How the Water Cycle Moves Water Around Earth
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The water in a cloud, a river and a glass may look like separate supplies, but they belong to one moving system. Water constantly changes location and state as it travels through the atmosphere, across land, underground and through living things.
Sunlight lifts water into the air
Most of Earth's water is stored in the oceans. Sunlight warms the surface and gives some water molecules enough energy to escape as water vapour. This change from liquid to gas is called evaporation.
Plants add water vapour through transpiration. They take in water through their roots, move it through their tissues and release some through tiny openings in their leaves. Scientists often group evaporation and transpiration together because both move water from the surface into the atmosphere.
Water also evaporates from lakes, rivers, soil and wet surfaces. The rate changes with temperature, wind, humidity and the amount of exposed water. A warm, windy and dry day usually supports faster evaporation than a cool, still and humid one.
Cooling builds clouds and precipitation
Water vapour is invisible. As moist air rises and cools, some vapour changes back into tiny liquid droplets or ice crystals. This process is condensation. Large collections of those droplets and crystals form clouds.
Cloud particles are very small and can remain suspended in moving air. As they collide and combine, they may grow heavy enough to fall. Depending on temperature and conditions, precipitation can arrive as rain, snow, sleet or hail.
Not every cloud produces precipitation that reaches the ground. Falling water can evaporate on the way down, and winds can move clouds far from the place where their moisture first entered the air. The cycle links distant regions, which is why water from one ocean can later fall on another part of a continent. Snow can remain stored on mountains for months before melting, so water may pause in one part of the cycle and move again in a different season.
Gravity brings water back downhill
Once water reaches the ground, gravity takes over. Some flows across the surface as runoff and enters streams, rivers, lakes and oceans. Some soaks into soil, where plants can use it. Water that moves deeper may become groundwater stored in spaces within rock and sediment. Groundwater can move slowly through rock and reappear at springs, feed rivers during dry weather or remain stored underground for years. Human activity can change these paths through paving, pumping, dams and drainage.
The main paths are easy to trace:
- •Evaporation and transpiration add vapour to the air.
- •Condensation forms cloud droplets and ice crystals.
- •Precipitation returns water to the surface.
- •Runoff carries water downhill.
- •Infiltration moves water into soil and rock.
The takeaway
The water cycle is not a simple circle with one fixed route. It is a network of changing paths powered by sunlight and gravity. Follow the changes of state and the movement downhill, and you can explain how water keeps travelling around the planet.