hail, in meteorology

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 sleet.

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