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Weather and climate

Water cycle

Ice crystals melt into water dropletsIce crystals melt into water dropletsOn Earth, water is neither created nor destroyed. The same water goes round and round in an endless cycle between the oceans, the air and the land. This is called the water cycle. About 97% of all the world’s water is held in the oceans. Driven by the heat of the Sun, water is evaporated from the ocean into atmosphere, where it cools, condenses and falls back to Earth again as rain or snow. It falls either directly back into the oceans, or on to land, from where it eventually flows back into the ocean.

Evaporation and condensation

A block diagram illustrating the water cycleA block diagram illustrating the water cycleThe Sun’s heat causes water from the oceans—along with lakes, rivers, plants and the ground itself—to evaporate (turn into vapour). The water lost to the atmosphere from both evaporation and transpiration, the release of water vapour through a plant's leaves, is called evapotranspiration. The warm water vapour rises high into the atmosphere. Moist air may be also forced to rise as it moves across high land. As the air starts to cool, the water vapour may then turn to liquid water or ice. It begins to condense (turns back to liquid) around tiny particles in the air, such as sea salt or dust. Millions of these tiny droplets gather together to form clouds. In the highest clouds, the water freezes into ice.

Nearly 97% of the world’s water held in the oceans. Another 2% is locked in ice caps and glaciers. The remaining 1% is fresh water in lakes, rivers, reservoirs and in the ground.

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