<P> The water cycle involves the exchange of energy, which leads to temperature changes . For instance, when water evaporates, it takes up energy from its surroundings and cools the environment . When it condenses, it releases energy and warms the environment . These heat exchanges influence climate . </P> <P> The evaporative phase of the cycle purifies water which then replenishes the land with freshwater . The flow of liquid water and ice transports minerals across the globe . It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation . The water cycle is also essential for the maintenance of most life and ecosystems on the planet . </P> <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> <Dl> <Dt> Precipitation </Dt> <Dd> Condensed water vapor that falls to the Earth's surface . Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet . Approximately 505,000 km (121,000 cu mi) of water falls as precipitation each year, 398,000 km (95,000 cu mi) of it over the oceans . The rain on land contains 107,000 km (26,000 cu mi) of water per year and a snowing only 1,000 km (240 cu mi). 78% of global precipitation occurs over the ocean . </Dd> <Dt> Canopy interception </Dt> <Dd> The precipitation that is intercepted by plant foliage eventually evaporates back to the atmosphere rather than falling to the ground . </Dd> <Dt> Snowmelt </Dt> <Dd> The runoff produced by melting snow . </Dd> <Dt> Runoff </Dt> <Dd> The variety of ways by which water moves across the land . This includes both surface runoff and channel runoff . As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses . </Dd> <Dt> Infiltration </Dt> <Dd> The flow of water from the ground surface into the ground . Once infiltrated, the water becomes soil moisture or groundwater . A recent global study using water stable isotopes, however, shows that not all soil moisture is equally available for groundwater recharge or for plant transpiration . </Dd> <Dt> Subsurface flow </Dt> <Dd> The flow of water underground, in the vadose zone and aquifers . Subsurface water may return to the surface (e.g. as a spring or by being pumped) or eventually seep into the oceans . Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures . Groundwater tends to move slowly and is replenished slowly, so it can remain in aquifers for thousands of years . </Dd> <Dt> Evaporation </Dt> <Dd> The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere . The source of energy for evaporation is primarily solar radiation . Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration . Total annual evapotranspiration amounts to approximately 505,000 km (121,000 cu mi) of water, 434,000 km (104,000 cu mi) of which evaporates from the oceans . 86% of global evaporation occurs over the ocean . </Dd> <Dt> Sublimation </Dt> <Dd> The state change directly from solid water (snow or ice) to water vapor . </Dd> <Dt> Deposition </Dt> <Dd> This refers to changing of water vapor directly to ice . </Dd> <Dt> Advection </Dt> <Dd> The movement of water--in solid, liquid, or vapor states--through the atmosphere . Without advection, water that evaporated over the oceans could not precipitate over land . </Dd> <Dt> Condensation </Dt> <Dd> The transformation of water vapor to liquid water droplets in the air, creating clouds and fog . </Dd> <Dt> Transpiration </Dt> <Dd> The release of water vapor from plants and soil into the air . Water vapor is a gas that cannot be seen . </Dd> <Dt> Percolation </Dt> <Dd> Water flows vertically through the soil and rocks under the influence of gravity </Dd> <Dt> Plate tectonics </Dt> <Dd> Water enters the mantle via subduction of oceanic crust . Water returns to the surface via volcanism . </Dd> </Dl>

The process by which water circulates from the oceans to the land masses is known as