Hail is solid precipitation in the form of balls or pieces of ice known as hailstones.
Hailstones fall from cumulonimbus clouds and are commonly spherical or conical in shape. Their diameter can range from 5 to 50mm or even more, however most hailstones are smaller than 25mm.
In thunderclouds, drops of water are continuously taken up and down though the cloud. When they go to the top of the cloud, it is very cold and they freeze. As the updraughts in thunderclouds are very big, they can keep these hailstones for a long time, so they get larger and larger by becoming coated with more and more ice.
When they get really big, the updraughts in the cloud cannot hold them up anymore and they fall to earth, and by this time they are big balls of ice, and don't have time to melt before they reach the ground. Hail can only be formed in this way, unlike snow which can be formed in fronts, and orographically too, just like rain.
When do we get hail?
Hail is most common in western parts of Britain, where it occurs most frequently in winter, presumably mainly due to polar maritime air masses generating convection over the North Atlantic and Irish Sea, which then spreads a fair way inland. In eastern England and south-east Scotland, hail is most frequent in spring, when temperatures are still relatively low and land-generated convection increases.
While Britain's most damaging hailstorms tend to occur during summer, these are relatively infrequent. Hail in summer is most common in inland parts of northern and eastern Britain, but much less frequent in those areas than during spring