Hail forms when tiny clumps of ice, kept aloft by strong updrafts, get blown through freezing thunderclouds until they are heavy enough to fall to earth. Most large thunderstorms create some hail, but the proper conditions must be present for the hailstones to grow large, freeze solid and then survive until they reach the ground. The ideal conditions for hail are tall clouds that reach high into the atmosphere, many swirling updrafts such as in a tornado and cold temperatures within and beneath the storm.
Starts with a Nucleus
A hailstone begins to form as an ice nucleus, a small cluster of supercooled water droplets or clumps of snow. This center might continue to accumulate ice, melt in the thundercloud and turn to rain or be smashed apart by other clusters. If a bug, a piece of dirt, a seed or another small particle gets blown up into the storm cloud, it creates another possible nucleus for a hailstone.
Layers of Ice
If the thunderstorm is cold and windy enough, this ice cluster will accumulate layers of ice the way a dipped candle accumulates layers of wax, through a process called accretion. Opaque, whitish layers form when icy droplets trap air bubbles and stick to the cluster. Clear layers have accreted large drops of supercooled water that freeze when they encounter the hailstone. Of course, much larger hailstones can be made when two smaller ones freeze together.
Gaining Weight
Hail can accrete more layers when the hailstone blows up through layers of the thunderstorm. Even heavy hailstones can be kept aloft by strong enough updrafts. When the hail falls back through the storm because of gravity, it accretes even more layers, until it is so heavy that it falls as precipitation. Hailstones form in most tall, cumulonimbus storm clouds that reach the colder upper atmosphere, but not all hail survives after it is out of the thunderstorm. A few outer layers frequently melt when the hail mixes with other precipitation, such as snow and rain.
Sizes
The size of fully formed hailstones can vary from pinheads to softballs. There are official size categories for hail that are useful for gauging the damage that they can cause. Some hailstones have been measured at more than 6 inches (15.24 cm) across and more than 1 pound (0.45 kg) in weight. Most hailstones, however, are less than 0.5 inches (1.27 cm) across.