Hail is a precipitation in the form of round or irregularly shaped lumps of ice called hailstones. Hailstones range from the size of peas to the size of oranges or larger. Most hailstones are smaller than 1 inch (2.5 centimeters) in diameter. Large hailstones can have bumps on their surfaces where they have grown more.
Hail can break windows, damage roofs, dent cars and airplanes, and occasionally injure or kill people. It causes hundreds of millions of dollars of damage to crops each year. Hail damage is greater when the wind is strong.
Hail is often observed from central Texas through the Great Plains states into Alberta. In the United States, it falls most frequently in southeastern Wyoming, western Nebraska, and eastern Colorado. Elsewhere in the world, hail often falls in Argentina, northern Italy, Kenya, South Africa, and the Caucasus region between the Black and Caspian seas.
Hailstones form in thunderstorm clouds and begin as frozen raindrops or snow pellets called hail embryos. Embryos originate in one part of the hailstorm and are then carried by air currents to the main region of hail growth. Hailstones develop as the embryos come into contact with supercooled water droplets, droplets that remain liquid at temperatures below freezing. As an embryo moves through the droplets, they strike its surface and freeze. An embryo grows into a hailstone as this freezing water accumulates on its surface.
Hailstones become large if they remain for a long time in parts of the hailstorm where there is a large amount of supercooled liquid water. Hailstones grow large if they are supported in the same cloud updraft for a long time. They also grow if they repeatedly fall out of an updraft but are then carried upward by other air currents. Hailstones fall to the ground when they leave the region of updrafts or become too heavy for the air currents to support. They fall at a speed of about 22 miles (35 kilometers) per hour or more.
