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This article is about precipitation. For other uses of the word 'Hail', see Wiktionary. Hail is a form of Precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds, nearly always cumulonimbus. HAIL FORMATION Hail forms on condensation nuclei such as dust, bugs, or ice crystals when supercooled water freezes on contact. 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 hail stones grow large enough, Latent Heat released by further freezing may melt the outer shell of the hail stone. The growth which follows, usually called wet growth, is more efficient because the liquid outer shell allows the stone to accrete other smaller hail stones in addition to supercooled droplets. Once a hailstone becomes too heavy to be supported by the storm's Updraft it falls out of the cloud. The reason rain can't fall, is typically because of the strong winds inside a thunderstorm cloud -- keeping the rain in and freezing it, making it hail that grows bigger and bigger. Even though temperatures at this height are below freezing, most of the cloud's moisture is super cooled. This means it remains liquid because it lacks a surface on which to freeze. As soon as a super cooled droplet collides with potential nuclei—ice crystals, frozen raindrops, dust, or salt—it freezes quickly IDEAL CONDITIONS FOR HAIL FORMATION Hail forms in strong Thunderstorm clouds, particularly those with intense Updraft s, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing (< 32F {Link without Title} ). The growth rate is maximized at about -13C, and becomes vanishingly small much below -30C 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 in elevated regions (mountains, plateaus, etc ..) since those locations are closer to the bottom of thunderstorms, and falling hail has less time to melt before reaching the ground. Sometimes, hail producing clouds are identifiable by their green colouration. [http://australiasevereweather.com/storm_news/arc1997.htm . Hail can do serious damage, notably to Automobile s, Skylight s, and Glass - Roof ed Structure s. Rarely, massive hailstones have been known to cause Concussion s or Fatal Head Trauma . Hailstones, while most commonly only a few Millimetre s in Diameter , can sometimes grow to several Centimetre s or occasionally even bigger. Pea or Golf ball-size hailstones are not uncommon in severe Storm s. The image to the right shows an aggregate hailstone with smaller stones visible. The Ruler shows the size of this hailstone as approximately 6 Cm , almost the size of a tennis ball. 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. China is also notorious for killer hailstorms. COSTLY OR DEADLY HAILSTORMS , compared to a U.S. Quarter , a coin with a diameter of 24.26 mm, in San Jose, California .]]
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