an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain. Drawing energy from the sea surface and maintaining its strength as long as it remains over warm water, a tropical cyclone generates winds that exceed 119 km (74 miles) per hour. In extreme cases winds may exceed 240 km (150 miles) per hour, and gusts may surpass 320 km (200 miles) per hour. Accompanying these strong winds are torrential rains and a devastating phenomenon known as the storm surge, an elevation of the sea surface that can reach 6 metres (20 feet) above normal levels. Such a combination of high winds and water makes cyclones a serious hazard for coastal areas in tropical and subtropical areas of the world. Every year during the late summer months (July–September in the Northern Hemisphere and January–March in the Southern Hemisphere), cyclones strike regions as far apart as the Gulf Coast of North America, northwestern Australia, and eastern India and Bangladesh.

Tropical cyclones are known by various names in different parts of the world. In the North Atlantic Ocean and the eastern North Pacific they are called hurricanes, and in the western North Pacific around the Philippines, Japan, and China the storms are referred to as typhoons. In the western South Pacific and Indian Ocean they are variously referred to as severe tropical cyclones, tropical cyclones, or simply cyclones. All these different names refer to the same type of storm.

Tropical cyclones are compact, circular storms, generally some 320 km (200 miles) in diameter, whose winds swirl around a central region of low atmospheric pressure. The winds are driven by this low-pressure core and by the rotation of the Earth, which deflects the path of the wind through a phenomenon known as the Coriolis force. As a result, tropical cyclones rotate in a counterclockwise (or cyclonic) direction in the Northern Hemisphere and in a clockwise (or anticyclonic) direction in the Southern Hemisphere.

The wind field of a tropical cyclone may be divided into three regions, as shown in the diagram-->. First is a ring-shaped outer region, typically having an outer radius of about 160 km (100 miles) and an inner radius of about 30 to 50 km (20 to 30 miles). In this region the winds increase uniformly in speed toward the centre. Wind speeds attain their maximum value at the second region, the eyewall, which is typically 15 to 30 km (10 to 20 miles) from the centre of the storm. The eyewall in turn surrounds the interior region, called the eye, where wind speeds decrease rapidly and the air is often calm. These main structural regions are described in greater detail below.

Horizontal winds within the eye, on the other hand, are light. In addition, there is a weak sinking motion, or subsidence, as air is pulled into the eyewall at the surface (see the diagram-->). As the air subsides, it compresses slightly and warms, so that temperatures at the centre of a tropical cyclone are some 5.5 °C (10 °F) higher than in other regions of the storm. Because warmer air can hold more moisture before condensation occurs, the eye of the cyclone is generally free of clouds. Reports of the air inside the eye being “oppressive” or “sultry” are most likely a psychological response to the rapid change from high winds and rain in the eyewall to calm conditions in the eye.

Friction at the surface, in addition to lowering wind speeds, causes the wind to turn inward toward the area of lowest pressure, as shown in the diagram-->. Air flowing into the low-pressure eye cools by expansion and in turn extracts heat and water vapour from the sea surface. Areas of maximum heating have the strongest updrafts, and the eyewall exhibits the greatest vertical wind speeds in the storm—up to 5 to 10 metres (16.5 to 33 feet) per second, or 18 to 36 km (11 to 22 miles) per hour. While such velocities are much less than those of the horizontal winds, updrafts are vital to the existence of the towering convective clouds embedded in the eyewall. Much of the heavy rainfall associated with tropical cyclones comes from these clouds.

In addition to deep convective cells (compact regions of vertical air movement) surrounding the eye, there are often secondary cells arranged in bands around the centre. These bands, commonly called rainbands, spiral into the centre of the storm. Both the primary cloud band associated with the eyewall and the outer rainbands can be seen in the diagram-->. In some cases the rainbands are stationary relative to the centre of the moving storm, and in other cases they seem to rotate around the centre. The rotating cloud bands often are associated with an apparent wobbling of the storm track. If this happens as the tropical cyclone approaches a coastline, there may be large differences between the forecast landfall positions and actual landfall.

A circulation system goes through a sequence of stages as it intensifies into a mature tropical cyclone, as is shown in the map-->. The storm begins as a tropical disturbance, which typically occurs when loosely organized cumulonimbus clouds in an easterly wave begin to show signs of a weak circulation. Once the wind speed increases to 36 km (23 miles) per hour, the storm is classified as a tropical depression. If the circulation continues to intensify and the wind speeds exceed 63 km (39 miles) per hour, then the system is called a tropical storm. Once the maximum wind speed exceeds 119 km (74 miles) per hour, the storm is classified as a tropical cyclone.

