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About Tropical Storms and Tornadoes
 Cyclones The term cyclone, in common use, is sometimes applied to a tornado. In the science of meteorology, however, the term has a different meaning. For meteorologists a cyclone - and its counterpart, an anticyclone - is a large-scale system of air circulation in the atmosphere in the zones between the equator and either of the poles. It can be considered as either producing or resulting from differences in air pressure in those zones. In a cyclone the central air pressure is lower than that of the surrounding environment, and the flow of circulation is clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. Cyclones are also characterized by low-level convergence and ascending air within the system. An anticyclone system has characteristics opposite to that of a cyclone. That is, an anticyclone's central air pressure is higher than that of its surroundings, and the air flow is counterclockwise in the Southern Hemisphere and clockwise in the Northern Hemisphere. Anticyclones are usually characterized by low-level divergence and subsiding air. Semipermanent Systems Semipermanent cyclone systems rarely vary during a season. One example is the Bermuda High in the northern subtropical region. Others include the Siberian High and the Aleutian Low, which dominate winter in the middle and high latitudes of Asia and North America. The subtropical high-pressure belts in the atmosphere coincide with the descending legs of the air-circulation mechanisms known as Hadley Cells. Subsiding air heats the atmosphere by adiabatic compression, producing an intense subsidence inversion within the first 2 kilometers (1.2 miles) of the atmosphere. The inversion, characterized by an extremely warm layer in the atmosphere, forms a stable lid that creates air-pollution problems in many cities. These semipermanent subtropical centers of high pressure develop as direct responses to surface heating anomalies, such as those produced by the differential heating of continents and oceans or by variations in the sea's surface temperature. Due to the effect of the Hadley cell, the subtropics remain at a fairly high pressure throughout the year. The centers change intensity and adjust their longitudinal position, however, to compensate for changing temperature and pressure gradients between land and ocean. Surface-pressure anomalies develop at higher latitudes by similar processes. During summer, land areas are considerably warmer than adjacent oceans, producing rising air over the land and subsidence over the oceans. The resulting pressure gradient causes cool ocean air to flow toward the warm land surface. The Coriolis effect deviates this flow, producing cyclonic flow over the land and anticyclonic flow over the sea. During winter the situation is reversed. The land cools quickly, having little stored heat. Consequently high-pressure regions form over the land, while low-pressure regions dominate the ocean. With the clear atmosphere of the subsident region, the land surface can continue cooling. The loss of heat is compensated for by an increase of energy that flows into the system, as a warm air flow, from the oceanic low-pressure region. When the amount of energy radiated to space matches the inflow, an equilibrium is reached, but by that time a very deep high-pressure region has developed. Transient Systems The second cyclonic group consists of transient cyclones and anticyclones associated with weather systems. Located in the equatorial and middle latitudes, they may grow, mature, and decay within a few days. Depressions in middle latitudes are cyclonic systems that develop rapidly and move eastward against the basic westerly flow, over distances from 500 to 2,000 kilometers (30 to 1,200 miles). Central pressures often fall below 990 millibars (mb). Inclement weather, strong winds (connected to the high-pressure gradient), and squalls are associated with such mid-latitude systems, which result from basic instabilities of a heated and rotating atmosphere. Because of the Coriolis effect, the upper tropospheric flow toward the pole in the Hadley cell is forced eastward, developing strong westerlies. The air accelerates as it moves progressively poleward. Because the winds are produced by pressure gradients, which in turn are functions of the temperature distributions, zones of strong winds ought to be associated with strong temperature gradients. Were this situation to continue, the wind and temperature gradients would build up an infinite potential-energy reservoir. If such a system is perturbed, however, so that cold air moves equatorward across the gradient and warm air moves poleward, rapid changes will ensue. As the light warm air overrides dense cold air and the latter undercuts warm air, a thermal circulation develops that taps the potential-energy store. The perturbation continues to grow, effectively relaxing the north-south temperature gradient and reducing the speed of the intense westerlies. This process, called a baroclinic instability, is the cause of most middle-latitude depressions. Subsequent development continues to move warm air poleward and cold air equatorward, producing adjacent pools of warm and cold air. The resultant large east-west temperature gradient produces a pressure distribution that causes a cyclonic circulation around the low-pressure center and an anticyclonic flow around the high. In the tropics, cyclonic systems known as tropical depressions may develop with central pressures less than 2 millibars lower than the environment. Associated with periods of intense rain, these systems usually move westward. Those which intensify significantly (pressures falling below 950 millibars) are called tropical cyclones or hurricanes. Because their horizontal scale is far less than that of their middle-latitude counterparts, the pressure gradient is tighter, resulting in more intense winds Hurricanes and Typhoons
