Hail

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This article is about the precipitation. For other uses, see Hail (disambiguation).


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Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones), also hail is a type of snow. Hailstones on Earth usually consist mostly of water ice and measure between 5 and 50 millimeters in diameter, with the larger stones coming from severe thunderstorms.^[1] 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. 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.

1 Hail formation
2 Ideal conditions for hail formation
3 Short term detection
4 Costly or deadly hailstorms
5 Gallery
6 See also
7 References
8 External links

[edit] Hail formation

Image:Granizo.jpg

A large hailstone, about 6 cm in diameter

Hail forms on condensation nuclei such as dust, insects, or ice crystals, when supercooled water freezes on contact. Hailstones are usually from the size of a small 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.

[edit] Ideal conditions for hail formation

Image:Hailstone.jpg

The largest hailstone ever measured, 17.8 cm (7.0 in) in diameter with a 47.6 cm (18.75 in) circumference.

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.^{[citation needed]} 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. 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]

Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to 15 centimetres and weigh more than half a kilogram (1.1 pounds)^[2]. Pea or golfball-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. [2][3]

[edit] Short term detection

To issue proper warnings and forecasts, the National Weather Service uses a network of NEXRAD doppler radars to detect hail. Hail size and probability can be determined from radar data by a computer by different algorithms. This, in combination with an analysis of the radar display is an accurate way of detecting hail. An analysis of the radar data would include viewing reflectivity data at multiple angles above ground level to check for hail development in the upper levels of the storm, and checking the Vertically Integrated Liquid (VIL). VIL and hail do have a relationship, although it varies with atmospheric conditions and therefore is not highly accurate. Radar data can also be complimented by a knowledge of current atmospheric conditions which can allow one to determine if the current atmosphere is conducive to hail development.

[edit] Costly or deadly hailstorms

Image:Hail clouds.jpg

Hail clouds often exhibit a characteristic green coloration.

Around the 9th century, several hundred pilgrims were killed by a massive hailstorm in Roopkund, Uttarakhand, India.[4]
March 29, 2000: Last known hail fatality in the United States - Juan Oseguera, a nineteen year old man who died from head injuries after being hit by a softball sized hailstone in Lake Worth, Texas [5]
July 11 1990, Denver, Colorado, USA, Softball-sized hail destroyed roofs and cars, causing $625 million in total damage.
September 7, 1991: a Labour 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.[6][7]
May 5 1995, Dallas and Fort Worth, Texas, USA, $1.1 billion insured losses.
April 14 1999, Sydney, New South Wales, Australia, $1.5 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. [8] Largest stone measured was 9.5cm.
May 18 2000, McHenry, Lake, northern Kane, and northern Cook County, Illinois, USA, $572 million [9]. 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. [10][11]
April 10 2001, St. Louis, Missouri, USA, Costliest in US history, largest swath of very large hail, >$1.9 billion insured losses.
July 19 2002, Henan Province, the People's Republic of China, 25 dead and hundreds injured.
June 22 2003, Aurora, Nebraska, USA, Largest hailstone on record falls. It has a 7-inch diameter and a circumference of 18.75 inches.
December 9 2007, Sydney, New South Wales, Australia, Severe thunderstorms that lasted barely 20 minutes caused immense damage in the North and Western Suburbs of Sydney. Worst hit were the suburbs of Blacktown, Castle Hill and neighbouring Baulkham Hills. Hail stones the size of golf balls damaged cars, windows and homes. Largest hail stones reached almost 10cm in diameter in Kings Langley.[12]

Also, a freak hailstorm hit Los Angeles County in December, 1967, blanketing the region much like a snowstorm. The storm also produced lightning, and one bolt struck an oil tank in Manhattan Beach, causing an explosion that covered much of the South Bay with the oil. The next hailstorm to hit the area was in 1979.

[edit] Gallery

Hailshaft.jpg Hail Shaft	Small hail, fractured to show internal structure.jpg Small hail that has been fractured to show internal structure; 246x magnification. The inset shows the original hail.	Bogota hailstorm.jpg Hailstorm in Bogotá, Colombia.	A field of hailstones.jpg A field littered with large hailstones right after a summer hailstorm.
DSC010250.jpg Hailstorm in Bogotá.	Hail On Ground.jpg Small transparent hail beads on the ground after a short spring storm.	Large Hailstons in Leipzig Jun06.jpg Large hailstones up to 5 cm in diameter with concentric rings. The coin diameter is 21.25 mm.	Hailsmtx.jpg April 20, 2006 hailstorm on San Marcos, Texas
IMAGEN-3801666-2.jpg November 4 Hailstorm, Winter in Bogotá, Colombia.	Hailstorm.jpg Hailstorm