Hail consists of large ice particles that have a layered structure, and are produced by intense thunderstorms that form in a very unstable air mass, that is, one that was relatively cool dry air overlying very warm and humid air. The unstable air is necessary to produce large updraft speeds -- fast enough to keep a developing hailstone from falling to the ground. Some of these updrafts can reach 60 mph or more. Small ice particles that form above the freezing level in the thunderstorm collect rain water on them, forming a water shell that slowly freezes. If these growing hailstones fall into another updraft, they can continue to grow, until they finally become too large for the updraft to carry them, or they get caught up in a downdraft, and they finally reach the ground.