 Hurricanes and typhoons are large and sometimes intensely violent storm systems. In meteorological terms, they are tropical cyclones that have maximum sustained winds of at least 120 kilometers per hour (75 miles per hour). Atlantic and eastern Pacific storms are called hurricanes, from the West Indian huracan ("big wind"), whereas western Pacific storms are called typhoons, from the Chinese taifun, "great wind." The primary energy source for a tropical cyclone is the latent heat released when water vapor condenses. Only extremely moist air can supply the energy necessary to spawn and maintain tropical storms, and only very warm air contains enough moisture. Tropical cyclones, therefore, form only over oceans with water temperatures of at least 27°C (80°F). After they have formed, such storms tend to intensify when passing over warmer water and weaken over colder water. Structure of the Storm The mature tropical cyclone is characterized by a circular pattern of stormclouds and torrential rains, whipped by winds that may reach velocities of 160 to 300 kilometers per hour (100 to 180 miles per hour) within a radius of 10 to 100 kilometers (6 to 60 miles) from the storm center. The winds diminish rapidly with increasing distance. At a radius of 500 kilometers (300 miles), wind speed is usually less than 30 kilometers per hour (18 miles per hour). The winds rotate in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. The heaviest precipitation occurs in this region of intense convection. Thunderstorms may produce rainfall rates of 250 millimeters (10 inches) a day. The release of latent heat associated with this rain maintains low pressure and strong winds. The total cloud system of a large tropical cyclone may have a diameter of up to about 3,200 kilometers (2,000 miles). At the center of the storm, within a "wall" of powerful winds, there is an "eye" - a cloud-free circular region of relatively light winds that has a diameter of 10 to 100 kilometers (6 to 60 miles). Surface pressure reaches its minimum in the eye. Typical values are 950 millibars, but values of less than 900 have been recorded. The sinking motion in the eye, which causes the clearing, also produces adiabatic warming and drying. Temperatures at 5 kilometers (3 miles) above sea level are typically 10°C (18°F) warmer than the tropical storm's environment. The very high-velocity winds surrounding the eye are maintained in strength by the large differences in horizontal pressure between the eye and the outer region of the storm. Although the winds themselves are responsible for much of the storm damage, the waves and tides generated by the wind often cause most of the damage to coastal areas. Because much of human activity near the coast is concentrated within a few meters above mean sea level, storm surges can result in considerable loss of life and property. Speed of Rotation The rapidly whirling tangential circulation of winds in a tropical cyclone can be explained by the conservation of angular momentum. Just as ice skaters spin faster as they bring their arms down, closer to the axis of rotation, so the air rotates faster as it is pulled in toward the center of the storm by the low pressure. Without friction, the wind would increase as the inverse of the distance from the center. Thus, a wind rotating at 5 kilometers per hour (3 miles per hour) at a radius of 500 kilometers (300 miles) would have a velocity of 250 kilometers per hour (160 miles per hour) if it reached a radius of only 10 kilometers (6 miles). Friction reduces the predicted speed, but the basic principle explains the high rotational velocities near the center. The air that spirals toward the center and rises in the intense convection in the wall of the eye turns outward in the upper troposphere (about 15 kilometers (10 miles) above sea level). As the air moves away from the center, its counterclockwise rotation slows, in accord with conservation of angular momentum. At a distance of about 300 kilometers (190 miles) from the center, the air acquires an anticyclonic (clockwise) rotation. Occurrence and Movement Tropical cyclones move with the large-scale wind currents in which they are embedded. The typical speed is 25 kilometers per hour (16 miles per hour), but some storms may race along at twice this speed. Others can remain stalled in the same location for several days. In the North Atlantic Ocean, tropical storms tend to develop primarily during the summer months of highest humidity and warmest water-surface temperatures, and often appear on into October. Occasional storms develop just before or after this period but only rarely in other months. Usually about five of these tropical cyclones become strong enough to be categorized as hurricanes. Typically these storms track from east to west at low latitudes, moving with the eastern winds of the large subtropical anticyclone that dominates that ocean area. As the storms approach the North American continental landmass, however, they often begin to take a more northerly tack as they curve around the western rim of the anticyclone. (Storms that do not curve in this way enter the Gulf of Mexico or cross