The Earth's Climate

Site Map
All the texts
Credits
Home page - See the Animation without Flash

The water cycle

The four reservoirs of the hydrosphere are: the seas and oceans, the continental surface and subterranean waters, the atmosphere and the biosphere. The constant exchange of water that takes place among between four compartments is called the external water cycle. Its driving force is the Sun, whose radiated thermal energy keeps the masses of water in constant movement.

The water cycle comprises all three of the possible forms of water: liquid water (liquid state) in rivers, oceans and clouds, water vapor (gaseous state) in the atmosphere, and ice (solid state) in the polar icecaps, glaciers and pack ice. The water present on Earth, whose quantity has remained unchanged for several billion years at about 1400 billion cubic kilometers, is constantly in motion, passing through the different physical states. The circulation of water takes place in several stages:
1. Evapotranspiration: water evaporates from the surface of the oceans, as well as from the continents, including evaporation from the ground and transpiration from plants, thus passing from the liquid to the gaseous state;
2. Condensation: evaporated water condenses in the atmosphere, passing from the gaseous to the liquid state, thus forming clouds;
3. Precipitation: water from the clouds falls back to the Earth's surface in the form of raindrops or snowflakes, depending on the air temperature;
4. Infiltration and run-off: the precipitated water infiltrates directly into the subsoil, eventually reaching the subterranean water table, or drains into rivers and streams, eventually reaching the oceans.

And water's long journey starts over again, with evapotranspiration, and so on.

On average, for all of the Earth's continents over an entire year, 65% of the precipitation that reaches the ground evaporates and the remaining 35% remains as run-off and infiltration water.

During this continuous cycle, water is stored in a number of "reservoirs." It remains for only one or two weeks in the atmosphere or the rivers and streams, a few thousand years in the oceans and glaciers, and can be stored for over a million years in the polar icecaps, for example in Antarctica. The water cycle contributes to the transfer of heat between the Earth's surface and the atmosphere. Evaporation is a transition from the liquid state, in which the water molecules are bonded together, to the gaseous state, in which the molecules are independent from each other. It takes energy to break the bonds that link the molecules together in the liquid phase. This energy is teken where the liquid water evaporates, which is why surface evaporation (from the oceans, ground and vegetation) results in the cooling of the surface in question. Conversely, when water vapor condenses in the atmosphere, the same quantity of energy is released into the ambient air. The evaporation–condensation cycle thus transfers heat from the planet's surface to the atmosphere. The warmed air is then transported by atmospheric circulation.

.