Precipitation

In <a href="/facts/Meteorology/xblFr28q">meteorology</a>, precipitation is any product of the <a href="/facts/Condensation/yvmkI08A">condensation</a> of atmospheric <a href="/facts/Water_vapor/DX5IUFG0">water vapor</a> that falls from clouds due to gravitational pull. The main forms of precipitation include <a href="/facts/Drizzle/Dk85tk47">drizzle</a>, <a href="/facts/Rain/MLqcQEyx">rain</a>, <a href="/facts/Rain_and_snow_mixed/j0JUXaIc">Rain and snow mixed</a> ("sleet" in Commonwealth usage), <a href="/facts/Snow/HEFKcgIB">snow</a>, <a href="/facts/Ice_pellets/tjzM3IsJ">ice pellets</a>, <a href="/facts/Graupel/00Y5szTD">graupel</a> and <a href="/facts/Hail/8sVew1sN">hail</a>. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor (reaching 100% <a href="/facts/Relative_humidity/Nwb28VlT">relative humidity</a>), so that the water condenses and "precipitates" or falls. Thus, <a href="/facts/Fog/mrIdyIc7">fog</a> and <a href="/facts/Mist/BywtmoJd">mist</a> are not precipitation; their water vapor does not condense sufficiently to precipitate, so fog and mist do not fall. (Such a non-precipitating combination is a <a href="/facts/Colloid/9EVucqND">colloid</a>.) Two processes, possibly acting together, can lead to air becoming saturated with water vapor: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called <a href="/facts/Shower_(precipitation)/YAzzQwBs">showers</a>.
<a href="/facts/Moisture/vubw4SpX">Moisture</a> that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed by the low temperature into clouds and rain. This process is typically active when freezing rain occurs. A <a href="/facts/Stationary_front/J4mgdJIY">stationary front</a> is often present near the area of freezing rain and serves as the focus for forcing moist air to rise. Provided there is necessary and sufficient atmospheric moisture content, the moisture within the rising air will condense into clouds, namely <a href="/facts/Nimbostratus_cloud/kxbRKvf4">nimbostratus</a> and <a href="/facts/Cumulonimbus/DPWNqTDE">cumulonimbus</a> if significant precipitation is involved. Eventually, the cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects. Where relatively warm water bodies are present, for example due to water evaporation from lakes, <a href="/facts/Lake-effect_snow/UHeegk1o">lake-effect snowfall</a> becomes a concern downwind of the warm lakes within the cold <a href="/facts/Cyclone/T8IEU0aP">cyclonic</a> flow around the backside of <a href="/facts/Extratropical_cyclone/4c5Y4hoJ">extratropical cyclones</a>. Lake-effect snowfall can be locally heavy. <a href="/facts/Thundersnow/Huqh8otc">Thundersnow</a> is possible within a cyclone's <a href="/facts/Extratropical_cyclone/4c5Y4hoJ">comma head</a> and within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within <a href="/facts/Windward/7LebHsNX">windward</a> sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. Most precipitation occurs within the tropics and is caused by <a href="/facts/Convection/E8vsCPhu">convection</a>.
Precipitation is a major component of the <a href="/facts/Water_cycle/wALHdKjJ">water cycle</a>, and is responsible for depositing most of the <a href="/facts/Fresh_water/xMP2TxwI">fresh water</a> on the planet. Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year: 398,000 cubic kilometres (95,000 cu mi) over oceans and 107,000 cubic kilometres (26,000 cu mi) over land. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in), but over land it is only 715 millimetres (28.1 in). Climate classification systems such as the <a href="/facts/K%C3%B6ppen_climate_classification/VDcr86hF">Köppen climate classification</a> system use average annual rainfall to help differentiate between differing climate regimes. <a href="/facts/Climate_change/rEN4BDE7">Global warming</a> is already causing changes to weather, increasing precipitation in some geographies, and reducing it in others, resulting in additional <a href="/facts/Extreme_weather/gzcNR4BC">extreme weather</a>.
Precipitation may occur on other celestial bodies. Saturn's largest <a href="/facts/Moons_of_Saturn/HA64rVRw">satellite</a>, <a href="/facts/Titan_(moon)/lg0l9PCH">Titan</a>, hosts <a href="/facts/Methane/kBHneYnh">methane</a> precipitation as a slow-falling <a href="/facts/Drizzle/Dk85tk47">drizzle</a>, which has been observed as <a href="/facts/Puddle/dYS9qrk1">rain puddles</a> at its equator and polar regions.

Precipitation open-in-new

Precipitation