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hail,precipitation in the form of pellets composed of ice or of ice and snow, occurring at any time of the year, usually during the passage of a cold front or during a thunderstorm. Small hailstones have a soft center and a single outer coat of ice. They are formed when the surfaces of snow clumps melt and refreeze or become coated with water droplets that subsequently freeze. Large hailstones usually have alternate hard and soft layers. There are various explanations of how these large stones form and grow. Some believe that they form in clouds when supercooled raindrops (i.e., drops chilled below the freezing temperature without solidifying) freeze on dust particles or snowflakes. These tiny hailstones are then blown repeatedly up and down by the winds in a cloud. Each time they are blown downward to a region whose temperature is above freezing, the stones collect more moisture, and each time they are blown upward to a region below freezing, the moisture solidifies into ice, and some snow may collect. The stones continue to grow, adding layer after layer, until they are too heavy to be supported by the winds and fall to the ground. In another explanation, it is suggested that hailstones continuously descend, gaining layers by passing through regions of the air that contain different amounts of water. Hailstones are spherical or irregularly spherical and usually vary in diameter up to 1-2 in. (1.3 cm); in rare cases hailstones having diameters up to 5 in. (12.7 cm) have been observed. Hail causes much damage and injury to crops, livestock, property, and airplanes. See sleetsleet,
precipitation of small, partially melted grains of ice. As raindrops fall from clouds, they pass through layers of air at different temperatures. If they pass through a layer with a temperature below the freezing point, they turn into sleet.
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Hail(hä`ēl, hīl), city (1993 pop. 176,757), N central Saudi Arabia. The city grew because of its location on a pilgrimage route from Iraq to Mecca. It was the capital of the independent emirate of Jabal Shammar, which Ibn Saud conquered in 1921.
a form of atmospheric precipitation consisting of spherical particles or pieces of ice (hailstones) measuring 5–55 mm and sometimes even more (hailstones measuring 130 mm and weighing 1 kg have been found). Hailstones are composed of transparent ice or layers of transparent ice at least 1 mm thick, alternating with translucent layers. In meteorology, hail is distinguished from granular snow, which consists of opaque white pellets measuring 2–5 mm, which are brittle and easily pulverized.
Hail usually falls during severe thunderstorms in the warm season, when the temperature at the surface of the earth is usually above 20° C, and it falls on a narrow belt a few kilometers wide (sometimes about 10 km) but tens and sometimes even hundreds of kilometers long. The accumulation of fallen hail is usually several centimeters and sometimes tens of centimeters thick. The fall may last from several minutes to half an hour, but most frequently it lasts between five and ten minutes and very rarely for about one hour. Between 500 and 1,000 hailstones fall in one minute on 1 sq m. Their density is 0.5–0.9 g per cu cm, and they fall at a speed of tens of meters per second.
Nuclei of hailstones are formed in supercooled clouds as a result of the random freezing of single droplets. The nuclei may then grow to a considerable size when supercooled droplets collide with them and freeze on them. Large hailstones can be formed only if there are strong upward currents in the clouds that prevent the hailstones from falling to earth for a long time.
Hail can cause a great deal of damage to agriculture by destroying crops and vineyards. In the USSR radar methods have been developed for determining the possible formation and danger of hail in clouds, and services for preventing hail have been established in Georgia, Moldavia, and other parts of the country. The prevention of hail is based on the principle of introducing special reagents into the cloud—usually lead iodide or silver iodide, which promote the freezing of supercooled drops. The reagent is introduced by means of rockets or shells fired into the supercooled part of the cloud. This produces an enormous number of artificial crystallizing nuclei on which the ice crystals begin to grow, and the supercooled water in the clouds, which is the main source of growth of the hailstones, is distributed over a considerably larger number of the stones. Consequently, they attain a much smaller size and melt completely or to a large extent in the warm layers of air before they reach the earth’s surface. Thus, hail is completely prevented, or its intensity and the size of its stones are substantially reduced.
REFERENCESZhenev, R. Grad. Leningrad, 1966. (Translated from French.)
Fizika oblakov i aktivnykh vozdeistvii: Trudy Vsesoiuznoi kon-ferentsii po aktivnym vozdeistviiam na gradovye protsessy, 26–29 marta 1968 g. Edited by G. K. Sulakvelidze and Kh. Kh. Medaliev. Leningrad, 1969.
I. P. MAZIN
soft hail and ice (small) hail, a form of atmospheric precipitation. Soft hail consists of granules 2-5 mm in diameter with snowlike structure; it is formed when snowflakes from the upper part of a cloud fall to an underlying cloud layer, which consists of minute supercooled drops. Ice (small) hail consists of granules 2-5 mm in diameter that are transparent at the surface and have a small white core; it is formed upon collision of soft hail with larger supercooled drops in a lower cloud layer. Soft hail and ice hail fall most often in the spring during unstable weather conditions.
a city in northern Saudi Arabia, situated in an oasis. Population, approximately 20,000. Hail is linked by roads with Riyadh and Medina. It is a center of commerce and handicrafts.