Water continually circulates between the surface of Earth and its atmosphere in what is called the hydrologic or water cycle. Responding to heat energy from the Sun, water in oceans, lakes, swamps, rivers, plants and even in your body can turn into water vapor. Water vapor in the atmosphere condenses as it cools to form clouds. