Hail
Hail is a form of precipitation
*balls or irregular lumps of
ice , 5 mm50 mm in diameter on average, with much larger reported from severe
thunderstorms,
*always produced by
cumulonimbus .
*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.
Encyclopedia
- 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 , 5 mm–50 mm in diameter on average, with much larger reported from severe thunderstorms,
- always produced by cumulonimbus .
- 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 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, are revealed. From these rings one can determine 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
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 -13 °C , and becomes vanishingly small much below -30 °C 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 , 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.
China is also notorious for killer hailstorms.
Hailstones, while most commonly only a few millimetres in
diameter, can sometimes grow to 15 centimetres and weigh more than half a
kilogram .
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.
Costly or deadly hailstorms
- Around the 8th century, several hundred pilgrims were killed by a massive hailstorm in Roopkund, Uttaranchal, India.
- July 11 1990, Denver, Colorado, USA, Softball-sized hail destroyed roofs and cars, causing $625 million in total damage.
- May 5 1995, Dallas and Fort Worth, Texas, USA, $1.1 billion insured losses.
- April 14 1999, Sydney, New South Wales, Australia, $1.6 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.
- 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 3 inches deep in many areas. There were 100 canceled flights, and train service was disrupted.
- April 10 2001, St. Louis, Missouri, USA, $1.9 billion insured losses.
- July 19 2002, Henan Province, China, 25 dead and hundreds injured.
- June 16 2006 saw a sudden hailstorm in Leipzig. Hailstones the size of golf balls ravaged the city.
- July 11 2006 a sudden severe outbreak in southern New England contains the 4th largest hailstorm in the United States for the past 50 years. Severe Thunderstorm Watch 591 was issued at 12:15. At around 1:30 four large thunderstorms popped up metro-west of Boston, MA. These thunderstorms were very slow moving, and quickly put most eastern MA, southern NH counties under severe thunderstorm warnings, and in a few cases tornado warnings. A tornado-warned supercell over Marblehead, MA produced straight line winds in excess of 98 MPH that piled up boats in the harbor. Hail up to 3.5" in diameter struck Exeter, NH. A video taken from an Exeter home shows a first-hand account of the severe hailstorm: .
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