AskDefine | Define hailstone

Dictionary Definition

hailstone n : small pellet of ice that falls during a hailstorm

User Contributed Dictionary



  1. A single ball of hail.


single ball of hail

Extensive Definition

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 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 formation

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.

Size scale

globalize section 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. The UK organisation, TORRO, also scales for both hailstones and hailstorms.

Costly or deadly hailstorms


Further reading

  • A Short Course in CLOUD PHYSICS
  • Hailstorms
  • Hailstorms of the United States
  • Hailstorms and Hailstone Growth
  • Ice and Hailstorms
hailstone in Arabic: بَرَد
hailstone in Aymara: Chhijchhi
hailstone in Bosnian: Grad (padavina)
hailstone in Bulgarian: Градушка
hailstone in Catalan: Calamarsa
hailstone in Czech: Kroupy (meteorologie)
hailstone in Danish: Hagl (nedbør)
hailstone in German: Hagel
hailstone in Modern Greek (1453-): Χαλάζι
hailstone in Spanish: Granizo
hailstone in Esperanto: Hajlo
hailstone in Basque: Txingor
hailstone in Persian: تگرگ
hailstone in French: Grêle
hailstone in Galician: Sarabia
hailstone in Croatian: Tuča
hailstone in Inuktitut: ᓇᑕᖅᑯᕐᓇᐃᑦ/nataqqurnait
hailstone in Italian: Grandine
hailstone in Hebrew: ברד
hailstone in Kurdish: Zîpik
hailstone in Latin: Grando
hailstone in Malayalam: ആലിപ്പഴം
hailstone in Dutch: Hagel (neerslag)
hailstone in Japanese: 雹
hailstone in Norwegian: Hagl
hailstone in Norwegian Nynorsk: Hagl
hailstone in Occitan (post 1500): Granissa
hailstone in Polish: Grad
hailstone in Portuguese: Granizo
hailstone in Romanian: Grindină
hailstone in Russian: Град
hailstone in Scots: Hail
hailstone in Sicilian: Gragnola
hailstone in Simple English: Hail
hailstone in Slovak: Krúpa (ľadovec)
hailstone in Slovenian: Toča
hailstone in Serbian: Град (падавина)
hailstone in Finnish: Rae
hailstone in Swedish: Hagel
hailstone in Tagalog: Hail
hailstone in Telugu: వడగళ్ళు
hailstone in Thai: ลูกเห็บ
hailstone in Vietnamese: Mưa đá
hailstone in Tajik: Жола
hailstone in Turkish: Dolu
hailstone in Ukrainian: Град
hailstone in Samogitian: Kroša
hailstone in Chinese: 冰雹
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