Hail forms when tiny clumps of ice, kept aloft by strong updraughts, get blown through freezing thunderclouds until they are heavy enough to fall to earth. Most large thunderstorms create some hail, but conditions must be met if the hailstones are to grow large, freeze solid, and survive until they reach the ground. 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.

Every hailstone begins to form as an ice nucleus, a small cluster of supercooled water droplets or clumps of snow. This center is called a graupel, and it may continue to accumulate ice, melt in the thundercloud and turn to rain, or be smashed apart by other graupels. If a bug, piece of bark, seed, or stick gets blown up into the storm cloud, it creates another possible nucleus for a hailstone.

If the thunderstorm is cold and windy enough, this graupel 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 graupel. 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.

Hail can accrete more layers when the hailstone blows up through layers of the thunderstorm. Even heavy hail will be kept aloft by strong enough updraughts. When the hail falls back through the storm due to gravity, it accretes even more layers, until it is so heavy it falls as precipitation. Hail forms in most tall, cumulonimbus storms that reach the colder upper atmosphere, but not all hail survives its trip once out of the thunderstorm.

The size of hail, once fully formed, varies from pinheads to softballs. A few outer layers frequently melt when the hail mixes with other warmer precipitation such as snow and rain. The National Weather Service has official size categories for hail that are useful for gauging the damage they can cause to crops. How hail forms gives us a window into the interior of a thunderstorm, helping meteorologists study the evolution of storms as well.