Hail is possible with most thunderstorms as it is produced by cumulonimbi (thunderclouds), usually at the leading edge of a severe storm system. Hail is possible within 2 nautical miles (3.7 km) of its parent thunderstorm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics.
Unlike ice pellets, hail stones are layered and can be irregular and clumped together. Hail is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 millimetre (0.039 in) thick, which are deposited upon the hail stone as it cycles through the cloud multiple times, suspended aloft by air with strong upward motion until its weight overcomes the updraft and falls to the ground.
There are methods available to detect hail-producing thunderstorms using weather satellites and radar imagery. Hail stones generally fall at higher speeds as they grow in size, though complicating factors such as melting, friction with air, wind, and interaction with rain and other hail stones can slow their descent through Earth's atmosphere. Severe weather warnings are issued for hail when the stones reach a damaging size, as it can cause serious damage to man-made structures and, most commonly, farmers' crops.
In North America, hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley." Hail in this region occurs between the months of March and October during the afternoon and evening hours, with the bulk of the occurrences from May through September. Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season.
During the Middle Ages, people in Europe used to ring church bells and fire cannons to try to prevent hail. After World War II, cloud seeding was done to eliminate the hail threat, particularly across Russia. Russia claimed a 50 to 80 percent reduction in crop damage from hail storms by deploying silver iodide in clouds using rockets and artillery shells. Their results have not been able to be verified. Hail suppression programs have been undertaken by 15 countries between 1965 and 2005. To this day, no hail prevention method has been proven to work.