The water cycle, or hydrologic cycle, is the movement of the Earth's water. Water is in constant movement, and undergoes a number of processes and property changes as it runs through the water cycle. At any given time, a water molecule may exist as liquid, vapor, or ice.
The water cycle is constant and has no real starting or ending point. The sun heats bodies of water on the earth, causing some of the water to evaporate, or change from liquid to gas form. Plants also help water change from liquid to gas through transpiration.
Water vapor returns to liquid form when it cools in a process known as condensation, resulting in clouds and fog. When the condensed water becomes heavy enough, it falls back to the earth in precipitation. Most precipitation is rain, but there are other forms as well, including hail, snow, and sleet.
Below freezing temperature, water takes the solid form of ice or snow. This water can change state through melting, becoming liquid water, or through sublimation, passing directly from a solid to a gaseous state with no intermediate liquid phase. Liquid water moving over the earth's surface, including melted snow or ice, is called runoff. Any movement of water that does not involve a change in material state is called advection. The movement of water from the earth's surface into the soil is called infiltration, and the movement of groundwater beneath the earth's surface is termed subsurface flow.
Some processes of the water cycle move very quickly, such as precipitation, while others can take millions of years, such as changes in lakes or glaciers. Any place that water can be stored during the water cycle is termed a reservoir. Some reservoirs are very short term, such as the atmosphere, in which water molecules remain for only nine days on average. Other reservoirs are extremely tenacious. Oceans have an average residence time of 3,200 years, and deep ground water can remain in the earth for over 10,000 years.