Hail, lumps of ice that fall from thunderstorm clouds. Individual lumps, called hailstones, are usually round and have a rough surface. Hailstones are normally one-fifth inch (5 mm) to two inches (5 cm) in diameter; about one-half inch (13 mm) is the most common size. Very large hailstones are often reported, but accurate measurements are rarely taken.
Hail can damage buildings, injure livestock, and ruin crops. Many farmers buy insurance against hail losses. Various methods have been tried to prevent hail or reduce its severity. The most promising method consists of scattering either dry ice or silver iodide crystals in clouds that threaten to produce hail. Under the proper conditions, the crystals increase the number of hailstones formed, reducing the amount of moisture available for hailstone growth. If the hailstones remain small enough, they melt before reaching the ground.
Hailstorms occur in warm months in temperate climates but are rare in the tropics. In the United States, hailstorms are most frequent on the western Great Plains and along the Rocky Mountains.
Hail is formed in the upper part of the cloud, two miles (3,200 m) or more above the ground, where the temperature is below 32 F. (0 C.). The hailstones are composed of concentric layers of hard, clear ice and soft, milky-looking ice. The layers are built up as the hailstone collects water droplets and snow crystals that freeze to it under varying conditions. Hailstones that are repeatedly carried up through the cloud by strong updrafts (rising air currents) can grow to a large size. Eventually the hailstones become so heavy that the updrafts cannot support them, and they fall to earth.
Ice pellets called sleet resemble very small hailstones but are really frozen raindrops. Sleet forms near the ground in cold weather.