Rain has been discussed at length because it is the most common form of precipitation. Snow occurs when the freezing level is so near the surface that aggregations of ice crystals do not have time to melt before reaching the ground. Generally, this means that the freezing level must be below 300 m. Mixed snow and rain ('sleet' in British usage) is especially likely when the air temperature at the surface is about 1.5°C. Snowfall rarely occurs with a surface air temperature exceeding 4°C.
Soft hail pellets (roughly spherical, opaque grains of ice with much enclosed air) occur when the Bergeron process operates in a cloud with a small liquid water content and ice particles grow mainly by deposition of water vapour. Limited accretion of small, supercooled droplets forms an aggregate of soft, opaque ice particles 1 mm or so in radius. Showers of such graupel pellets are quite common in winter and spring from cumulonimbus clouds.
Ice pellets may develop if the soft hail falls through a region of large liquid water content above the freezing level. Accretion forms a casing of clear ice around the pellet. Alternatively, an ice pellet consisting entirely of transparent ice may result from the freezing of a raindrop or the refreezing of a melted snowflake.
True hailstones are roughly concentric accretions of clear and opaque ice. The embryo is a raindrop carried aloft in an updraft and frozen. Successive accretions of opaque ice (rime) occur due to impact of supercooled droplets, which freeze instantaneously. The clear ice (glaze) represents a wet surface layer, developed as a result of very rapid collection of supercooled drops in parts of the cloud with large liquid water content, which has subsequently frozen. A major difficulty in early theories was the necessity to postulate violently fluctuating upcurrents to give the observed banded hailstone structure. Modern thunderstorm models successfully account for this; the growing hailstones are recycled by a traveling storm (see Chapter 9I). On occasions, hailstones may reach giant size, weighing up to 0.76 kg each (recorded in September 1970 at Coffeyville, Kansas). In view of their rapid fall speeds, hailstones may fall considerable distances with little melting. Hailstorms are a cause of severe damage to crops and property when large hailstones fall.
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