Hydrology studies the distribution of water on Earth's surface, its interaction with other natural substances and the role that it plays in plant and animal life. The continuous exchange of water between land and the atmosphere is called the hydrological cycle. By a number of factors, the first among them is the heat radiated by the sun, the water evaporates from the soil, by expanses of water and living organisms, and then condense and fall as rain or snow. Most of the water that reaches the Earth's surface in the form of rain, or in general, precipitation collects water from rivers and streams and then flows directly in the seas; the remaining fraction, on the other hand, penetrates into the soil, which helps to keep the soil moist, is absorbed by plant roots, or seeps underground fueling the flap and then returning to the surface through springs. The total amount of water on Earth is estimated at 1.5 billion km ³. Of this, 97.4% consists of salt water (Oceans and seas) and the remaining 2.6% from fresh water on land that, for the most part, is "trapped" in glaciers and enclosed in groundwater; of this, only a small fraction of 0.015%, i.e. approximately 11 ml km³ (that found in rivers, lakes, in the atmosphere as water vapor and in living forms) is available for humans. Between these different "tanks" is a constant circulation of water under the three forms of liquid, solid (ice) and steam; much of the energy necessary to this process comes from the sun that provides the heat required for evaporation. The evaporated water from the oceans is transported in part on land by atmospheric movements and there arrives in the form of rain or snow (precipitation). About one-third of this water returns to the oceans through surface or scrolling underground percolation. The remainder reaches the atmosphere through evaporation or transpiration by plants (evapotranspiration). The water cycle is the set of phenomena that keeps constant water reserves present on Earth:
· The evaporation of water determines the formation of clouds.
· The clouds are driven by the winds.
· Lowering temperature causes condensation of water and ice in suspension, and then rain.
· Back on the ground, in the form of rain or snow, the water can evaporate from the soil directly, or through the perspiration of the trees; or you can scroll or infiltrate the underground.
· Through the springs and rivers the water flows down to the sea.
· The new evaporation does resume the cycle.
Factories, homes and cars that burn fossil fuels, releasing into the atmosphere, sulfur trioxide and nitrogen oxides. For solar energy effect these substances react with water to form sulphuric acid and nitric acid. The acid water coming to Earth in the form of rain. Plants and animals are severely damaged by acid rain. Earth, air and water are connected in the water cycle. Not only the authorities, but also individual citizens must actively engage to reduce pollution.