Hail is the only frozen precipitation that forms in warm weather. It is produced by powerful thunderstorms that result when a cold air mass clashes with a warm air mass. The precipitation in thunderstorm clouds begins as ice crystals. These crystals attract water droplets to form raindrops and melt as they fall to the earth. However, in some storms, the upward wind currents are so strong that ice crystals are blown back up several thousand feet into colder air, where another layer of ice forms around them. The crystals bounce up and down, as if they are on a "trampoline of air", growing larger and larger. When the crystals become so heavy that the upward currents can no longer support them, they fall to the ground as ice pellets that we call hail. Wind patterns usually form hailstones into balls; but they can also appear in other shapes, such as cones, discs, stars, pyramids, or just weird, pointy blobs.

  The more violent the thunderstorm, the stronger the updrafts and the larger the hailstones are. Although most hailstones measure less than an inch in diameter, golfball-size hail is not unusual in "Hail Alley," the area made up of northern Colorado, southeastern Wyoming, and western Nebraska. Occasionally, halistones are the size of baseballs, softballs, or even grapefruit. Obviously, hailstones of almost any size can be dangerous and destructive – shattering windows, smashing automobiles, and injuring, or even killing, people and animals.

  Perhaps the most unusual hailstone ever reported was the one known as "the human hailstone." This event occurred in Germany in 1930 and gives us a terrifying look at the enormous forces at work in a thunderstorm. As sixteen glider pilots flew in a contest to see who could reach the highest altitude, strong winds carried them into storm clouds that were forming over the mountains. Sensing the danger of the powerful updraft, fourteen of the pilots quickly steered their planes out of the current and out of trouble. But two pilots were pulled into the storm system and trapped in currents of swirling air that lifted them to an altitude of 40,000 feet. Their gliders were blasted apart by the winds, and both pilots tried to parachute down to safety. One of them made it. The other was carried up into a cloud, where heavy layers of ice formed on him. He then fell 7 miles to his death – A HUMAN HAILSTONE!