Precipitation has been defined as water in liquid or solid forms falling to the earth. According to Foster, precipitation is deposition of atmospheric moisture and is perhaps the most important phase of the hydrologic cycle.
Atmospheric moisture may be precipitated either is solid or liquid states. Rain, snow, hail and sleet, etc. are the common forms of precipitation. However, ‘precipitation’ does not include some other forms of condensed moisture, namely, fog, dew and frost.
The first step in precipitation is condensation. The process of condensation involves a change from water vapour to liquid, while the process of precipitation involves the falling out of that water as rain, snow, hail or some other hydrometer.
The most interesting and intriguing aspect is the fact that there are more periods of cloudiness without the occurrence of precipitation than with it.
Sometimes raindrops or snowflakes are formed from cloud droplets or ice crystals and, at other times, they are not; this is still one of the great mysteries in meteorology.