The Collision-Coalescence theory explains that condensation starts around hydroscopic nuc1ei earlier than the saturation point. The size of hydroscopic nuclei determines the fall velocity. When the droplets grow into raindrop size precipitation occurs. Rain, drizzles, snow sleet and hail indicate the different form of precipitation. The precipitations are classified on the basis of the conditions under which large masses of moist air actually induced to rise to higher elevation.
Forms of precipitation - Precipitation results when condensation is occurring rapidly within a cloud. Rain is formed when cloud droplets in large numbers are caused to coalesce in to drops too large to remain suspended in the air.
The drops may then grow by colliding with other drops and joining with, them to become as large as 0.25 in (7 mm) in diameter; but above this size they are unstable and break into smaller drops. Falling droplets less than 0.02 in (0.5 mm) in diameter make a drizzle.
Sleet, as the term is used in the United States, consists of pellets of ice produced from freezing of rain. The raindrops form in an upper, warmer layer, but fall into an underlying cold air layer. (Elsewhere in the English-speaking world sleet means a mixture of rain and snow.)
Snow consists of masses of crystals of ice, grown directly from the water vapour of the air, where air temperature is below freezing. Individual snow crystals, which can be carefully caught upon a black surface and examined with magnifying glass, develop in six-sided, flat crystals or as prisms. They display infinite variations in their beautiful symmetrical patterns.
Hail consists of rounded lumps of ice, having an internal structure of concentric layers, much like an onion. Ordinarily the ice is not clear but has a frosted appearance. Hailstones range from 0.2 to 2 in (0.5 to 5 cm) in diameter and may be extremely destructive to crops and light buildings. Hail occurs only from the cumulonimbus cloud type, inside of which are extremely strong updrafts of air. Raindrops are carried up to high altitudes, are frozen into ice pellets, then fall again through the cloud. Suspended in powerful updrafts, the hailstone grows by the attachment and freezing of droplets, much as ice accumulates on the leading edge of an airplane wing. Eventually the hailstone escapes from the updrafts and falls to earth.
When rain falls upon a ground surface that is covered by an air layer of below-freezing temperature, the water freezes into clear ice after striking the ground or other surfaces such as trees, houses, or wires. The coating of ice that results is called a glaze, and an icing storm is said to have occurred. Actually no ice falls, so that ice glaze is not a form of precipitation. Icing storms cause great damage, especially to telephone and power wires and lo tree limbs. Roads and sidewalks are made extremely hazardous.
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