In
meteorology,
precipitation is any product of the condensation of
atmospheric water vapor that falls under gravity.
The main forms of precipitation include
drizzle,
rain,
sleet,
snow,
graupel and
hail. Precipitation occurs when a local portion of the atmosphere becomes saturated with water vapour, so that the water condenses and "precipitates". Thus,
fog and
mist are not precipitation but suspensions, because the water vapour does not condense sufficiently to precipitate. Likewise,
frost does not precipitate out of the atmosphere but rises from the ground; so it, too, is not precipitation.
Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air. Generally, precipitation will fall to the surface; an exception is
Virga which evaporates before reaching the surface. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a
cloud. Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops. Unlike raindrops, snowflakes grow in a variety of different shapes and patterns, determined by the
temperature and
humidity characteristics of the air the snowflake moves through on its way to the ground. While snow and ice pellets require temperatures close to the ground to be near or below freezing, hail can occur during much warmer temperature regimes due to the process of its formation.
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