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Hail
Hail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 millimetres (0.20 in) and 150 millimetres (5.9 in) in diameter, with the larger stones coming from severe thunderstorms. The METAR reporting code for hail 5 millimetres (0.20 in) or greater in diameter is GR, while smaller hailstones and graupel are coded GS. 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.
Causes of Hails:
Hailstorms form within a unusually unstable air mass, that is, an air mass in which the temperature falloff with height is much greater than normal. The unstable air is necessary to produce large updraft speeds -- fast enough to keep a developing hailstone from falling to the ground. Some of these updrafts can reach 60 mph or more.
Frequent occurrence:
Like tornadoes, there is a region of the North American continent where hail is most commonly found. In the United States, this area extends from the area where Colorado, Nebraska and Wyoming meet south and east through the High Plains and into Oklahoma and Texas.
Across Canada, hail falls frequently during the summer months across the Province of Alberta, especially in the portion east of the Rocky Mountains.
Weather expert Bryan Neaton of The Weather Network adds that all of these areas share a common thread of being downwind of large mountain ranges.Thus, across the world, it could be expected that hail producing thunderstorms could also be found east of large mountain range where cold and warm air frequently intersect.
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