<P> The sun , which drives the water cycle , heats water in oceans and seas . Water evaporates as water vapor into the air . Ice and snow can sublimate directly into water vapour . Evapotranspiration is water transpired from plants and evaporated from the soil . The water vapour molecule H 2O has less density compared to the major components of the atmosphere , nitrogen and oxygen , N 2 and O 2 . Due to the significant difference in molecular mass , water vapor in gas form gains height in open air as a result of buoyancy . However , as altitude increases , air pressure decreases and the temperature drops ( see Gas laws ) . The lowered temperature causes water vapour to condense into a tiny liquid water droplet which is heavier than the air , such that it falls unless supported by an updraft . A huge concentration of these droplets over a large space up in the atmosphere become visible as cloud . Fog is formed if the water vapour condenses near ground level , as a result of moist air and cool air collision or an abrupt reduction in air pressure . Air currents move water vapour around the globe , cloud particles collide , grow , and fall out of the upper atmospheric layers as precipitation . Some precipitation falls as snow or hail , sleet , and can accumulate as ice caps and glaciers , which can store frozen water for thousands of years . Most water falls back into the oceans or onto land as rain , where the water flows over the ground as surface runoff . A portion of runoff enters rivers in valleys in the landscape , with streamflow moving water towards the oceans . Runoff and water emerging from the ground ( groundwater ) may be stored as freshwater in lakes . Not all runoff flows into rivers , much of it soaks into the ground as infiltration . Some water infiltrates deep into the ground and replenishes aquifers , which can store freshwater for long periods of time . Some infiltration stays close to the land surface and can seep back into surface - water bodies ( and the ocean ) as groundwater discharge . Some groundwater finds openings in the land surface and comes out as freshwater springs . In river valleys and floodplains , there is often continuous water exchange between surface water and ground water in the hyporheic zone . Over time , the water returns to the ocean , to continue the water cycle . </P>
the process by which water circulates from the oceans to the land masses is known as
precipitation