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Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones). 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. The METAR code for hail 5 mm or greater in diameter is GR, while smaller hailstones and graupel are coded GS. Unlike ice pellets, they are layered and can be irregular and clumped together.

Hail formation

A large hailstone, about 6 cm (2.36 in) in diameter

Hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust. The storm's updraft blows the hailstones to the upper part of the cloud. The updraft dissipates and the hailstones fall down, back into the updraft, and are lifted up again. The hailstone gains an ice layer and grows increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls out of the cloud.

In large hailstones, latent heat released by further freezing may melt the outer shell of the hailstone. The hailstone then may undergo 'wet growth', where the liquid outer shell accretes other smaller hailstones.

Ideal conditions for hail formation

The largest hailstone ever measured, 17.8 centimetres (7.0 in) in diameter with a 47.6 centimetres (19 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 mid-latitudes 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 mid-latitudes 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, Houston, Texas, and St. Louis, Missouri) have gained the nickname "Hailstorm Alley" among meteorologists for the frequency of hailstorms and their severity.^[2]

Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to 15 centimetres (6 in) and weigh more than .5 kilograms (1.1 lb)^[3]. Pea or golf ball-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.^[4][5]

Short term detection

In the United States, to issue proper warnings and forecasts, 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.

Size scale

The examples and perspective in this section may not represent a worldwide view of the subject. Please improve this article or discuss the issue on the talk page.

Hailstone size is often reported as compared to known objects rather than by reporting the actual diameter. Below is a table of commonly used objects for this purpose.^[6] The UK organisation, TORRO, also scales for both hailstones and hailstorms.^[7]

Costly or deadly hailstorms

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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.^[8]
• December 1967, A hailstorm hit Los Angeles County, 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.^{citation needed}
• 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, Alberta, Canada. Thirteen additional hailstorms between 1981 and 1998 caused an estimated $600 million in damage in the Calgary area alone.^[9][10]
• May 5, 1995, Dallas and Fort Worth, Texas, USA, $1.1 billion insured losses, total storm damage reported at around $2 billion. The storms produced hail about the size of softballs.^[11][12][13]
• 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.^[14] Largest stone measured was 9.5 cm.^[15]
• March 29, 2000, The last known hail fatality in the United States occurs. The victim was Juan Oseguera, a nineteen-year-old man who died from head injuries after being hit by a softball sized hailstone in Lake Worth, Texas. ^[16]
• May 18, 2000, McHenry, Lake, northern Kane, and northern Cook County, Illinois, USA, $572 million ^[17]. 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 (76 mm) deep in many areas. There were 100 canceled flights, and train service was disrupted. ^[18][19]
• April 10, 2001, St. Louis, Missouri, USA, $2.0 billion+. The costliest hailstorm in US history struck the I-70 corridor of eastern Kansas, across Missouri, into southwestern Illinois.^[20]
• July 19, 2002, Henan Province, the People's Republic of China, 25 dead and hundreds injured.
• The largest hailstone on record fell on June 22, 2003 in Aurora, Nebraska, USA. It has a 7-inch (180 mm) diameter and a circumference of 18.75 inches (476 mm).
• April 20 – April 21, 2006, San Marcos, Texas, USA, hail of sizes up to 10 cm^[21][22] results in 10,000 auto claims, 7,000 homeowner and commercial property claims ranging between $100-160 million insured losses.^[23][24] One woman was hospitalized.^[25] The storm was especially costly at the San Marcos Outlet Malls and a nearby Toyota dealership.^[26][27]
• June 28 – June 29, 2006, Villingen-Schwenningen and suburbs, Baden-Württemberg, Germany, supercell thunderstorms, severe damage by very large hailstones (softball-size), causing € 150 million damage, more than 100 fatalities.
• December 9, 2007, Sydney, New South Wales, Australia, Severe thunderstorms 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 10 cm in diameter in Kings Langley.^[28]

Gallery

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Hail Shaft	Small hail that has been fractured to show internal structure; 246x magnification. The inset shows the original hail.	Hailstorm in Bogotá, Colombia.	A field littered with large hailstones right after a summer hailstorm.
Hailstorm in Bogotá.	Large hailstones up to 5 centimetres (2 in) in diameter with concentric rings. The coin diameter is 21 millimetres (0.8 in).	April 20, 2006 hailstorm on San Marcos, Texas	Flood caused by the November 4, 2007 Hailstorm in Bogotá, Colombia.
Hailstorm

See also

• Sleet
• Graupel
• Snow grains
• Hail cannon

References

Further reading

• Rogers and Yau (1989). A Short Course in CLOUD PHYSICS. Massachusetts: Butterworth-Heinemann. ISBN 0-7506-3215-1. 
• Jim Mezzanotte (2007). Hailstorms. Gareth Stevens Publishing. ISBN 978-0836879124. 
• Snowden Dwight Flora (2003). Hailstorms of the United States. Textbook Publishers. ISBN 978-0758116987. 
• Narayan R. Gokhale (1974). Hailstorms and Hailstone Growth. State University of New York Press. ISBN 978-0873953139. 
• Duncan Scheff (2001). Ice and Hailstorms. Raintree Publishers. ISBN 978-0739847039. 

External links

• Hail Research Information Center
• Images of hail and hailstorms
• Images of a major giant hail event in Brazil
• Hail factsheet
• Using sound to break up hail
• Youtube Video of large hailstorm in progress
• Social & Economic Costs of Hail from "NOAA Socioeconomics" website initiative

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Hail

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