The hydrologic, or water cycle circulates the waters in nature, continuously changing them from one state to another. The path of this complex cycle, which has no beginning and no end, may stretch to a height of 15 kilometres above the Earth's surface, or reach to a depth of 1 kilometre beneath the Earth's crust. In simple terms, what happens is that water in the oceans and on land evaporates, rises into the atmosphere and is finally returned to Earth as snow or rainfall. The latter part of the cycle is known as the precipitation process. Water that percolates down through the soil is stored naturally underground and will eventually return to the sea, by way of streams, to be evaporated once more.

Together, evaporation, transpiration, and sublimation, plus volcanic emissions, account for almost all the water vapour in the atmosphere that isn’t inserted through human activities. While evaporation from the oceans is the primary vehicle for driving the surface-to-atmosphere portion of the hydrologic cycle, transpiration is also significant. For example, a cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day.

After the water enters the lower atmosphere, rising air currents carry it upward, often high into the atmosphere, where the air is cooler. In the cool air, water vapour is more likely to condense from a gas to a liquid to form cloud droplets.