Precipitation composed of chunks or lumps of ice formed in strong updrafts in cumulonimbus clouds. Individual lumps are called hailstones. Most hailstones are spherical or oblong, some are conical, and some are bumpy and irregular. Diameters range from 0.2 to 6 in. (5 to 150 mm) or more. That is, the largest stones are grapefruit or softball size, and the smallest are pea size.

Very often hailstones are observed to be made of alternating rings of clear and white ice . These rings indicate the growth processes of the hail. The milky or white portion of the growth occurs when small cloud droplets are collected by the hailstone and freeze almost instantaneously, trapping bubbles of air between the droplets and creating a milky appearance. The clear portion is formed when many droplets are collected so rapidly that a film of water spreads over the stone and freezes gradually, giving time for any trapped air bubbles to escape from the liquid.

The most favorable conditions for hail formation occur in the mountainous, high plains regions of the world. Hailstorms normally have relatively high, cool cloud bases and very strong updrafts within the clouds to carry the hailstones into the cooler regions of the cloud, where maximum growth occurs. Both small ice particles and supercooled liquid water (liquid water at temperatures below 32°F or 0°C) are needed for the ice particles to grow into hailstones. Cloud physics Precipitation (meteorology)