Hailstone

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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 millimetres in diameter, with the larger stones coming from severe thunderstorms.[1] Hail is always 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. Small hailstones are less than 5 mm in diameter, and are reported as SHGS. Unlike ice pellets, they are layered and can be irregular and clumped together.

Hail formation

Enlarge picture
A large hailstone, about 6 cm in diameter
Hail forms on condensation nuclei such as dust, insects, or ice crystals, when supercooled water freezes on contact. Hailstones are usually from the size of a pea to the size of a golfball. 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 heat released by further freezing may melt the outer shell of the hailstone. The growth that follows, usually called wet growth, is more efficient because the liquid outer shell allows the stone to accrete other smaller hailstones in addition to supercooled droplets. These winds hold the rain and freeze it. As the process repeats, the hail grows increasingly larger. Once a hailstone becomes too heavy to be supported by the storm's updraft it falls out of the cloud. When a hailstone is cut in half, a series of concentric rings, like that of an onion, is revealed. These rings reveal the total number of times the hailstone had traveled to the top of the storm before falling to the ground.

Ideal conditions for hail formation

Enlarge picture
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 becomes vanishingly small much below -30 °C (-22 °F) as supercooled water droplets become rare. For this reason, hail is most common in midlatitudes 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 midlatitudes 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 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) have gained the nickname "Hailstorm Alley" among meteorologists for the frequency of hailstorms and their severity.[1]
Enlarge picture
Hail clouds often exhibit a characteristic green coloration.
Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to 15 centimetres 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]

Costly or deadly hailstorms

Enlarge picture
April 20, 2006 hailstorm on San Marcos, Texas
Also, a freak 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.

Gallery




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.

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 21.25 mm.

April 20, 2006 hailstorm on San Marcos, Texas

Hailstorm


See also

References

1. ^ Weather Glossary (html). Weatherzone.
2. ^ Video accompanying entry for "hail" in Britannica Online, Academic Edition
  • Rogers and Yau (1989). A Short Course in CLOUD PHYSICS. Massachusetts: Butterworth-Heinemann. ISBN 0-7506-3215-1. 

External links



'''
  • For the frozen precipitation, see Hail
  • For the city in Saudi Arabia, see Ha'il
  • For the mathematical problem known as the 'hailstone problem', see Collatz conjecture
  • For the "Hail Mary" prayer, see Hail Mary

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precipitation (also known as hydrometeor) is any product of the condensation of atmospheric water vapor that is deposited on the earth's surface. It occurs when the atmosphere (being a large gaseous solution) becomes saturated with water vapour and the water condenses and
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