3rd March 2006
Hail
Source: Excerpt from The Book " Weather "
Distribution: Widespread, most common in cooler middle latitudes. Cause: Formation of lumps of ice in a thundercloud. Associated Weather: Thunder, lightning, and rain. Hazard Warning: Large stones are a threat to aircraft, people, and property. Perhaps the most destructive form of precipitation is hail. These frozen lumps of ice are produced by thunderstorms and are responsible for injury and property damage worth millions of dollars around the world each year.
Hail is produced when supercooled water droplets are circulated within the updraft area of a cumulonimbus cloud. As the droplets pass through areas of differing temperature and humidity, different types of ice build up on them. Where temperatures are just below freezing and there are many supercooled water droplets, layers of clear ice form. In colder parts of the cloud, where there are fewer and smaller water droplets, freezing is so rapid that bubbles of air are trapped, forming opaque rime ice. This layering effect is enhanced as the hailstone alternately melts and freezes as it encounters warmer and colder air.
Most hailstones are around the size of a pea; however, some grow as large as golf balls, or even oranges. The size and number of layers will depend on how long the hailstone remains in the storm --- hailstones consisting of 25 ice layers have been recorded.
Hail will eventually fall from the cloud if it becomes too heavy to be supported by the updrafts, if the updrafts weaken, or if it is thrown clear of the updraft area. Hail formation requires the strong updrafts associated with spring and summer storms. However, hail is unlikely to reach the ground in tropical areas as the high temperatures will melt the stones. Hail-producing storms are therefore most common around the middle latitudes, with parts of central North America reporting the highest incidence.
Freak Hail
Throughout history, there are records of freak hailstorms, producing enormous stones. In 1888, in northern India, baseball-sized hail reportedly killed 250 people, as well as large numbers of livestock. More recently, in 1986, a hailstorm over Bangladesh produced 2.25 pound (1 kg) stones, which killed 92 people. Hail causes a great deal of damage to property. Cars are particularly vulnerable and insurance claims run to hundreds of millions of dollars each year.
Hailstorms also present a significant aviation hazard, although the advent of onboard radar has lessened this risk. Signs of hail in an approaching storm include a greenish tinge at the base of the cloud, or a whitening of any rainfall. If hail looks likely, move people and pets indoors. Fortunately, hailstorms tend to be brief events, as they occur in only a small part of a moving storm.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
