Understanding Precipitation

Precipitation is the collective name for moisture in the liquid, supercooled and solid forms that falls from the atmosphere.
The cloud particles that form as a result of condensation/sublimation are very small and usually remain suspended in the atmosphere. In a cloud formed by condensation in very stable air, there is little vertical motion to sustain the small cloud droplets and the may escape and drift to earth as drizzle. When the vertical motion associated with the cloud is stronger, the cloud droplets may be too small to escape and must grow sufficiently in size and weight to overcome the vertical currents before they can fall as precipitation.

All of the theories of precipitation formation dictate that there are ice crystals somewhere in the cloud for all types of precipitation (except drizzle) which means that any cloud from which rain is falling must have built up well above the freezing level. Also, there must be a relationship between the intensity of the precipitation and the stability of the air in the parent cloud. Large droplets indicate strong vertical motion while small droplets, such as drizzle, indicate very little vertical motion.

Precipitation and Cloud Types

Rain, Freezing Rain, Ice Pellets	Nimbostratus, Altostratus	Stable
Drizzle, Freezing Drizzle,		Stratus				Stable
Snow Grains

Snow					Nimbostratus, Altocumulus	Stable
					Stratocumulus

Rain Shower, Snow Shower		Cumulonimbus, Towering		Unstable
					Cumulus, Altocumulus
					Castellanus

Ice Crystals				Stratus				Stable

Snow Pellets, Ice Pellet Showers	Cumulonimbus			Unstable
Hail

Forms of Precipitation

Liquid Forms
Rain reaching the ground in middle latitude is usually melted snow. Even in the summer the layers of the atmosphere where precipitation forms (from 3-5km) are cold enough to produce snow. The freezing level in the summertime is typically 3km. Snow that forms above this level melts in a deep layer of above freezing temperatures and reaches the ground as rain.

Drizzle forms in above freezing conditions in a deep layer of stratus not far from the ground. Although the layer is deep, so far as stratus goes, it is quite shallow as compared with the depth of ordinary rain clouds and therefore the drizzle drops have little chance to grow larger by collision.

Freezing Forms
A wide variety of precipitation is possible when layers of air that are above freezing occur. Freezing rain or freezing drizzle occur when a shallow layer of subfreezing air exists at the ground. Rain or drizzle falling into this layer from the warmer air aloft freezes upon contact with subfreezing objects.

Frozen Forms
In situations where the temperature is below freezing from the ground on upwards frozen precipitation will not melt as it falls. If liquid precipitation from warm air aloft falls through a layer of below freezing air based at the surface it does not turn to snow. If the layer is shallow we have seen that it will produce freezing precipitation at the surface. If the layer is deeper, frozen drizzle will produce snow grains which do not bounce when the hit the ground, while frozen rain will yield ice pellets which arrive at the surface with considerable noise as they hit things.

At very low temperatures, ice crystals may fall from the base of clouds or from clear skies. In the latter case, water vapour changes directly into ice via sublimation. The source of the water vapour may be natural such as vast moving water or the result of human activity such as the burning of fossil fuels. In either case, the ice crystals are so light that they seem to float in the air glittering in the sunlight or moonlight as they slowly drift earthwards.

The formation of hail is associated with the updraft and downdraft structure of thunderstorms. Hail is formed when ice particles are lifted several times by strong updrafts through layers of supercooled waterdrops. With each passage, the hailstones accumulate a coating of ice. The hailstones grow in this way until they become too heavy to be supported by the rising air. The large size of hailstones enables them to fall through deep warm layers of air near the ground, so hail is mainly associated with cumulonimbus clouds and is a summer phenomenon.

Understanding Convection
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