What is a hurricane? A hurricane is a type of tropical cyclone (the general term for all circulating weather systems which form over the tropical waters). A tropical cyclone is a warm-core low pressure system which is not a front, that develops over the tropical or subtropical waters, and has an organized circulation. Tropical cyclones are classified in one of three categories: Tropical Depression: An organized system of clouds and thunderstorms with a defined circulation, and maximum sustained winds of 38 mph (33 knots) or less. Tropical Storm: An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 mph to 73 mph (34-63 knots). Hurricane: An intense tropical weather system with a well-defined circulation, producing maximum sustained winds of 74 mph (64 knots) or greater. In the western Pacific, hurricanes are called "typhoons" and similar storms in the Indian Ocean are called "cyclones". How are hurricanes classified? Hurricanes are classified by their wind speed, using the Saffir-Simpson hurricane scale. The stronger the wind, the higher the category given, and the more damage the hurricane can cause. A hurricane with winds of 150 mph has four times the energy of a hurricane with 75 mph winds. Hurricanes range from a Category 1 to Category 5, as seen in the table below: Saffir-Simpson Scale Category Wind Speed(kt) Wind Speed(mph) Central Pressure (mbs) Central Pressure (in) Damage 1 64 - 82 74 - 95 > 980 mb > 28.50 hg Minimal 2 83 - 95 96 - 110 965-979 mb 28.50-28.91 hg Moderate 3 96 - 113 111 - 130 945-964 mb 27.91-28.47 hg Extensive 4 114 - 135 131 - 155 920-944 mb 27.17-27.88 hg Extreme 5 > 135 > 155 less than 919 mb less than 27.17 hg Catastrophic A major hurricane is defined as a category 3 or higher hurricane. Why is it called "hurricane"? The word hurricane comes from the Carib Indians of the West Indies, who called this storm a huracan. Supposedly, the ancient Tainos tribe of Central America called their god of evil "Huracan". Spanish colonists modified the word to hurricane. When do hurricanes form? Almost all tropical storms and hurricanes in the Atlantic basin (which includes the Gulf of Mexico and Caribbean Sea) form between June 1 and November 30, known as hurricane season. On rare occasions, a tropical disturbance can form a little earlier or later than this. Where do hurricanes form? Although tropical systems can form anywhere in the Atlantic basin during hurricane season, some areas are more prone to develop tropical storms and hurricanes at certain times of season. Early in the season (June through early July), tropical cyclones occur almost exclusively in the Gulf of Mexico and western Caribbean Sea. By early July, the formation area gradually shifts eastward and continues eastward through late July. By late August, formation is over a broad area extending eastward to near the Cape Verde Islands (a few hundred miles west of the westernmost tip of north Africa). The formation area then moves westward in mid-September, returning to the western Caribbean Sea by early October. How do hurricanes form? Hurricanes form in the warm waters of tropical and sub-tropical oceans, seas, or Gulf of Mexico. Most hurricanes evolve from tropical disturbances. A tropical disturbance is a discrete system of organized convection (showers or thunderstorms), that originate in the tropics or subtropics, does not migrate along a frontal boundary, and maintains its identity for 24 hours or more. The seedlings of hurricanes can come from: Easterly Waves: Also called tropical waves, this is an inverted trough moving generally westward in the tropical easterlies. A trough is defined as a region of relative low pressure. The majority of hurricanes form from easterly waves. West African Disturbance Line (WADL): This is a line of convection (similar to a squall line) which forms over West Africa and moves into the Atlantic Ocean. WADL's usually move faster than tropical waves. TUTT: A TUTT is a trough, or cold core low in the upper atmosphere, which produces convection. On occasion, one of these develops into a warm-core tropical cyclone. Old Frontal Boundary: Remnants of a polar front can become lines of convection and occasionally generate a tropical cyclone. This will occur early or late in the hurricane season in the Gulf of Mexico or Caribbean Sea. Once a disturbance forms and convection develops, it can become more organized under certain conditions. If the disturbance moves or stays over warm water (at least 80F), and upper level winds remain weak, the disturbance can become more organized. How are individual hurricanes identified? When a tropical depression forms, it is identified by a number identical to the number of depressions which have formed during the hurricane season. When a tropical depression intensifies to a tropical storm, it is given a name, and this name is kept if and when the storm intensifies into a hurricane. The first tropical storm is given a name starting with "A", the next storm "B" and so on through the alphabet. The names alternate between male and female names. Moreover, the names now reflect the three main languages of the area: English, Spanish and French. The same names are used every five years. On occasion, an extremely powerful and/or destructive hurricane will have its name retired. How will I know when a hurricane threatens my area? Four times a day during the hurricane season, the Tropical Prediction Center (TPC) issues a product called the "Tropical Weather Outlook". This product discusses