water cycle

The water cycle, also known as the hydrologic cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go, in and out of the atmosphere. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. In so doing, the water goes through different phases: liquid, solid (ice), and gas (vapor).

The water cycle involves the exchange of heat, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate. By transferring water from one reservoir to another, the water cycle purifies water, replenishes the land with freshwater, and transports minerals to different parts of the globe. It is also involved in reshaping the geological features of the Earth, through such processes as erosion and sedimentation. Finally, the water cycle figures significantly in the maintenance of life and ecosystems on Earth.



http://en.wikipedia.org/wiki/File:HydrologicalCycle1.png

2013-04-23 20:40:15 補充：
hopefully I can help you :)

First, water of river and sea are evoporated by sunlight. The water floats to the sky. It forms clouds since the temperature on the sky is lower. If more and more water were evoporated, more clouds are formed. Next, if they are too heavy, then raining is occured. The water finally back to the rivers and seas. The cycle happended again.

Hope I can help you~