hail - Hail formation, Ideal conditions for hail formation, Costly or deadly hailstorms

A form of precipitation comprising small balls or pieces of ice, which may reach up to 50 mm/2 in diameter. It is generally associated with rapidly rising convection currents in low latitudes, or the passage of a cold front in temperate latitudes. Hail storms can cause considerable damage to crops and property.

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.

Hail formation

Hail forms on condensation nuclei such as dust, insects, or ice crystals, when supercooled water freezes on contact. If the hailstones grow large enough, latent heat released by further freezing may melt the outer shell of the hailstone. As the process repeats, the hail grows increasingly larger.

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 (< 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. 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.

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). Hail can do serious damage, notably to automobiles, skylights, glass-roofed structures, and most commonly, farmers' crops.

Costly or deadly hailstorms

Around the 8th century, several hundred pilgrims were killed by a massive hailstorm in Roopkund, Uttaranchal, India. July 28, 1981: softball-sized hail hits Calgary, Alberta, Canada, causing $150 million in damage. July 11, 1990, Denver, Colorado, USA, Softball-sized hail destroyed roofs and cars, causing $625 million in total damage. 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. Hail was 3 inches deep in many areas. 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. Hail up to 3.5" in diameter (between teacup and grapefruit size) struck Exeter, NH.

Gallery

Hail Shaft	Small hail that has been fractured to show internal structure; The inset shows the original hail.	A field littered with large hailstones right after a summer hailstorm.
Small transparent hail beads on the ground after a short spring storm.	Small hail from a thunderstorm, compared to a U.S. quarter, a coin with a diameter of 24.26 mm, in San Jose, California.	Large hailstones up to 5 cm in diameter with concentric rings.