Hail Facts
Hail is a form of precipitation in a form of balls or irregular lumps of ice (hailstones), 5 mm–50 mm in diameter on average, although much larger reported from severe thunderstorms. It is always produced by thunderclouds and is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 mm thick. Hail forms on condensation nuclei such as dust, insects, or ice crystals, when supercooled water freezes on contact. Once a hailstone becomes too heavy to be supported by the storm's updraft it falls out of the cloud.
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 (< 32 °F (0 °C)). 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. 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.
Costly or deadly hailstorms
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.
September 7, 1991: a Labour Day thunderstorm caused $400 million worth of insurable damage in Calgary. Thirteen additional hailstorms between 1981 and 1998 caused an estimated $600 million in damage in the Calgary area alone.
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.
June 22, 2003, Aurora, Nebraska, USA, Largest hailstone on record falls. It has a 7-inch diameter and a circumference of 18.75 inches.
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.
