Precipitation includes all forms of additional water, in the liquid or solid state, that fall or form on the Earth’s surface.
They can be subdivided into direct precipitation, such as rain, snow and hail and occult precipitation, such as dew and frost, that do not derive from clouds but form directly on contact with the Earth’s surface.
Liquid precipitation, or rain, takes place when drops of water present in a cloud grow bigger and bigger until they are too heavy to remain in the cloud and therefore fall to the ground.
The mechanisms by which the cloud drops grow bigger are several: by absorbing water in an over-saturated atmosphere or, specially by coalescence, that occurs when drops collide against each other on falling.
The limit that separates cloud drops from rain drops is around 100 micron, but generally rain drops are much bigger, at times greater than 2000 micron.

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