Hail is solid precipitation in the form of balls or pieces of ice known as hailstones.
How does hail form?
Hail forms in thundercloud when drops of water are continuously taken up and down though the cloud by updraughts and downdraughts. 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 the hailstones 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, in these type of convective clouds, unlike snow which can also be formed in weather fronts, and by ascending up hills and mountains, just like rain can.
When do we get hail?
Hail is most common in western parts of Britain, where it occurs most frequently in winter. The land is cold compared to the sea at this time of year, so showers form over the North Atlantic and Irish Sea, driven by the heat energy in the sea. Once the showers reach the cold land, they tend to lose their driving force (the heat) and therefore will tend to die out before they get too far inland. However, sometimes if the atmospheric conditions are right higher up in the air, the showers can continue even over the cold land, and they may affect central and eastern parts of the UK too.
In eastern England and south-east Scotland, hail is most frequent in spring, when temperatures are still relatively low (i.e. low enough to freeze the droplets in the cloud) but the temperature of the land is mild enough to generate the shower clouds over the land.
While Britain's most damaging hailstorms tend to occur during summer, these are relatively infrequent. In these situations, the hot land surface has enough energy to generate really tall shower clouds, and the tops of these clouds are high up enough (and therefore cold enough) to form the hailstones. Hail in summer is most common in inland parts of northern and eastern Britain, but much less frequent in those areas during spring.
How big can hailstones get?
Hailstones 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. The largest hailstones tend to form when shower clouds cluster together to form ‘multicell’ storms, which are common in the United States.