Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones). Hailstones usually consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe thunderstorms. 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 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 collects other smaller hailstones.
Ideal conditions for hail formation
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 . The growth rate is maximized at about , and becomes vanishingly small much below 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." Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season.
Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to and weigh more than . 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.
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.
Hailstone size is reported in some countries as compared to known objects rather than by reporting the actual diameter. Below is a table of commonly used objects for this purpose. The UK organisation, TORRO
, also scales for both hailstones and hailstorms.
Common coin sizes
|| Canadian |
|Cent (or "Penny")
|Five cents (Nickel)
|50 Cents/Half Dollar
Costly or deadly hailstorms
- Around the 9th century, several hundred pilgrims were killed by a massive hailstorm in Roopkund, Uttarakhand, India.
- The practice of "weather-shooting" in Austria in the nineteenth century, as a hail prevention measure, was discredited by Joseph Maria Pernter.
- 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.
- July 11 1990, Denver, Colorado, USA, Softball-sized hail destroyed roofs and cars, causing $625 million in total damage ($1 billion in damage adjusted to 2007 dollars ).
- 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.
- 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.
- 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. Largest stone measured was 9.5 cm.
- 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.
- May 18 2000, McHenry, Lake, northern Kane, and northern Cook County, Illinois, USA, $572 million . 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 deep in many areas. There were 100 canceled flights, and train service was disrupted.
- 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.
- 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 diameter and a circumference of .
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- (2007). Hailstorms. Gareth Stevens Publishing. ISBN 978-0836879124.
- (2003). Hailstorms of the United States. Textbook Publishers. ISBN 978-0758116987.
- (1974). Hailstorms and Hailstone Growth. State University of New York Press. ISBN 978-0873953139.
- (2001). Ice and Hailstorms. Raintree Publishers. ISBN 978-0739847039.