● The temperature of the surface layer of ocean water must be 26.5 °C (80 °F) or warmer, and this warm layer must be at least 50 metres (150 feet) deep.

● A preexisting atmospheric circulation must be located near the surface warm layer.

● The atmosphere must cool quickly enough with height to support the formation of deep convective clouds.

● The middle atmosphere must be relatively humid at a height of about 5,000 metres (16,000 feet) above the surface.

● The developing system must be at least 500 km (300 miles) away from the Equator.

● The wind speed must change slowly with height through the troposphere—no more than 10 metres (33 feet) per second between the surface and an altitude of about 10,000 metres (33,000 feet).

A tropical cyclone that remains over the ocean and moves into higher latitudes will change its structure and become extratropical (extratropical cyclone) as it encounters cooler water, as is shown in the map-->. The transformation from a tropical to an extratropical cyclone is marked by an increase in the storm's diameter and by a change in shape from circular to comma- or v-shaped as its rainbands reorganize. An extratropical cyclone typically has a higher central pressure and consequently has lower wind speeds. Extratropical cyclones, which are fueled by a north-to-south variation of temperature, weaken and dissipate in a few days.

Tropical cyclones typically bring large amounts of water into the areas they affect. Much of the water is due to rainfall associated with the deep convective clouds of the eyewall and with the rainbands of the outer edges of the storm. Rainfall rates are typically on the order of several centimetres per hour with shorter bursts of much higher rates. It is not uncommon for totals of 500 to 1,000 mm (20 to 40 inches) of rain to be reported over some regions. Rainfall rates such as these may overwhelm the capacity of storm drains, resulting in local flooding. Flooding may be particularly severe in low-lying regions such as in Bangladesh and the Gulf Coast of the United States. It is also a problem in areas where mountains and canyons concentrate the rainfall, as occurred in 1998 when floods caused by rains from Hurricane Mitch washed away entire towns in Honduras.

atmosphere saffir-simpson Australian scale of cyclone intensityA wide range of wind speeds is possible between tropical cyclones of minimal strength and the most intense ones on record, and tropical cyclones can cause damage ranging from the breaking of tree limbs to the destruction of mobile homes and small buildings. To aid in issuing warnings to areas that may be affected by a storm, and to indicate the severity of the potential threat, numerical rating systems have been developed based on a storm's maximum wind speed and potential storm surge. For tropical systems in the Atlantic and eastern Pacific, the Saffir-Simpson hurricane scale is used (see the table (atmosphere saffir-simpson)). This scale ranks storms that already have reached hurricane strength. A similar scale used to categorize storms near Australia includes both tropical storms and tropical cyclones (see the table (Australian scale of cyclone intensity)). Though these two scales have different starting points, the most intense rating in each—category 5—is similar. Numerical ranking scales are not utilized in any of the other ocean basins.

Hurricane names for the North Atlantic Ocean Hurricane names for the eastern North Pacific OceanIn the United States, names given to hurricanes during World War II corresponded to radio code names for the letters of the alphabet (such as Able, Baker, and Charlie). In 1953 the U.S. National Weather Service began to identify hurricanes by female names, and in 1978 a series of alternating male and female names came into use. The lists of names are recycled every six years—that is, the 2003 list is used again in 2009, the 2004 list in 2010, and so on—as is shown in the table (Hurricane names for the North Atlantic Ocean) of tropical cyclone names for the North Atlantic and the table (Hurricane names for the eastern North Pacific Ocean) of names for the eastern North Pacific. Names of very intense, damaging, or otherwise newsworthy storms are retired. Names that will not be used again include Gilbert, a 1988 category 5 hurricane that had the lowest central atmospheric pressure (888 millibars) ever recorded in the Atlantic. Also retired is Mitch (Mitch, Hurricane), the name of a category 5 hurricane that stalled off the coast of Honduras for two days in 1998 before slowly moving inland, inundating Central America with heavy rain and causing mudslides and floods that took nearly 10,000 lives. Another notable storm whose name has been retired was Hurricane Ivan, which reached category 5 on three separate occasions during its long life cycle in September 2004. Ivan almost completely destroyed all agricultural infrastructure in Grenada, wrecked much of that year's crops in Jamaica, leveled 1.1 million hectares (2.7 million acres) of timber in Alabama, and caused almost 100 deaths along its path.

atmosphere w n pacificPacific and Indian basin storms are named according to systems established by regional committees under the auspices of the World Meteorological Organization. Each region maintains its own list of names, and changes to the list (such as retiring a name) are ratified at formal meetings. Two or more lists of names are alternated each year for several regions, including the central North Pacific (i.e., the Hawaii region), the western North Pacific and South China Sea (see the table (atmosphere w n pacific)), the southern Indian Ocean west of 90° E, the western South Pacific Ocean, and Australia's eastern, central, and northern ocean regions. In some areas, such as the northern Indian Ocean, tropical cyclones are given numbers instead of names.

