Hailstones form in thunderstorms out of small graupel particles (compacted snow). The updrafts and downdrafts in a storm cause the particles to be cycled by being blown upwards into the anvil of the storm, falling in the downdraft, captured by an updraft again, and so on. During each cycle the hailstone accretes ice crystals (in the anvil) and water droplets (in the lower part of the cloud). This is why hailstones have a layered structure. The core of a hailstone is usually a graupel particle, or a cluster of graupel. In severe thunderstorms, hailstones may grow as big as 8 to 10 cm before finally falling out of the cloud. Typically though, hail is around 1cm (0.4") in diameter or smaller.
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