What is Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Unlike graupel, which is made of rime, and ice pellets, which are smaller and translucent, hail stones, on Earth, consist mostly of water ice and measure between 5 and 200 millimetres (0.20 and 7.9 in) in diameter. The METAR reporting code for hail 5 mm (0.20 in) or greater is GR, while smaller hailstones and graupel are coded GS. Hail is possible within most thunderstorms as it is produced by cumulonimbi (thunderclouds), and within 2 nautical miles (3.7 km) of the parent storm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. In the mid-latitudes, hail forms near the interiors of continents, while in the tropics, it tends to be confined to high elevations.
There are methods available to detect hail-producing thunderstorms using weather satellites and weather 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 human-made structures and, most commonly, farmers' crops.
Any thunderstorm which produces hail that reaches the ground is known as a hailstorm. Hail has a diameter of 5 millimetres (0.20 in) or more. Hail stones can grow to 15 centimetres (6 in) and weigh more than 0.5 kilograms (1.1 lb). 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, suspended aloft by air with strong upward motion until its weight overcomes the updraft and falls to the ground. Although the diameter of hail is varied, in the United States, the average observation of damaging hail is between 2.5 cm (1 in) and golf ball-sized (1.75 in).
Stones larger than 2 cm (0.75 in) are usually considered large enough to cause damage. The Meteorological Service of Canada will issue severe thunderstorm warnings when hail that size or above is expected. The US National Weather Service has a 2.5 cm (1 in) or greater in diameter threshold, effective January 2010, an increase over the previous threshold of .75 inch hail. Other countries will have different thresholds according local sensitivity to hail; for instance grape growing areas could be adversely impacted by smaller hailstones. Hailstones can be very large or very small, depending on how strong the updraft is: weaker hailstorms produce smaller hailstones than stronger hailstorms (such as supercells).
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. These types of strong updrafts can also indicate the presence of a tornado. The growth rate is maximized where air is near a temperature of mius 13 degrees C.
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