PRECIPITATION
The atmospheric air always contains moisture. Evaporation
from the oceans is the major source (about 90%) of the atmospheric moisture for
precipitation. Continental evaporation contributes only about 10% of the
atmospheric moisture for precipitation. The atmosphere contains the moisture
even on days of bright sun-shine. However, for the occurrence of precipitation,
some mechanism is required to cool the atmospheric air sufficiently to bring it
to (or near) saturation. This mechanism is provided by either convective
systems (due to unequal radiative heating or cooling of the earth’s surface and
atmosphere) or by orographic barriers (such as mountains due to which air gets
lifted up and consequently undergoes cooling, condensation, and precipitation)
and results into, respectively, convective and orographic precipitations. Alternatively,
the air lifted into the atmosphere may converge into a low-pressure area (or cyclone)
causing cyclonic precipitation. Artificially induced precipitation requires
delivery of dry ice or silver iodide or some other cloud seeding agent into the
clouds by aircrafts or balloons.
The common forms of precipitation are drizzle or mist (water droplets of diameters less than
0.5 mm), rain
(water drops of size
between 0.5 mm and 6.0 mm), snow
(ice crystals combining
to form flakes with average specific gravity of about 0.1), sleet (rain water drops, falling through
air at or below freezing temperatures, turned to frozen rain drops), and hail (precipitation in the form of ice
balls of diameter more than about 8 mm). Most of the precipitation, generally,
is in the form of rains. Therefore, the terms precipitation and rainfall are considered
synonymous. Rainfall, i.e.,
liquid
precipitation, is considered light when the rate of rainfall is upto 2.5 mm/hr,
moderate when the rate of rainfall is between 2.5 mm/hr and about 7.5 mm/hr,
and heavy when the rate of rainfall is higher than about 7.5 mm/hr.
Types of Precipitation
It can be divided into three categories depending upon the form such as:
1. Liquid Water
Ex :- Drizzle , Rain
2. Ice
Ex :- freezing rain , freezing drizzle
3. Liquid water freezes when comes in contact with the surface.
Ex :- Snow, Ice Needles, Hail, Graupel, Sleet
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