohio-State Hurricane FAQ