Tropical oceans spawn approximately 80 tropical storms annually, and about two-thirds are severe (category 1 or higher on the Saffir-Simpson scale of intensity). Almost 90 percent of these storms form within 20° north or south of the Equator. Poleward of those latitudes, sea surface temperatures are too cool to allow tropical cyclones to form, and mature storms moving that far north or south will begin to dissipate. Only two tropical ocean basins do not support tropical cyclones, because they lack waters that are sufficiently warm. The Peru Current in the eastern South Pacific and the Benguela Current in the South Atlantic carry cool water Equatorward from higher latitudes and so deter tropical cyclone development. The Pacific Ocean generates the greatest number of tropical storms and cyclones, as can be seen in the map-->. The most powerful storms, sometimes called supertyphoons, occur in the western Pacific. The Indian Ocean is second in the total number of storms, and the Atlantic Ocean ranks third.

Tropical cyclones in both the Northern and Southern Hemispheres tend to move westward and drift slowly poleward, as shown in the map-->. Their motion is due in large part to the general circulation of the Earth's atmosphere. Surface winds in the tropics, known as the trade winds, blow from east to west, and they are responsible for the general westward motion of tropical cyclones. For the poleward movement, two other factors are responsible. One is the presence of large-scale regions of subsiding air, known as subtropical highs, over the oceans poleward of the trade winds. These regions of high atmospheric pressure have anticyclonic circulations (that is, clockwise circulation in the Northern Hemisphere and counterclockwise in the Southern), so that winds on the western edges of these large-scale circulations move toward the poles. The second factor is the Coriolis force, which becomes progressively stronger at higher latitudes. The diameter of a tropical cyclone is large enough for the Coriolis force to influence its poleward side more strongly, and hence the tropical cyclone is deflected toward the pole. Once a tropical cyclone moves poleward of the subtropical high, it begins to move eastward under the influence of the middle-latitude westerlies (which blow toward the east). When the motion of a tropical cyclone changes from westward to eastward, the tropical cyclone is said to recurve.

An array of geostationary satellites (those that remain over a fixed position on the Earth) is operated by a number of countries. Each of these satellites provides continuous displays of the Earth's surface in visible light (see the photograph-->) and in infrared wavelengths. It is the latter that are most important in tracking the stages of tropical cyclone development. Infrared images show the temperatures of cloud tops, thus allowing the loosely organized convection associated with easterly waves to be detected by the presence of cold, high clouds. They also show the deep, organized convection characteristic of a tropical cyclone. Satellite images not only show a storm's location but also can be used to estimate its intensity because certain cloud patterns are characteristic of particular wind speeds.

A general overview of the physical mechanisms of tropical cyclone formation and behaviour, with particular emphasis on Hurricane Andrew of 1992, is presented by Roger A. Pielke, Sr., and Roger A. Pielke, Jr., Hurricanes: Their Nature and Impacts on Society (1997). For a comprehensive survey of tropical cyclones, including threat assessment and prediction, see Robert H. Simpson and Herbert Riehl, The Hurricane and Its Impact (1981). John M. Williams and Iver W. Duedall, Florida Hurricanes and Tropical Storms (1997), discusses tropical storms and cyclones affecting Florida since 1871. A classic work on the structure and mechanics of tropical cyclones is Gordon E. Dunn and Banner I. Miller, Atlantic Hurricanes, rev. ed. (1964). C. Donald Ahrens, Meteorology Today, 6th ed. (1999), is an introductory textbook containing a chapter on tropical cyclones.The formation and tracking of Hurricane Gilbert is recorded in Hurricane! (1989), a video documentary produced for the American Public Broadcasting Service's NOVA series. Hurricanes (1998), produced for the History Channel's Wrath of God series, provides footage of three great cyclones that struck the United States in the 20th century.A technically advanced discussion of all aspects of tropical cyclones is given by Gary R. Foley, Hugh E. Willoughby, John L. McBride, Russell L. Elsberry, Isaac Ginis, and L. Chien, Global Perspectives on Tropical Cyclones, published by World Meteorological Organization (1995). Also at an advanced level, the factors controlling variability of Atlantic hurricanes, seasonal and long-term forecasts, and the impact of hurricanes on the insurance industry is presented by James B. Elsner and A. Birol Kara, Hurricanes of the North Atlantic: Climate and Society (1999).Joseph A. Zehnder