United States Search and Rescue Task Force
Picture of Hurricane From Space - Note The "Eye" In The Center
What is a Hurricane?
Hurricanes are high speed windstorms accompanied by torrential rains. They begin over the ocean where air rising from warm seas creates a severe low pressure zone. This zone draws air to it with such force the winds rotate around the core (the 'eye') at up to 185 mph. The 'eye' averages about 20 miles in diameter while the hurricane may be up to a few hundred miles in diameter. After the onset of a hurricane at sea, the storm moves slowly toward one of the poles and loses force as it moves into cooler areas or over land. Hurricanes are usually predicted by meteorologists well in advance of their occurrence.
A hurricane is a type of tropical cyclone, the general term for all circulating weather systems over tropical waters (counterclockwise in the Northern Hemisphere). Tropical cyclones are classified as follows:
Hurricanes are products of a 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, 10 tropical storms, of which six become hurricanes, develop over the Atlantic Ocean, Caribbean Sea, or Gulf of Mexico from June to November. 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 on the Saffir-Simpson Hurricane Scale.
Saffir-Simpson Hurricane Scale
Florence 1988 (LA)
Kate 1985 (FL Panhandle)
Alicia 1983 (N TX)
Andrew 1992 (S FL)
|5||> 155||Catastrophic||Camille 1969 (LA/MS)
Labor Day Hurricane 1935 (FL Keys)
In the eastern Pacific, hurricanes start forming by mid-May. In the Atlantic, Caribbean, and Gulf of Mexico, hurricanes season starts in June. For the United States, 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.
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.
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. For example as hurricane moves ashore, a 15-foot surge added to the normal 2-foot tide creates a storm tide of 17 feet. This mound of water, topped by battering waves, moves ashore along an area of the coastline as much as 100 miles wide. The combination of the storm surge, battering waves, and high winds is deadly.
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, downing trees and power lines and causing massive disruption.
Widespread torrential rains often in excess of 6 inches can produce deadly and destructive floods. This is the major threat to areas well inland.
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.
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.
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.
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 90 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.
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.
Geostationary satellites orbiting the earth at an altitude of about 22,000 miles above the equator provide imagery both day and night. The satellite imagery helps provide estimates of the location, size, and intensity of a storm and its surrounding environment.
The US Air Force Reserve provides most of the operational reconnaissance. Pilots fly aircraft into the core of a hurricane to measure wind, pressure, temperature, and humidity as well as to provide an accurate location of the center of the hurricane. The National Oceanic and Atmospheric Administration also flies aircraft into hurricanes to aid scientists in better understanding these storms and to improve forecast capabilities. The NOAA flights also provide operational support as required.
When a hurricane gets close to the coast, it is monitored by land-based weather radars. The National Weather Service is currently installing Doppler weather radars across the country which will add new dimensions to hurricane warning capabilities. They will provide detailed information on hurricane wind fields and their changes. Local NWS offices will be able to provide more accurate short-term warnings for floods, tornadoes, and inland high winds.
The National Hurricane Center uses several different numerical computer models to aid in forecasting the path, speed, and strength of hurricanes. Data from weather satellite sensors, reconnaissance aircraft, and other sources are fed into these computer models. The National Hurricane Center also has a computer storm surge model. This model provides guidance on storm surge height and the extent of flooding it will cause.
What To Listen For....
The National Weather Service continuously broadcasts updated hurricane advisories that can be received by NOAA Weather Radios sold in many stores. The average range is 40 miles, depending on topography. Your National Weather Service recommends purchasing a radio that has both a battery backup and a tone-alert feature which automatically alerts you when a watch or warning is issued.
During the Storm
Only stay in a home if you have NOT been ordered to leave. Stay inside a well constructed building. In structures, such as a home, examine the building and plan in advance what you will do if winds become strong. Strong winds can produce deadly missiles and structural failure.
National Oceanic & Atmospheric Administration (NOAA)
In-Depth 19 Page Hurricane Information (Adobe Acrobat file which does take time to load)
Devastation Caused by Hurricane
Total Destruction of Trailer Park by Hurricane Andrew
Devastation Caused by Hurricane