Thunderstorms cause hail when strong winds push raindrops upward into the atmosphere where the extremely cold air supercools the water and causes it to freeze into spheres of ice. This can occur several times, with balls of ice falling and then being lifted by updrafts, collecting condensation as they go. This results in a distinct layering in hailstones each time a layer of liquid water freezes on the surface.
In the right weather conditions, the ability of hailstones to rise and fall in columns of air, growing as they do, can be very dangerous. Hailstones can reach up to six inches in diameter and nearly two pounds in weight in extreme conditions, and can fall at up to 90 miles per hour. Crops, buildings and cars all suffer heavy damage from large hail. Occasionally animals such as livestock are killed by large hail. In addition to these dangers, hail is often a precursor to other severe weather conditions such as tornadoes.
Hail is generally a brief phenomenon, lasting only a few minutes, but there are dangerous exceptions. Hail that lasts longer can deposit several inches of ice on the ground. In 1984, a Denver, CO hailstorm lasted for almost an hour and deposited nearly 2 feet of ice on the ground.