Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones). Hailstones usually consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe thunderstorms. Hail is only produced by cumulonimbi (thunderclouds), usually at the front of the storm system, and is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 mm thick. The METAR code for hail 5 mm or greater in diameter is GR, while smaller hailstones and graupel are coded GS. Unlike ice pellets, they are layered and can be irregular and clumped together.
Hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust. The storm's updraft blows the hailstones to the upper part of the cloud. The updraft dissipates and the hailstones fall down, back into the updraft, and are lifted up again. The hailstone gains an ice layer and grows increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls out of the cloud.
In large hailstones, latent heat released by further freezing may melt the outer shell of the hailstone. The hailstone then may undergo 'wet growth', where the liquid outer shell collects other smaller hailstones.
Hail forms in strong thunderstorm clouds, particularly those with intense updrafts, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing . The growth rate is maximized at about , and becomes vanishingly small much below as supercooled water droplets become rare. For this reason, hail is most common in mid-latitudes during early summer where surface temperatures are warm enough to promote the instability associated with strong thunderstorms, but the upper atmosphere is still cool enough to support ice. Accordingly, hail is actually less common in the tropics despite a much higher frequency of thunderstorms than in the mid-latitudes because the atmosphere over the tropics tends to be warmer over a much greater depth. Also, entrainment of dry air into strong thunderstorms over continents can increase the frequency of hail by promoting evaporational cooling which lowers the freezing level of thunderstorm clouds giving hail a larger volume to grow in.
Hail is also much more common along mountain ranges because mountains force horizontal winds upwards (known as orographic lifting), thereby intensifying the updrafts within thunderstorms and making hail more likely. One of the most notorious regions for large hail is the mountainous northern India and Bangladesh, which have reported more hail-related deaths than anywhere else in the world and also some of the largest hailstones ever measured. Mainland China is also notorious for killer hailstorms. In North America, hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley." Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season. Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to and weigh more than . Pea or golf ball-sized hailstones are not uncommon in severe storms. Hail can do serious damage, notably to automobiles, skylights, glass-roofed structures, and most commonly, farmers' crops. Rarely, massive hailstones have been known to cause concussions or fatal head trauma. Sometimes, hail-producing clouds are identifiable by their green colouration.
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