school energy&environment

Water cycle

In the oceans, water is in the liquid state. Solar heating causes a portion of surface water to evaporate and turn into steam, go up into the atmosphere and be carried by the wind. When a mass of water, that is already rich in water vapour, receives more water and saturates, or when it encounters a colder mass of air, water vapour condenses in the atmosphere, i.e. vapour turns into water again (or snow and ice, depending on how cold it is). This is how precipitations originate, through which liquid or solid water (rain, snow or hail) partly reaches the continents and partly gets straight back to the oceans. The water fallen through precipitations on the emerged land still has to go a long, often winding way before going back to the oceans and close the cycle. Some of this water seeps through the soil, and part of it remains in the soil, part feeds the water-bearing stratum (deep flow) to resurface later in rivers or springs. Part of the water that had remained in the soil evaporates into the atmosphere, while part of it is absorbed by plant roots and carried through to the leaves to be released again into the atmosphere through transpiration. These two processes together are called evapo-transpiration. Finally, some of the water fallen through precipitations remains on the Earth’s surface and originates lakes and rivers, through which it gets straight back to the seas and oceans (surface flow).