the location of tropical disturbances and cyclones, along with a forecast movement and intensification, weakening or dissipation. When a tropical cyclone forms, the TPC will issue separate advisories to the public and marine communities. Public and marine tropical depression advisories (for winds below 38 mph), tropical storm advisories (for winds 39-73 mph), or hurricane advisories (for winds 74 mph or greater) are formal messages, giving warning information and details on tropical cyclone location, intensity and movement. Advisories are issued every six hours, or every two or three hours when a tropical storm or hurricane approach a coastal area. If a tropical storm, or hurricane poses an imminent threat to a certain area, the TPC will either issue a watch or a warning: Tropical Storm Watch: This will be issued when tropical storm conditions, including sustained winds of 39 to 73 mph pose a possible threat to a specified coastal area generally within 36 hours. Hurricane Watch: This will be issued for a specified coastal area when a hurricane or an incipient hurricane condition is a potential threat generally within 36 hours. Tropical Storm Warning: This will be issued when tropical storm conditions, including sustained winds of 39 to 73 mph, are expected in specified coastal area generally within 24 hours. Hurricane Warning: A hurricane warning is issued when sustained winds of 74 mph or higher, caused by a hurricane, are expected in a specified coastal area in generally within 24 hours. What are the main threats from a hurricane when it approaches? Storm Surge: This is the greatest hurricane threat to life and property, and it can be devastating. 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 stronger the hurricane and the shallower the offshore water, the higher the storm surge will be. Storm Tide: The storm tide is the combination of the storm surge and the normal astronomical tide. For example, a normal tide of 2 feet added to a 12 foot storm surge results in a 14 foot storm tide. The "worst-case scenario" for an approaching hurricane is for the storm surge to arrive around high astronomical tide. 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 storm surge, battering waves, and high winds can be devastating and deadly. Winds and Squalls: A squall is a significant increase in the average or sustained wind speed which will usually be observed with a convective line of showers and thunderstorms. Hurricane force winds can destroy poorly constructed buildings and mobile homes. Debris, such as roofing material, traffic signs, aluminum siding, and small outdoor items can become flying missiles. Remember, a hurricane with winds of 150 mph has four times the energy of a hurricane with winds of 75 mph. Heavy Rain and Flooding: Heavy rain and flooding is a threat to inland areas as well as coastal communities. Rainfall in excess of 6 inches is likely in many hurricanes, and can produce deadly and devastating floods. Tornadoes: These tornadoes most often occur in thunderstorms embedded in rain bands well away from the center of the hurricane, but can also occur just outside the center of the hurricane. What can be expected when a hurricane impacts an area? The first precursor of a incoming hurricane is the pre-hurricane squall line. The line resembles a line of showers and thunderstorms along a mid latitude cold front. Gusty winds accompany the showers and thunderstorms. The line is usually 100 to 200 miles ahead of the eye, but can be as much as 500 miles ahead of the eye in very large hurricanes. The next part of a hurricane to affect the area are the outer convective bands. The typical hurricane has two or three of these bands (occasionally more in large hurricanes) which are comprised of cells resembling ordinary thunderstorms. These bands are in advance of the main rain shield. Wind gusts are higher in these bands than in the pre-hurricane squall line. The rain shield is a solid or nearly solid area of rain that typically becomes heavier as the eye wall approaches. The wind, both sustained and peak gusts, keeps increasing as you move through the rain shield toward the storm's center or eye. Another portion of the hurricane are the convective rings and bands. Also called spiral bands, when these pass through a location, the wind speed increases by as much as 50%, accompanied by a significant increase in the rainfall rate. When tornadoes and downbursts occur, they are likely to come from convective rings and bands. Between convective bands, an area of stratiform rings and bands exist. Very light (if any) rain usually occurs while this area of the hurricane passes. The eye wall will precede the hurricane's center. The eye wall is an organized band of convection that immediately surrounds the center. The fiercest winds and most intense rainfall typically occur near the eye wall. Finally, the center of the hurricane or eye will pass over the area. The eye is usually a relatively calm center in the hurricane. The winds are light, the skies are partly cloudy or even clear and rain-free. The diameter of the eye can be as small as 5 miles or as large as 100 miles, averaging about 20 miles. Does this mean that the hurricane is over? NO! Once the eye passes, the area will again experience the eye wall, spiral bands, and so on. Many residents can be tricked into believing that the eye is the end of the hurricane. This can be disastrous. Hurricane Awareness -- National Weather Service Corpus Christi | Revised August 2000.