Water Cycle
The continuous movement of water between atmosphere, land, and oceans through evaporation, condensation, precipitation, and runoff.
The water cycle (hydrologic cycle) is the continuous circulation of water among the atmosphere, oceans, land, and living organisms. It is powered by solar energy: sunlight evaporates water from oceans and lakes, plants release water vapor through transpiration, the vapor rises and cools, condenses into clouds, and returns as precipitation — rain or snow. From there, water flows downhill as runoff into rivers and lakes, infiltrates the ground into aquifers, or evaporates again. The cycle has no beginning or end; it is a closed loop of storage and flow between reservoirs.
The main reservoirs are the oceans (about 97% of Earth's water), ice sheets and glaciers (about 2%), groundwater, and the small fractions in lakes, rivers, soil, the atmosphere, and living things. Residence times vary enormously: a water molecule stays in the atmosphere only about nine days, in a river a few weeks, in a lake years to decades, in groundwater centuries, and in ice sheets up to hundreds of thousands of years. The atmosphere holds only a tiny fraction of the total, yet it is the fastest-moving part of the cycle and the source of all fresh water on land.
Evaporation is the largest transfer: about 86% of atmospheric water vapor comes from the oceans, and the remaining 14% from land and transpiration. Condensation requires cooling, which is why precipitation concentrates where air rises — over mountains (orographic rainfall), along weather fronts, and in convective storms. The global circulation of water in the atmosphere is coupled to the jet stream, which steers weather systems and transports moisture across continents.
The cycle is sensitive to climate: a warmer atmosphere holds more moisture, intensifying both evaporation and heavy precipitation — the same feedback that strengthens the greenhouse effect. Human activity alters the cycle directly through dams, irrigation, and groundwater pumping, and indirectly through land-use change. Understanding the water cycle is therefore central to water management, agriculture, flood prediction, and climate science.
Tags
earth science evaporation hydrology precipitation