Saturday, June 26, 2010
Hail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 millimetres (0.20 in) and 150 millimetres (5.9 in) in diameter, with the larger stones coming from severe thunderstorms. The METAR reporting code for hail 5 millimetres (0.20 in) or greater in diameter is GR, while smaller hailstones and graupel are coded GS. Hail is possible with most thunderstorms as it is produced by cumulonimbi (thunderclouds), usually at the leading edge of a severe storm system. Hail is possible within 2 nautical miles (3.7 km) of its parent thunderstorm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics. Hail formation is most common during the summer months in the afternoon and evening hours of the day.