<P> The sun, which drives the water cycle, heats water in oceans and seas . Water evaporates as water vapor into the air . Some ice and snow sublimates directly into water vapor . Evapotranspiration is water transpired from plants and evaporated from the soil . The water molecule H O has smaller molecular mass than the major components of the atmosphere, nitrogen and oxygen, N and O, hence is less dense . Due to the significant difference in density, buoyancy drives humid air higher . As altitude increases, air pressure decreases and the temperature drops (see Gas laws). The lower temperature causes water vapor to condense into tiny liquid water droplets which are heavier than the air, and fall unless supported by an updraft . A huge concentration of these droplets over a large space up in the atmosphere become visible as cloud . Some condensation is near ground level, and called fog . </P> <P> Atmospheric circulation moves water vapor 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 cap s and glacier s, 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 by passing the liquid state . </Dd> <Dt> Deposition </Dt> <Dd> This refers to changing of water vapor directly to ice . </Dd> <Dt> Advection </Dt> <Dd> The movement of water 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 . </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> <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>

During which part of the water cycle does water move from earth’s surface to the atmosphere