Definition · Plain-language
Precipitation
Precipitation is any form of water — liquid or frozen — that falls from clouds to the ground, including rain, snow, sleet and hail.
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Water falling from the sky
In everyday speech precipitation means rain, but to a scientist it is any water that falls from the atmosphere to the surface, whether liquid or frozen. It is distinct from condensation, which is water vapour turning to liquid in the air to make cloud or dew, and from humidity, which is vapour still suspended as gas. Precipitation specifically means the water has formed droplets or crystals heavy enough to overcome the rising air and fall. It is one of the central processes that moves water through the environment.
How precipitation forms
Precipitation begins with rising, cooling air. As moist air rises it cools, and once it reaches saturation the water vapour condenses onto tiny airborne particles to form cloud droplets or ice crystals. These are far too small and light to fall; they must grow first. Droplets collide and merge, and ice crystals gather more vapour and clump together, until they are heavy enough that gravity overcomes the updraughts holding them aloft. Then they fall. Whether they reach the ground as rain, snow or another form depends largely on the temperatures they pass through on the way down.
The different types
Precipitation takes several forms set by temperature. Rain is liquid water; drizzle is the same but in very fine drops. Snow forms when water vapour freezes directly into ice crystals that fall before melting. Sleet is rain that freezes into ice pellets as it falls through a cold layer, while freezing rain stays liquid until it hits a cold surface and ices over. Hail forms in tall storm clouds, where strong updraughts repeatedly lift ice pellets through freezing layers so they grow into hard, layered lumps before falling.
Key facts
At a glance
- Definition: water (liquid or frozen) falling from clouds to the ground
- Types: rain, drizzle, snow, sleet, freezing rain and hail
- Forms when: cloud droplets or ice crystals grow heavy enough to fall
- Set by: the temperatures the falling water passes through
- Not the same as: condensation (vapour to liquid) or humidity (vapour in air)
- Role: returns water from the atmosphere in the water cycle
Common misconceptions
What people often get wrong
Often heard: Precipitation just means rain.
Actually: Precipitation covers all water falling from clouds, including snow, sleet, hail, drizzle and freezing rain. Rain is only its most familiar liquid form.
Often heard: Precipitation and condensation are the same thing.
Actually: Condensation is water vapour turning into liquid droplets in the air, forming cloud. Precipitation is the later stage where those droplets or crystals grow heavy enough to fall to the ground.
Often heard: Snow and hail form the same way, just at different times of year.
Actually: Snow forms when vapour freezes into crystals that fall while cold. Hail forms inside tall storm clouds in any season, where updraughts repeatedly lift ice through freezing layers until it grows into solid lumps.
