Water on Earth is in a continuous state of change. The water cycle includes processes such as evaporation, precipitation, melting and freezing.
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- Elevations of the Earth's surface rising abruptly, higher than 500 meters (1,640 ft) above sea level.
- The place where underground water flows to the surface.
The water deposit of the Earth is like a huge perpetual motion machine. Approximately 1.4 billion km³ (3.698x10²⁰ gal) of water is processed in its cycle every year. (About 30% of water cannot take part in the circular process, as this amount is in a chemically bound state.)
What keeps the process moving? Principally, the energy of solar radiation. This causes the continuous change of state of water: it evaporates – condensates – freezes – melts. When absorbing heat, ice melts; ice and snow (=solid water) go from a solid to a liquid (=water) state; and finally water evaporates, turning into vapor (=aeriform water).
When releasing heat, water freezes into ice; water vapor condensates, forming clouds, and resulting in precipitation. Water cannot stay in any of its states permanently neither in the atmosphere, nor on the surface. Gravity moves water on lands: on the surface downwards from higher areas towards lower areas, or under the surface. Water may continue its journey even under the surface, until it finds its way to the surface as a spring.
During its cycle, water wanders around several layers of the Earth, connecting them by exchanging their water content. Water in the atmosphere is renewed every 8 days, while the world ocean every 3,500 years, water stored in continental ice every 12,000 years, and the water supply situated under the surface gets exchanged every 1,400 years.
vaporization by the vegetation
vaporization by the vegetation
The Sun’s energy warms up lakes and seas, causing the water to evaporate from the surface and from plants. The vapor rises into the colder regions of the atmosphere where it condenses and forms clouds.
The condensation occurs on the surface of small dust particles found in the air. Clouds may be composed of water droplets and ice crystals. The wind blows clouds and vapor towards land, where the formation of further clouds begins. The condensed water droplets start to grow; when they reach a size that prevents them from remaining in the air, they fall in the form of rain.
Rain clouds are dark in color and hang low, while large storm clouds are anvil-shaped. When the temperature is above 0 °C (32 °F), it rains, when it falls below 0° (32 °F), it snows.
A part of the rain or snow seeps into the soil, while the rest flows into surface waters. Rivers carry the water into lakes and seas, and so it returns to the starting point, where the process starts all over again due to the effect of solar radiation.