Water is in continuous motion. The Sun warms the Earth and oceans, causing water to evaporate and enter the air. Plants also release water into the air through a process called transpiration. As this moisture rises, it cools and condenses to form clouds.
These clouds release water as precipitation in the form of rain, sleet, hail, and snow. This precipitation enters lakes, rivers, and streams. Some water also enters the ground to fill the space between particles of soil, sand, and rocks in the Earth, thus forming ground water. Ground water, which flows very slowly, and surface water, which flows quickly, both move toward the oceans. Some water is used by [living] plants, animals, and people and some is returned to the air through transpiration. The remainder returns to the oceans where it again evaporates to repeat the Hydrologic Cycle.
