From SKYbrary Wiki
(RA) Rain is the condensation of atmospheric water vapour into drops of water heavy enough to fall to the surface of the earth.
Rainfall occurs when the air becomes saturated, either through cooling or by the addition of water, to form a cloud. Condensed water molecules coalesce to form droplets; this process continues until the droplets are of sufficient size to fall as rain.
Cooling of the air may be caused by a variety of means, for example, due to up-currents or when the air moves over a rising surface. Moisture may be added when the air passes over water (e.g. sea or a lake).
When water droplets falling are very small, this is called Drizzle.