Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones), 5 mm–50 mm in diameter on average, with much larger hailstones from severe thunderstorms[1]. Hail is always produced by cumulonimbus (thunderclouds), and is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 mm thick. Small hailstones are less than 5 mm in diameter, and are reported as SHGS. Unlike ice pellets, they are layered and can be irregular and clumped together.
Contents
1Hail formation
2Ideal conditions for hail formation
3Costly or deadly hailstorms
4Gallery
5See also
6References
7External links
Hail formation
Hail forms on condensation nuclei such as dust, insects, or ice crystals, when supercooledwater freezes on contact. Hailstones are usually from the size of a pea to the size of a golfball. In clouds containing large numbers of supercooled water droplets, these ice nuclei grow quickly at the expense of the liquid droplets because the saturation vapor pressure over ice is slightly less than the saturation vapor pressure over water. If the hailstones grow large enough, latent heat released by further freezing may melt the outer shell of the hailstone. The growth that follows, usually called wet growth, is more efficient because the liquid outer shell allows the stone to accrete other smaller hailstones in addition to supercooled droplets. These winds hold the rain and freeze it. As the process repeats, the hail grows increasingly larger. Once a hailstone becomes too heavy to be supported by the storm's updraft it falls out of the cloud. When a hailstone is cut in half, a series of concentric rings, like that of an onion, is revealed. These rings reveal the total number of times the hailstone had traveled to the top of the storm before falling to the ground.
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 (0 °C (32 °F)). The growth rate is maximized at about -13 °C (9 °F), and becomes vanishingly small much below -30 °C (-22 °F) as supercooled water droplets become rare. For this reason, hail is most common in midlatitudes 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 midlatitudes 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 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. Certain locations in North America (such as the area around Calgary, Alberta) have gained the nickname "Hailstorm Alley" among meteorologists for the frequency of hailstorms and their severity.[1]
September 7, 1991: a Labor Day thunderstorm caused $400 million worth of insurable damage in Calgary. Thirteen additional hailstorms between 1981 and 1998 caused an estimated $600 million in damage in the Calgary area alone.[7][8]
April 141999, Sydney, New South Wales, Australia, $1.6 billion. 20,000 properties and 40,000 vehicles were damaged during the storm with more than 25 aircraft damaged at Sydney Airport, one person was killed while fishing after getting struck by lightning and several other people were injured. It was the costliest hailstorm to hit an Australian populated city. [9]
May 182000, McHenry, Lake, northern Kane, and northern Cook County, Illinois, USA, $572 million [10]. Golfball-, baseball-, and softball-sized hail damaged roofs, cars, patio furniture, skylights, and windows in the area's worst and most widespread hailstorm in 30 years. Around 100,000 homes lost power. Hail was 3 inches deep in many areas. There were 100 canceled flights, and train service was disrupted. [11][12]
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