Hail

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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 150 millimeters in diameter, with the larger stones coming from severe and dangerous thunderstorms. It sometimes, though not always, occurs during a tornado warning but not as a result of a tornado itself. Hail can occur within any thunderstorm.^[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.

Contents 1 Formation 2 Short term detection 3 Size scale 4 Costly or deadly hailstorms 5 Other hailstorms 6 Gallery 7 See also 8 References 9 Further reading 10 External links

[edit] Formation

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. 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.

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.

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. 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. In North America, hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley." ^[2] Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season. ^[3]

Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to 15 centimetres (6 in) and weigh more than 0.5 kilograms (1.1 lb)^[4]. 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. Hail-producing clouds are often identifiable by their green coloration.^[5][6]

[edit] Short term detection

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 complimented 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^[7][8]. The use of differential reflectivity (Z_dr), in combination with horizontal reflectivity (Z_h) has lead to a variety of hail classification algorithms^[9].

The examples and perspective in this section deal primarily with the United States and do not represent a worldwide view of the subject. Please improve this article or discuss the issue on the talk page.

In the United States, to issue proper warnings and forecasts, National Weather Service uses a network of NEXRAD doppler radars to detect hail.

[edit] 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.^[10] The UK organisation, TORRO, also scales for both hailstones and hailstorms.^[11]

Common coin sizes

	U.S. coin size comparisons	Canadian
Dime		18.03 millimetres (0.710 in)
Cent (or "Penny")	0.75 inches (19 mm)[12]	19.05 millimetres (0.750 in)
Five cents (Nickel)	0.88 inches (22 mm)[13]	21.2 millimetres (0.83 in)
Quarter dollar	1.00 inch (25 mm)[14]	23.88 millimetres (0.940 in)
Dollar		26.5 millimetres (1.04 in)
50 Cents/Half Dollar	1.25 inches (32 mm)[15]	27.13 millimetres (1.068 in)
Two Dollars		28 millimetres (1.1 in)

Other Objects

Object	Diameter
Pea	0.25 inches (6.4 mm)[16]
Marble (small)	0.50 inches (13 mm)[17]
Walnut/Ping-pong ball	1.50 inches (38 mm)[18]
Golf ball	1.75 inches (44 mm)[19]
Lime/Hen egg	2.00 inches (51 mm)[20]
Tennis ball	2.50 inches (64 mm)[21]
Cricket ball	71 millimetres (2.8 in)
Baseball	2.75 inches (70 mm)[22]
Apple/Teacup	3.00 inches (76 mm)[23]
Grapefruit	4.00 inches (102 mm)[24]
Softball	4.50 inches (114 mm)[25]
Computer CD	128 millimetres (5.0 in)

[edit] Costly or deadly hailstorms

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.^[26] The largest hailstone ever recorded fell in 2003 in Aurora, Nebraska, USA.

[edit] Other hailstorms

On the afternoon of June 14, 2009, a major hail storm struck parts of Greater Sudbury, Ontario, Canada enough to cover the ground like snow and flooded some streets. Minor damage was reported. ^{[27] [28]}

[edit] Gallery

This section may require cleanup to meet Wikipedia's quality standards. Please improve this section if you can. (April 2008)

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 in San Marcos, Texas	Hailstorm
The result of a hailstorm in the Hudson Valley of New York

[edit] See also

• Ice pellets
• Graupel
• Snow grains
• Hail cannon

[edit] References

[edit] 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. 

[edit] 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)

Wikimedia Commons has media related to: Hail

Look up Hail or hail in Wiktionary, the free dictionary.