over Central America.) As they reach higher latitudes and come under the influence of the westerlies, they usually turn toward the northeast, often missing the continent. This turn to the northeast is called recurvature. Typhoons of the western Pacific Ocean develop almost exclusively in a band between latitudes 6 degrees and 35 degrees, both North and South of the equator. Those in the Northern Hemisphere occur most frequently in the period from July to November. Once developed, such a typhoon generally tracks northwestwardly while it remains in the zone of the trade winds. Thereafter the storms most commonly recurve in a northeastward direction, generally picking up speed as they enter the wind zone of prevailing westerlies. Typhoons are observed most often in the general vicinity of the South China Sea, but devastating storms have also frequently occurred in the Bay of Bengal. Tornadoes
 The word tornado is probably derived from the Spanish tronada ("thunderstorm"). Tornadoes are also popularly called twisters or cyclones and are characterized by rapidly rotating columns of air hanging from cumulonimbus clouds. They are generally observed as tube- or funnel-shaped clouds. At ground level they usually leave a path of destruction only about 50 meters (170 feet) wide and travel an average of only about 8 to 24 kilometers (5 to 15 miles). Ground contact is often of an intermittent nature - lasting usually less than a couple of minutes in any particular area - because the funnel skips along. Tornadoes generally exhibit a certain characteristic cycle of behavior between formation and final disappearance. The first sign of a tornado may be a strong whirlwind of dust from the ground surface, often in conjunction with the appearance of a short funnel growing from the storm cloud above it. The funnel then becomes more organized and descends further from the cloud, sometimes touching the ground. (The winds forming the funnel generally move counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, but exceptions are observed.) The funnel as a whole commonly moves forward slowly but can travel at speeds greater than 30 meters (100 feet) per second. The tornado eventually becomes fragmented and dissipates. Causes and Classification
 Tornadoes are the result of great instability in the atmosphere and are often associated with severe thunderstorms (above). The full details of the formation of tornadoes are not known. The existence of a strong updraft, such as that generated by a severe thunderstorm, and the conservation of angular (rotational) momentum, however, are fundamental considerations. The falling of rain or hail probably drags air from aloft, and the resultant inrush of air tightens the rotational motion. The tornado cyclone is an area of low pressure about 8 to 24 kilometers (5 to 15 miles) in diameter, with wind speeds of approximately 240 kilometers per hour (150 miles per hour) or less. At the cyclone's center is the tornado proper, a funnel that becomes visible as it picks up surface matter. The funnel exhibits exceedingly high winds and low pressures. Such winds can pick up and hurl objects with terrible force and cause tremendous damage to structures insufficiently well built to resist them. Wind speeds of 800 kilometers per hour (500 miles per hour) or more have been inferred. In the United States, tornadoes are most often associated with conditions in advance of cold fronts, and weather forecasts include tornado alerts when these conditions arise. Tornadoes can occur, however, ahead of warm fronts or even behind cold fronts. Tornadoes also occur frequently in association with hurricanes. A tornado that begins on land and then crosses water may be called a waterspout. A waterspout is a funnel cloud extending from the base of a cloud to the sea surface. Its formation requires high surface temperatures and humidities. Waterspouts are common in all equatorial oceans and inland seas. Like tornadoes, they are part of convective cloud systems, and they result from similar formation processes. Rapid updrafts within the cloud cause an inflow of moist low-level air that spirals faster and faster toward the updraft region. The water in a waterspout comes from condensation of water vapor in the inflow air and not from rising seawater. Whereas tornadoes are usually associated with foul weather and possess great destructive power, waterspouts may occur in fine weather with small clouds and are rarely strong enough to cause damage. The term waterspout, however, is applied more commonly to a less intense form of tornado activity that originates over a body of water and is not necessarily associated with storm activity. The water in the spout comes from condensation, not from the water below. Tornadoes are now classified on the Fujita-Pearson scale, which links maximum wind speed, path length, and path width. A 0,0,0 tornado would have maximum wind speeds of below 117 kilometers per hour (73 miles per hour), a path length of less than 1.6 kilometers (1 miles), and a path width of no greater than 16 meters (53 feet); a 5,5,5 tornado would have values of 420 to 512 kilometers per hour (261 to 318 miles per hour), 161 to 507 kilometers (100 to 315 miles), and 1.6 to 5.0 kilometers (1.0 to 3.1 miles), respectively. The Fujita scale for damaging winds uses only the first digit of the Fujita-Pearson scale. The table below lists the Fujita scale :
