hail

Large hailstones up to 5 centimetres (2 in) in diameter with concentric rings

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 150 millimeters in diameter, with the larger stones coming from severe and dangerous thunderstorms. The METAR reporting code for hail 5 mm or greater in diameter is GR, while smaller hailstones and graupel are coded GS. Hail can occur within any thunderstorm,^[1] and is only produced by cumulonimbi (thunderclouds), usually at the leading edge of a storm system. Hail formation requires environments of strong, upward motion and lowered heights of the freezing level. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics.

Unlike ice pellets, they 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 multiple times, suspended aloft by strong upward motion until its weight overcomes the updraft and falls to the ground. There are methods available to detect hail-producing thunderstorms using weather satellites and radar imagery. Severe weather warnings are issued for hail when the stones reach a damaging size, as it can cause serious damage to automobiles, aircraft, skylights, glass-roofed structures, and most commonly, farmers' crops. Hail is normally produced by tornadic thunderstorms.

Formation

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

Like other precipitation, hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust or dirt. 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. Hail has a diameter of 5 millimetres (0.20 in) or more.^[2] Stones just larger than golf ball-sized are one of the most frequently reported hail sizes.^[3] Hailstones can grow to 15 centimetres (6 in) and weigh more than 0.5 kilograms (1.1 lb).^[4] 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 collects other smaller hailstones.^[5] 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 from the cloud.^[6]

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).^[2] Hail-producing clouds are often identifiable by their green coloration.^[7][8] 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 within continental interiors of the mid-latitudes, as hail formation is considerably more likely when the freezing level is below the altitude of 11,000 feet (3,400 m).^[9] 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. 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. Hail in the tropics occurs mainly at higher elevations.^[10]

Preferred regions

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 also experiences significant hailstorms. In North America, hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley." ^[11] Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season. ^[12]

Short term detection

Hail clouds often exhibit a characteristic green coloration.

Doppler Weather radar is a very useful tool to detect the presence of hail producing thunderstorms. Traditionally, hail size and probability can be determined from radar data by a computer using 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 complemented by a knowledge of current atmospheric conditions which can allow one to determine if the current atmosphere is conducive to hail development.

More recently, the polarization properties of weather radar returns have been analyzed to differentiate between hail and heavy rain^[13][14]. The use of differential reflectivity (Z_dr), in combination with horizontal reflectivity (Z_h) has lead to a variety of hail classification algorithms^[15]. Visible satellite imagery is beginning to be used to detect hail, but false alarm rates remain high using this method.^[16]

Size scale

The examples and perspective in this section may not represent a worldwide view of the subject. Please improve this article and 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.^[17] The UK organisation, TORRO, also scales for both hailstones and hailstorms.^[18]

Hailstones ranging in size from Pea to Nickel

Damage

Hailstorm in Bogotá, Colombia.

Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, and most commonly, farmers' crops. Rarely, massive hailstones have been known to cause concussions or fatal head trauma. Hailstorms have been the cause of costly and deadly events throughout history. One of the earliest incidents occurred around the 9th century in Roopkund, Uttarakhand, India.^{[citation needed]} The largest hailstone ever recorded fell in 2003 in Aurora, Nebraska, USA.^{[citation needed]}

Other hailstorms

On the night of July 7th, 2009 approximately around midnight, a very large thunderstorm hit a city in Suburban New York City in Yonkers, blanketing the ground with over two inches of hail and snow causing 17,000 without power, many down trees, flash flooding, and a mudslide. ^[20][21]

See also

• Ice pellets
• 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
• Hail factsheet
• Using sound to break up hail
• The Economic Costs of Hail Storm Damage NOAA Economics
• Images
  • Hail and hailstorms
  • Major hail event in Brazil
  • Youtube video of large hailstorm in progress
  • NOAA Hail Reports on Google map (non commercial)

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