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The Water Cycle

Water can exist in three states or phases, depending upon conditions of temperature and pressure: gas (…), liquid (…) and solid (…). (…)

The changes from one state to another constitute the Water Cycle, indispensable to life on Earth. (…)

The oceans contain 97% of the Earth’s water. The other 3% is fresh water. (…)

The Sun causes water to evaporate. A small fraction is also changed to the gaseous state by the transpiration of plants. The water vapor accumulates in the atmosphere.

Note that water vapor is invisible. It is when it cools down and condenses into fine droplets and crystals of ice that it once again becomes visible, in the form of clouds or fog. (…)

The droplets and crystals grow, and end up falling back down to Earth in the form of rain or snow, notably when masses of air collide with elevated parts of the local relief.

A small part of the fallen water drips downward, rejoins bodies of water and returns to the sea in a matter of days or weeks.  But the much greater part accumulates in glaciers or infiltrates the soil to arrive at the water table.

Water can stay there for several thousands of years. (…)

Water is not equally distributed over the surface of the globe. Nor is potable water, meaning water that is safe to drink: a billion people are deprived of this.

In addition, agriculture requires ever growing amounts of water in order to feed what soon will be  7 billion human beings. 

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