From Wikipedia, the free encyclopedia

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (February 2008)

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 iscomposed of transparent ice or alternating layers of transparent andtranslucent ice at least 1 mm thick. Small hailstones are less than 5mm in diameter, and are reported as SHGS. Unlike ice pellets, they are layered and can be irregular and clumped together.

[edit] Hail formation

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

Hail forms on condensation nuclei such as dust, insects, or ice crystals, when supercooled waterfreezes on contact. Hailstones are usually from the size of a small peato the size of a golf ball. 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 heatreleased by further freezing may melt the outer shell of the hailstone.The growth that follows, usually called wet growth, is more efficientbecause the liquid outer shell allows the stone to accrete othersmaller hailstones in addition to supercooled droplets. These windshold the rain and freeze it. As the process repeats, the hail growsincreasingly larger. Once a hailstone becomes too heavy to be supportedby the storm's updraftit falls out of the cloud. When a hailstone is cut in half, a series ofconcentric rings, like that of an onion, is revealed. These ringsreveal the total number of times the hailstone had traveled to the topof the storm before falling to the ground.

[edit] Ideal conditions for hail formation

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 becomesvanishingly small much below -30 °C (-22 °F) as supercooled waterdroplets become rare. For this reason, hail is most common inmidlatitudes during early summer where surface temperatures are warmenough 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 muchhigher frequency of thunderstorms than in the midlatitudes because theatmosphere over the tropics tends to be warmer over a much greaterdepth.^{[citation needed]} Also, entrainmentof dry air into strong thunderstorms over continents can increase thefrequency of hail by promoting evaporational cooling which lowers thefreezing level of thunderstorm clouds giving hail a larger volume togrow 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 hailmore likely. One of the most notorious regions for large hail is themountainous northern India and Bangladesh,which have reported more hail-related deaths than anywhere else in theworld 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.[1]

Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to 15 centimetres (6 inches)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 NEXRADdoppler radars to detect hail. Hail size and probability can bedetermined from radar data by a computer by different algorithms. This,in combination with an analysis of the radar display is an accurate wayof detecting hail. An analysis of the radar data would include viewingreflectivity data at multiple angles above ground level to check forhail development in the upper levels of the storm, and checking theVertically Integrated Liquid (VIL). VIL and hail do have arelationship, although it varies with atmospheric conditions andtherefore is not highly accurate. Radar data can also be complimentedby a knowledge of current atmospheric conditions which can allow one todetermine if the current atmosphere is conducive to hail development.

[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 ratherthan by reporting the actual diameter. Below is a table of commonlyused objects for this purpose.^[3]

[edit] Costly or deadly hailstorms

To meet Wikipedia's quality standards, this article or section may require cleanup because it is in a list format that may be better presented using prose. You can help by converting this section to prose, if appropriate. Editing help is available. (February 2008)

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]
• The last known hail fatality in the United States occurred March 29, 2000.The victim was Juan Oseguera, a nineteen-year-old man who died fromhead 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 Daythunderstorm caused $400 million worth of insurable damage in Calgary.Thirteen additional hailstorms between 1981 and 1998 caused anestimated $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, total storm damage reported at around $2billion. The storms produced hail about the size of softballs.^[4][5][6]
• April 14, 1999, Sydney, New South Wales, Australia,$1.5 billion. 20,000 properties and 40,000 vehicles were damaged duringthe storm with more than 25 aircraft damaged at Sydney Airport, oneperson was killed while fishing after getting struck by lightning andseveral other people were injured. It was the costliest hailstorm tohit an Australian populated city. [8] Largest stone measured was 9.5cm.[9]
• May 18, 2000, 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 mostwidespread hailstorm in 30 years. Around 100,000 homes lost power. Hailwas 3 inches deep in many areas. There were 100 canceled flights, andtrain service was disrupted. [11][12]
• The costliest hailstorm in US history occurred April 10, 2001 in St. Louis, Missouri, USA. >$1.9 billion insured losses.
• 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 diameter and a circumference of 18.75 inches.
• December 9, 2007, Sydney, New South Wales, Australia,Severe thunderstorms that lasted barely 20 minutes caused immensedamage in the North and Western Suburbs of Sydney. Worst hit were thesuburbs 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.[13][14]

A 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.^{[citation needed]}

[edit] Gallery

This section may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can (February 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.	Small transparent hail beads on the ground after a short spring storm.	Large hailstones up to 5 cm in diameter with concentric rings. The coin diameter is 21 mm.	April 20, 2006 hailstorm on San Marcos, Texas
Flood caused by the November 4, 2007 Hailstorm in Bogotá, Colombia.	Hailstorm