Claim: Antarctica is dry.

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 portion of the atmosphere becomes saturated with water vapor , so that the water condenses and `` precipitates '' . Thus , fog and mist are not precipitation but suspensions , because the water vapor does not condense sufficiently to precipitate . Two processes , possibly acting together , can lead to air becoming saturated : 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 `` showers . ''   Moisture that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain . This process is typically active when freezing rain is occurring . A stationary front is often present near the area of freezing rain and serves as the foci for forcing and rising air . Provided necessary and sufficient atmospheric moisture content , the moisture within the rising air will condense into clouds , namely stratus and cumulonimbus . 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 , lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones . Lake-effect snowfall can be locally heavy . Thundersnow is possible within a cyclone 's comma head and within lake effect precipitation bands . In mountainous areas , heavy precipitation is possible where upslope flow is maximized within windward 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 . The movement of the monsoon trough , or intertropical convergence zone , brings rainy seasons to savannah climes .   Precipitation is a major component of the water cycle , and is responsible for depositing the fresh water on the planet . Approximately 505000 km3 of water falls as precipitation each year ; 398000 km3 of it over the oceans and 107000 km3 over land . Given the Earth 's surface area , that means the globally averaged annual precipitation is 990 mm , but over land it is only 715 mm . Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes .   Precipitation may occur on other celestial bodies , e.g. when it gets cold , Mars has precipitation which most likely takes the form of frost , rather than rain or snow .