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Tropical cyclone - Wikipedia. A tropical cyclone is a rapidly rotating storm system characterized by a low- pressure center, a closed low- level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane (),[1][2][3]typhoon (), tropical storm, cyclonic storm, tropical depression, and simply cyclone.[4] A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean and a typhoon occurs in the northwestern Pacific Ocean; while in the south Pacific or Indian Ocean, comparable storms are referred to simply as “tropical cyclones” or “severe cyclonic storms”.[4]“Tropical” refers to the geographical origin of these systems, which form almost exclusively over tropical seas. Cyclone” refers to their winds moving in a circle,[5] whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and blowing clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation.

This energy source differs from that of mid- latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. Tropical cyclones are typically between 1. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator.[6] Tropical cyclones are also almost completely absent from Earth’s southwestern quartersphere, mainly because the shapes of the African and South American continents permit the Benguela and Humboldt Currents to cover ocean basins as far north as 5˚N with excessively cool water. These powerful cold currents also produce much stronger vertical wind shear in the South Atlantic and Southeast Pacific, which typically prevent tropical depressions and minor storms there from developing into cyclones and prevent even the waters of the Brazil Current from being so hot as analogous western boundary currents or ocean gyres. Also, the African easterly jet and areas of atmospheric instability which gives rise to cyclones in the Atlantic Ocean and Caribbean Sea, along with the Asian monsoon and Western Pacific Warm Pool, are feature of the Northern Hemisphere and Australia.

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Coastal regions are particularly vulnerable to the impact of a tropical cyclone, compared to inland regions. The primary energy source for these storms is warm ocean waters, therefore these forms are typically strongest when over or near water, and weaken quite rapidly over land. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to severe pressure changes), and the potential of spawning tornadoes. Tropical cyclones also draw in air from a large area—which can be a vast area for the most severe cyclones—and concentrate the precipitation of the water content in that air (made up from atmospheric moisture and moisture evaporated from water) into a much smaller area. This continual replacement of moisture- bearing air by new moisture- bearing air after its moisture has fallen as rain, may cause extremely heavy rain and river flooding up to 4.

Though their effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperatelatitudes, which may play an important role in modulating regional and global climate. Physical structure. Diagram of a Northern hemisphere hurricane. Tropical cyclones are areas of relatively low pressure in the troposphere, with the largest pressure perturbations occurring at low altitudes near the surface. On Earth, the pressures recorded at the centers of tropical cyclones are among the lowest ever observed at sea level.[7] The environment near the center of tropical cyclones is warmer than the surroundings at all altitudes, thus they are characterized as “warm core” systems.[8]Wind field.

The near- surface wind field of a tropical cyclone is characterized by air rotating rapidly around a center of circulation while also flowing radially inwards. At the outer edge of the storm, air may be nearly calm; however, due to the Earth’s rotation, the air has non- zero absolute angular momentum. As air flows radially inward, it begins to rotate cyclonically (counter- clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere) in order to conserve angular momentum. At an inner radius, air begins to ascend to the top of the troposphere. This radius is typically coincident with the inner radius of the eyewall, and has the strongest near- surface winds of the storm; consequently, it is known as the radius of maximum winds.[9] Once aloft, air flows away from the storm's center, producing a shield of cirrus clouds.[1. The previously mentioned processes result in a wind field that is nearly axisymmetric: Wind speeds are low at the center, increase rapidly moving outwards to the radius of maximum winds, and then decay more gradually with radius to large radii.

However, the wind field often exhibits additional spatial and temporal variability due to the effects of localized processes, such as thunderstorm activity and horizontal flow instabilities. In the vertical direction, winds are strongest near the surface and decay with height within the troposphere.[1. Eye and center. NASA animation of Hurricane Arthur in 2. GPM satellite data. At the center of a mature tropical cyclone, air sinks rather than rises. For a sufficiently strong storm, air may sink over a layer deep enough to suppress cloud formation, thereby creating a clear “eye”.

Weather in the eye is normally calm and free of clouds, although the sea may be extremely violent.[1. The eye is normally circular in shape, and is typically 3. The cloudy outer edge of the eye is called the “eyewall”.

The eyewall typically expands outward with height, resembling an arena football stadium; this phenomenon is sometimes referred to as the stadium effect.[1. The eyewall is where the greatest wind speeds are found, air rises most rapidly, clouds reach to their highest altitude, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.[1. Watch The 39 Steps Online there. In a weaker storm, the eye may be obscured by the central dense overcast, which is the upper- level cirrus shield that is associated with a concentrated area of strong thunderstorm activity near the center of a tropical cyclone.[1. The eyewall may vary over time in the form of eyewall replacement cycles, particularly in intense tropical cyclones. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward, which is believed to rob the primary eyewall of moisture and angular momentum. When the primary eyewall weakens, the tropical cyclone weakens temporarily.

The outer eyewall eventually replaces the primary one at the end of the cycle, at which time the storm may return to its original intensity.[1. Rapid deepening. On occasion, tropical cyclones may undergo a process known as rapid deepening, a period in which the minimum sea- level pressure of a tropical cyclone decreases by 4. In order for rapid deepening to occur, several conditions must be in place.

Water temperatures must be extremely high (near or above 3. C, 8. 6 °F), and water of this temperature must be sufficiently deep such that waves do not upwell cooler waters to the surface. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted.

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