Hail

[3] Ice pellets generally fall in cold weather, while hail growth is greatly inhibited during low surface temperatures.

Hail formation requires environments of strong, upward motion of air within the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level.

[citation needed] Hail is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 mm (0.039 in) thick, which are deposited upon the hailstone as it travels through the cloud, suspended aloft by air with strong upward motion until its weight overcomes the updraft and falls to the ground.

[10] Other countries have different thresholds according to local sensitivity to hail; for instance, grape-growing areas could be adversely impacted by smaller hailstones.

[11] The growth rate of hailstones is impacted by factors such as higher elevation, lower freezing zones, and wind shear.

Former theory suggested that hailstones were subjected to multiple descents and ascents, falling into a zone of humidity and refreezing as they were uplifted.

[citation needed] The storm's updraft, with upwardly directed wind speeds as high as 110 mph (180 km/h),[13] blows the forming hailstones up the cloud.

As the hailstone ascends, it passes into areas of the cloud where the concentration of humidity and supercooled water droplets varies.

The hailstone's growth rate changes depending on the variation in humidity and supercooled water droplets that it encounters.

This means that generally the larger hailstones will form some distance from the stronger updraft, where they can pass more time growing.

[16] Hail can also undergo "dry growth", in which the latent heat release through freezing is not enough to keep the outer layer in a liquid state.

Hail forming in this manner appears opaque due to small air bubbles that become trapped in the stone during rapid freezing.

One of the more common regions for large hail is across mountainous northern India, which reported one of the highest hail-related death tolls on record in 1888.

[28] Hail in this region occurs between the months of March and October during the afternoon and evening hours, with the bulk of the occurrences from May through September.

[29] To the north of this area and also just downwind of the Rocky Mountains is the Hailstorm Alley region of Alberta, which also experiences an increased incidence of significant hail events.

[30] The triple border region between the Brazilian states of Paraná, Santa Catarina, and Argentina, in southern Brazil is another area known for damaging hailstorms.

[31] Hailstorms are also common in parts of Paraguay, Uruguay, and Bolivia that border the high-frequency hail regions of northern Argentina.

[32] The high frequency of hailstorms in these areas of South America is attributed to the region's orographic forcing of convection, combined with moisture transport from the Amazon and instability created by temperature contrasts between the surface and upper atmosphere.

[33] Traditionally, hail size and probability can be estimated from radar data by computer using algorithms based on this research.

More recently, the polarization properties of weather radar returns have been analyzed to differentiate between hail and heavy rain.

[36] Visible satellite imagery is beginning to be used to detect hail, but false alarm rates remain high using this method.

[38] Using objects such as hen's eggs, peas, and marbles for comparing hailstone sizes is imprecise, due to their varied dimensions.

[41] In the United States, the National Weather Service reports hail size as a comparison to everyday objects.

Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, livestock, and most commonly, crops.

Hail is a common nuisance to drivers of automobiles, severely denting the vehicle and cracking or even shattering windshields and windows unless parked in a garage or covered with a shielding material.

One of the earliest known incidents occurred around the 9th century in Roopkund, Uttarakhand, India, where 200 to 600 nomads seem to have died of injuries from hail the size of cricket balls.

The hailstones, described as between the size of bumble bees and ping pong balls, were accompanied by rain and high winds.

[59] Research focused on four individual days that accumulated more than 5.9 inches (15 cm) of hail in 30 minutes on the Colorado front range has shown that these events share similar patterns in observed synoptic weather, radar, and lightning characteristics,[60] suggesting the possibility of predicting these events prior to their occurrence.

The joint project's goal is to enlist the help of the general public to develop a database of hail accumulation depths.

[61] During the Middle Ages, people in Europe used to ring church bells and fire cannons to try to prevent hail, and the subsequent damage to crops.