THUNDERSTORM


Meaning of THUNDERSTORM in English

any violent, short-lived atmospheric disturbance almost always associated with cumulonimbus clouds (dense rain clouds of great vertical extent) and accompanied by thunder and lightning. In most cases, such storms generate strong, gusty winds, heavy rain, and, occasionally, hail or tornadoes. A deep layer of moist air and a vertical temperature distribution favourable to the development of intense local overturning are essential for thunderstorm formation. The rate of decrease of air temperature with height is known as the temperature lapse rate. In general, steep lapse rates (i.e., rapid decreases of temperature with height) are most favourable for thunderstorms, because it is under these conditions that an air parcel made warmer than its environment is capable of rising through the atmosphere by virtue of buoyancy forces. After any given parcel of air is lifted to the point of condensation at the cloud base, the released latent heat of condensation will be added to the parcel. This will assist further ascent. In the upper parts of the thundercloud, where snow and ice crystals are found, the latent heat of fusion will also contribute to sustaining the updraft currents. The average thunderstorm consists of three to five convective cells, each of which undergoes a life cycle characterized by changes in the direction and magnitude of the strong, nearly vertical air currents. Three stages in this life cycle are recognized. In the initial, or cumulus, stage, the cell contains strong upward-moving air currents that extend from below the cloud base to the cloud top. The updrafts average speeds of about 4 to 8 m (13 to 26 feet) per second. If the air is sufficiently unstable, there will be a continual supply of energy for the updraft and the cell will continue to develop in size and intensity. With increase in size comes an increase in total cloud-water content. After the establishment of an active updraft, precipitation particles will grow to a size at which they become important both in the further development of the cell and in the formation of electrical-charge centres. The precipitation usually will be in the form of snow and snow pellets in the upper, colder parts of the cell and of raindrops in its lower, warmer regions. At this time the cell passes into the mature stage of development. The centres of electrical charge become well established during the mature stage. The lightning that invariably accompanies a thunderstorm is a crash discharge between an upper positive centre and a lower negative centre or between one of these and the ground. Each lightning flash generally consists of several strokes in rapid sequence, with time separations of a few hundredths of a second. The sound of thunder originates in a compression wave formed along the highly heated path of a lightning stroke. Each portion of the path of a stroke releases intense sound waves, which travel different paths to the observer. In the mature stage, both upward- and downward-moving air currents are present within a component cell. The updraft of the initial stage gives way to downdraft as the size and number of precipitation particles increase. This reversal of the updraft is also aided by evaporational cooling, which occurs as clear air from outside the cloud is mixed into the cloud by the turbulent motions. At first, the downdraft is small and located on one side of the updraft. Gradually, however, it works its way across the cell and squeezes off the updraft, at which time the cell goes into the final, or dissipating, stage. At the ground, the onset of heavy rain marks the beginning of the mature stage. The downdraft, which coincides closely with the rain area, reaches the surface a few minutes after the first rain. Because of the continued evaporation, air in the downdraft is cold, which accounts for its great downward speed. As the downdraft nears the ground, it is diverted and spreads outward in all directions as a cold, gusty wind blowing from the rain area. A thunderstorm cell in the dissipating stage is filled throughout by weak subsiding air and light precipitation. Rain at the Earth's surface is much less intense than was the case during the mature stage. The cold outflow gradually subsides as a result of the waning downdraft. The time duration of a thunderstorm is somewhat variable, but observations indicate that the first two stages of development usually last about 15 to 20 minutes each. During these stages, the visual cloud will frequently reach heights of 11,000 to 20,000 m (36,000 to 66,000 feet) and horizontal dimensions of several kilometres. The duration of a cell in the final stage varies over wide limits. Some storms end abruptly, whereas others may linger for several hours as a gentle rain from a large, formless cloud mass. These differences result from different conditions of the air masses in which the clouds form. The anvil-shaped, upper region so characteristic of thunderclouds is formed when the rising cloud air encounters a thermally stable layer in the tropospherethe portion of the atmosphere below the stratosphere, extending about 11 to 16 km (7 to 10 miles) from the Earth's surface. Thunderstorms have been known to occur in almost every part of the world. They are, however, rare in the polar regions and infrequent poleward of latitudes 50 N and 50 S. In the United States the areas of maximum thunderstorm activity are the Florida peninsula and the coast of the Gulf of Mexico (7080 days per year) and the mountains of New Mexico (5060 days). Central Europe and Asia average 20 to 60 thunderstorm days annually. It has been estimated that at any one moment there are about 1,800 thunderstorms in progress throughout the world. a short-lived storm that is produced by clouds of great vertical extent and that is always accompanied by lightning and thunder. A thunderstorm frequently produces strong, gusty winds, heavy rain, and occasionally hail. Sometimes, particularly in the mountainous areas of the western United States, water drops from a thunderstorm are small, and they evaporate before reaching the ground as precipitation. Probable distribution of thundercloud charges. Almost invariably, thunderstorms are associated with cumulonimbus clouds, which are dense rain clouds with exceptional vertical development that look like mountains or enormous towers. In an advanced stage, their summits have a smooth, fibrous appearance and occasionally resemble a huge anvil (Figure 1). The base of a cumulonimbus cloud is usually dark because it has great depth. Additional reading Joe R. Eagleman, Severe and Unusual Weather (1983), presents an overview of storms. Types of storms, their attendant phenomena, and their effects are treated in Edwin Kessler (ed.), ThunderstormsA Social, Scientific, & Technological Documentary, 3 vol. (198182); R.H. Golde (ed.), Lightning, 2 vol. (1977); and Martin A. Uman, Lightning (1969, reissued 1984), a fundamental work. Louis J. Battan The Editors of the Encyclopdia Britannica

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