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Southern AER

A Quarterly Activity Bulletin of The South Carolina Department of Natural Resources-Southeast Regional Climate Center
Spring 1999
Volume 5, No. 1

What is the hydrologic cycle?

The hydrologic cycle, or water cycle, is the circulation and recycling of the earth's water. Water is a unique substance because it has the ability to exist in three forms: solid (ice), liquid, and gas (water vapor). This unique ability is extremely important to the water cycle process. Seventy percent of the earth= s surface is covered in water. The majority of the water found on earth, 97.5%, is in the oceans, while 2.4% is on land, and less than .001% is in the atmosphere. The paragraphs below describe the journey water takes through the hydrologic cycle as it is recycled from the earth= s surface, into the atmosphere, and then to the earth again.

Step One: Water Leaves the Earth

Evaporation is the physical process that occurs when water is transformed from a liquid into a gas. As the heat from the sun warms surface water, the molecules that make up the water begin to vibrate faster. When the molecules gain enough speed, they will lift into the air above. As a gas, water is no longer restricted to the surface of the earth, and travels upward into the atmosphere. About 80-85% of the water leaving the earth is due to the evaporation that occurs over oceans. The remaining 15-20% comes from the evaporation of inland water (lakes, ponds, streams, and soil moisture) and a process called transpiration. Transpiration is the process by which plants add moisture to the air. The water that is absorbed by the root systems of plants move upward through the stems and enters the air through tiny openings on the underside of leaves.

Step Two: Condensation

The next step of the hydrologic cycle is condensation, or the change water from vapor to liquid. As warm, moist air rises in the atmosphere, it cools because the temperature of the atmosphere decreases with height. As the air cools, the speed of water molecules slow down to the point where they no longer have sufficient speed or energy to remain a gas. At this point, the water vapor condenses on tiny particles in the air called cloud nuclei, and clouds are formed. Clouds are made up of these water droplets and/or ice crystals. Winds move water vapor and clouds around the earth.

Step Three: Water Returns to the Earth

When the air becomes saturated, or cannot hold any more moisture, precipitation occurs. There are many forms of precipitation such as rain, snow, sleet, and hail. Some of the precipitation may evaporate back into the air before ever reaching the ground. When precipitation reaches the earth, some of the water soaks into the ground through openings in the soil and rock. This water is stored as groundwater in the water table and in aquifers. The water that is stored under ground moves slowly, eventually emptying into bodies of water where it can be evaporated once again. Some water is soaked up by vegetation where it is eventually released back into the atmosphere through transpiration. The water that does not evaporate, transpire, or penetrate the surface to become groundwater collects in puddles or runs off into streams, rivers, lakes and oceans. There the continuous process known as the water cycle begins once again.


1.    What is the hydrologic cycle?

2.    What unique quality does water have that is extremely important to the water cycle process?

3.   As molecules get warmer, do they move faster or slower?

4.    How much of the earth's surface is covered in water?

5.    What is evaporation?

6.    As altitude (height above the earth) increases, does temperature increase or decrease?

7 .      What is evaporation from the stems and leaves of plants called?

8 .      How is water vapor moved around the earth?

9 .      Why does it rain?

10.   Name four types of precipitation.

11.   What happens to water that is stored under ground?

12.   Draw a picture of the water cycle, labeling the following processes: Evaporation, Condensation, Precipitation

Internet Resources on the Hydrologic Cycle
bulletWater Science Glossary of Terms
bulletWater - EPA
bulletEarth's Water - EPA
bulletWater Resources of the United States
bulletWater Science for Schools
bulletEducation Resources - Water Resources
bulletThe Hydrologic Cycle - The University of Illinois at Urbana-Champaign
bulletThe Global Water Cycle
bulletWhat is ground water?
bulletThe Water Cycle

Permission is granted for the reproduction of materials contained in this bulletin.

Southern AER
Southeast Regional Climate Center
S.C. Department of Natural Resources
1201 Main Street, Suite 1100
Columbia, South Carolina 29201

The South Carolina Department of Natural Resources prohibits discrimination on the basis of race, color, sex, national origin, disability, religion, or age. Direct all inquiries to the Office of Human Resources, P.O. Box 167, Columbia, SC 29202.