Occurrence The greatest incidence of tornadoes is generally assumed to be in North America, and especially in the Mississippi Valley. On an equal area basis, however, other countries, such as Italy, New Zealand, and the United Kingdom, exceed or at least challenge the incidence rate of the United States. In actual numbers observed, Australia ranks second to the United States. The United States is notable for the incidence of severe tornadoes of scale 4 or 5. Tornadoes occurring in the tropics are usually extremely weak and often begin as waterspouts. The Stockholm (Sweden) and Saint Petersburg (Russia) areas appear to be the northernmost regions that experience tornadoes. Within the United States, Texas records the greatest number, usually about 15 to 20 percent of the nation's annual total of about 1,000. On an area basis, however, Texas ranks ninth, far behind Oklahoma, Kansas, and Massachusetts. A rather steady increase in the annual total has been observed, probably as a result of the improving reporting system. The seasonal maximum occurs in the spring and early summer, although tornadoes have been reported in all months. The height of the tornado activity is in early spring in the southern United States, later across the more northerly regions, and in July in western Canada. Tornadoes most frequently occur during the middle and late afternoon. There is a large interannual variation, as well. Lightning
 Lightning is a natural, short-lived, high-current electrical discharge in the atmosphere. Its path length is normally several kilometers. The cumulonimbus clouds of thunderstorms are the most common producers of lightning, but it is also produced by cumulus, stratus, and other clouds, including snowstorms, sandstorms, and clouds over erupting volcanoes. Lightning can also occur in clear air within a few kilometers of a storm. More than half of all discharges occur within a cloud. The rest generally take place between clouds and the ground, with occasional cloud-to-air or cloud-to-cloud discharges. The latter may include "superbolts" up to 100 kilometers long and many times more powerful than typical bolts. The optical discharges called upflashes that sometimes also occur between large storms and the clear air above are events that scientists do not yet well understand. At any one time about 2,000 thunderstorms may exist worldwide, producing lightning flashes at a total rate of 100 per second. In an average year, about 100 to 200 persons are killed and several hundred injured by lightning in the United States alone, a death rate exceeding deaths caused by hurricanes and tornadoes. Total property losses resulting from lightning in the United States range as high as several hundred million dollars per year, and lightning also causes an estimated 10,000 forest fires each year. When lightning-caused fires occur away from human habitation they have beneficial aspects as well, however, including the propagation of seeds of certain plant species and the recycling of nutrients. Cause Before lightning can occur, a charge separation large enough to cause electrical breakdown of air must develop. Thunderstorms are generally negatively charged at the base and positively charged in higher regions. One theory holds that the principle mechanism for separating electric charge is the vertical separation of larger charged droplets of water or ice (raindrops, hailstones, and so on) from differentially charged smaller droplets, as a result of their different settling velocities within a cloud. Another theory holds that small cloud particles and droplets are the principal charge carriers, and that the main mechanism for the separation of charge is the variable convective air motions within a cloud, which carry some particles upward and others downward. Occurrence and Formation Discharges between clouds and the ground cause the greatest destruction. Most are initiated from the negatively charged base of a cloud, but a few (and often more powerful) discharges are initiated from positively charged higher regions, most commonly in winter storms. Strokes initiated from the ground are rarer, the more frequent being from positively charged tall structures or mountain peaks; negative upward strokes are rarest of all. The most familiar lightning strokes are the negative flashes from cloud to ground. They start near the base of a cloud as an invisible discharge called the stepped leader, which moves downward in discrete, microsecond steps about 50 meters (165 feet) long. It is believed to be initiated by a small discharge near the cloud base, releasing free electrons that move toward the ground. When the negatively charged stepped leader approaches to within 100 meters (330 feet) or less of the ground, a leader moves up from the ground - especially from objects such as buildings and trees - to meet it. Once the leaders have made contact, the visible lightning stroke, called the return stroke, propagates upward from the ground along the path of the stepped leader. Several subsequent strokes can occur along the original main channel in less than a second. These strokes continue until the charge center in the lower part of the cloud is eliminated. The explosive heating and expansion of air along the leader path produces a shock wave that is heard as thunder. Ball Lightning Ball lightning, a little-understood phenomenon, is generally spherical, from 1 to more than 100 centimeters (0.4 to more than 40 inches) in diameter; it usually lasts less than 5 seconds. The balls are reported to move at a few meters per second and to decay silently or with a small explosion.
 © Copyright 2002, 2003 Seelendran Naidoo All rights reserved.
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