Hail

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Hail, form of precipitation consisting of roughly spherical pellets of ice and snow usually combined in alternating layers. True hailstones occur only at the beginning of thunderstorms and never when the ground temperature is below freezing. Raindrops or snow pellets formed in cumulonimbus clouds are swept vertically in the turbulent air currents characteristic of thunderstorms. The hailstone grows by the repeated collisions of these particles with supercooled water, that is, water that is colder than its freezing point yet remains in liquid form. This water is suspended in the cloud through which the particle is traveling. When the particles of hail become too heavy to be supported by the air currents, they fall to earth. Hailstones range in diameter from 2 mm to 13 cm (w to 5 in); the larger ones are sometimes very destructive. Often several hailstones freeze together into a large, shapeless, heavy mass of ice and snow.

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