Other countries have different thresholds according to local sensitivity to hail for instance, grape-growing areas could be adversely impacted by smaller hailstones. The US National Weather Service has a 2.5 cm (0.98 in) diameter threshold, effective January 2010, an increase over the previous threshold of 0.75 in (1.9 cm) hail. The Meteorological Service of Canada issues severe thunderstorm warnings when hail that size or above is expected. Stones larger than 2 cm (0.80 in) are usually considered large enough to cause damage. Although the diameter of hail is varied, in the United States, the average observation of damaging hail is between 2.5 cm (0.98 in) and golf-ball-sized 4.4 cm (1.75 in). Hail is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 mm (0.039 in) thick, which are deposited upon the hailstone as it travels through the cloud, suspended aloft by air with strong upward motion until its weight overcomes the updraft and falls to the ground. ![]() Unlike ice pellets, hailstones are layered and can be irregular and clumped together. Hailstones can grow to 15 cm (6 in) and weigh more than 0.5 kg (1.1 lb). An ice crystal with a diameter of >5 mm (0.20 in) is considered a hailstone. Severe weather warnings are issued for hail when the stones reach a damaging size, as it can cause serious damage to human-made structures, and, most commonly, farmers' crops.Īny thunderstorm which produces hail that reaches the ground is known as a hailstorm. Hailstones 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 hailstones can slow their descent through Earth's atmosphere. There are methods available to detect hail-producing thunderstorms using weather satellites and weather radar imagery. In the mid-latitudes, hail forms near the interiors of continents, while, in the tropics, it tends to be confined to high elevations. Hail formation requires environments of strong, upward motion of air within the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Hail is possible within most thunderstorms (as it is produced by cumulonimbus), as well as within 2 nmi (3.7 km) of the parent storm. The METAR reporting code for hail 5 mm (0.20 in) or greater is GR, while smaller hailstones and graupel are coded GS. Unlike other forms of water ice precipitation, such as graupel (which is made of rime ice), ice pellets (which are smaller and translucent), and snow (which consists of tiny, delicately crystalline flakes or needles), hailstones usually measure between 5 mm (0.2 in) and 15 cm (6 in) in diameter. Ice pellets generally fall in cold weather, while hail growth is greatly inhibited during low surface temperatures. ![]() It consists of balls or irregular lumps of ice, each of which is called a hailstone. It is distinct from ice pellets (American English "sleet"), though the two are often confused.
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