Hail is produced inside storm clouds and is made of solid ice. Hailstones can grow to dangerous sizes before they fall and have to be at least 5 mm (0.2 in) across to be considered as hail. If the particles of opaque ice are smaller than this they are known as 'soft hail' (graupel or snow pellets).

Hail occurs when ice pellets falling from the top of the cloud collect a film of moisture as they descend. The moisture freezes when the pellets are carried back to the top by upcurrents. As this process is repeated the hailstones grow in size and each trip through the cloud adds a new layer of ice. Once hailstones become heavy enough they will fall to the ground and can be a really dangerous weather hazard. Most are smaller than 2.5 cm (1 in) in diameter, but some can be as big as tennis balls or even grapefruit size. The largest can weigh a little under 1 kilo and fall at more than 50 metres per second. Hail storms can devastate entire fields of crops in a few minutes, flattening and bruising the growing plants. The biggest hailstones in the world tend to fall in north-east India. In Britain there were last hailstorms with tennis ball-size hail stones in Horsham in 1958 (5 September) and Wokingham in 1959.