描述導致在大氣中出現凝結現象的冷卻作用。解釋為什麼該等作用本身不能單獨產生降水。舉出例子幫助說明。

The water cycle — technically known as the hydrological cycle — is the continuous circulation of water within the Earth's hydrosphere, and is driven by solar radiation. This includes the atmosphere, land, surface water and groundwater. As water moves through the cycle, it changes state between liquid, solid, and gas phases. Water moves from compartment to compartment, such as from river to ocean, by the physical processes of evaporation, precipitation, infiltration, runoff, and subsurface flow.

Description
The water cycle is the continuous movement of water over, above, and beneath the Earth's surface. It is powered by solar energy, and because it is a cycle, there is no beginning or end. As water moves around in the hydrosphere, it changes state among liquid, vapour, and ice. The time taken for water to move from one place to another varies from seconds to thousands of years, and the amount of water stored in different parts of the hydrosphere ranges up to 1.37 billion km³, which is contained in the oceans. Despite continual movement within the hydrosphere, the total amount of water at any one time remains essentially constant.

Movement of water takes place by a variety of physical and biophysical processes. The two processes responsible for moving the greatest quantities of water are precipitation and evaporation, transporting 505,000 km³ of water each year. The flow of water along rivers transports an intermediate amount of water, and sublimation of ice directly to vapour transports relatively very little. The different processes are as follows.

Precipitation is condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet.
Approximately 505,000 km³ of water fall as precipitation each year, 398,000 km³ of it over the oceans.
Canopy interception is the precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground.
Snowmelt refers to the runoff produced by melting snow.
Runoff includes the variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
Infiltration is the flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater.
Subsurface Flow is the flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (eg. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.
Evaporation is the transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Approximately 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration.[citation needed] Total annual evapotranspiration amounts to approximately 505,000 km³ of water, 434,000 km³ of which evaporates from the oceans.
Sublimation is the state change directly from solid water (snow or ice) to water vapor.
Advection is the movement of water—in solid, liquid, or vapour states—through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.
Condensation is the transformation of water vapour to liquid water droplets in the air, producing clouds and fog

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