Definition: The movement of water in all of its phases (gas, liquid, solid) from the Earth to the atmosphere and back to the Earth.
As the sun evaporates (liquid to gas[vapor]) liquid water from the surface of the earth (oceans, lakes and rivers), air motions move this water vapor into the atmosphere. The water vapor is transported both in the horizontal and vertical, eventually into areas where condensation (vapor to liquid) occurs. Condensation produces clouds, some of which produce precipitation (liquid or solid) whereby the liquid reaches the ground and flows back into the oceans, lakes and rivers. This constant circulation of water from the earth to the atmosphere and back again is called the hydrologic cycle.
The hydrologic cycle is very complicated. Some of the water which condenses into clouds and precipitates actually evaporates before reaching the Earth's surface. Some precipitation is intercepted by buildings or vegetation, where it continues to drip long after the storm has ended. This same vegetation can also place water vapor into the atmosphere via a process called transpiration. Humans can place water vapor into the atmosphere by perspiration or by simply exhaling. If all of the vapor in the atmosphere were to suddenly condense and fall out as rain, it would cover the entire Earth with one inch of water.
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