The Hydrologic Cycle

One of the main ways water affects climate is through the hy-drologic cycle. The hydrologic cycle is "the pilgrimage of water as water molecules make their way from the earth's surface to the atmosphere and back again."1 In the hydrologic cycle, water is constantly moving between different states (solid, liquid, and gas) and different sites on the earth (the ocean, the atmosphere, and the land).

As an example of how the hydrologic cycle works, imagine a glass of water outside a home on a warm day. Some of the water in that glass will evaporate and go into the atmosphere as water vapor. It may stay in the atmosphere for a while, and then eventually fall as rain into a nearby lake. That lake may in turn supply that same home with water. Eventually, when the faucet is turned on, the very same water molecules that were in that glass of water may be reused.

The transformation of water into vapor and back to water— the hydrologic cycle—is powered by the sun. It is the sun that provides the heat energy that turns liquid water into vapor. Specifically, "Heating of the ocean water by the sun is the key process that keeps the hydrologic cycle in motion."2 Most of the water in the atmosphere—as much as 90 percent—comes from evaporation from the oceans and other large bodies of water, such as lakes.3 A smaller amount comes from sublimation, which occurs when, near the freezing point at certain pressures, snow or ice turns directly into water vapor without first turning into water.

According to the NASA Web site Earth Observatory, "A cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day" That is about as much water as would be needed for a 19-hour shower.

A significant amount of water vapor in the air comes from plants. Plants take in water through their roots and let it out through pores called stomates on their leaves. This release of water from plants is called transpiration. According to the NASA Web site Earth Observatory, "a cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day."4 That is about as much water as would be needed for a 19-hour shower.

Once water is in the atmosphere, it rises, cools, and condenses, turning into clouds and eventually falling back to the earth. The water may fall directly into the ocean, or it may fall on land, to be intercepted by plants, or to percolate into the ground, or to end up in streams or lakes or other bodies of water. Eventually, the water evaporates again, and the cycle continues.

The water cycle is broadly the same everywhere: Water turns to vapor and rises into the air; vapor turns to water and falls to the earth. Individual variations in the cycle have an enormous effect on local climates, however. For example, around the Great Lakes in winter, large amounts of water evaporate from the lakes,


Water Source

Percent of Total Water

Oceans, Seas, and Bays


Ice Caps, Glaciers, and Permanent Snow








Soil Moisture


Ground Ice and Permafrost










Swamp Water




Biological Water




Source: "Where Is Earth's Water Located?" USGS Water Science for Schools, July 15, 2009.

and this moisture is blown toward shore as clouds. "By the time these clouds reach the shoreline, they are filled with snowflakes too large to remain suspended in the air and, consequently, they fall along the shoreline as precipitation," according to the Web site WW2010 maintained by the University of Illinois at Chicago.

The precipitation that falls near the lake is often referred to as lake-effect snow, and it can be extremely heavy. For instance, in November 1996, Cleveland, Ohio, received more than 50 inches (127cm) of snow over the course of two to three days due to the lake effect.5

As the lake effect demonstrates, variations in the hydrologic cycle can have substantial effects on the amount of precipitation. Later chapters will look at how global warming may affect the water cycle and rainfall.

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