Hail
Encyclopedia : H : HA : HAI : Hail
- This article is about precipitation. For other uses of the word 'Hail', see Wiktionary.
Hail is a form of precipitation
- balls or irregular lumps of ice (hailstones), 5 mm–50 mm in diameter on average, with much larger reported from severe thunderstorms,
- always produced by cumulonimbus (thunderclouds).
- composed of transparent ice or alternating layers of transparent and translucent ice at least 1 mm thick,
- small hailstones are >5 mm in diameter, and will be reported as SHGS unlike Ice Pellets they are layered and can be irregular and clumped together.
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 that 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. These winds hold the rain and freeze it. As the process repeats, the hail grows increasingly larger. When a hail stone is cut in half, a series of concentric rings, like that of an onion, are revealed. From these rings we can determine the total number of times the hail stone had traveled to the top of the storm before falling to the ground.
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 updrafts, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing (< 32F [0C]). 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.[link] [link].
Hail can do serious damage, notably to automobiles, skylights, and glass-roofed structures. Rarely, massive hailstones have been known to cause concussions or fatal head trauma.
Hailstones, while most commonly only a few millimetres in diameter, can sometimes grow to several centimetres or occasionally even bigger. Pea or golfball-size hailstones are not uncommon in severe storms.
16th June 2006 saw a sudden hailstorm in Leipzig, right in the middle of summer. Hailstones the size of golf balls ravaged the city. Shattered windscreens and dented hoods probably led to a small boom in the car repair business.
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
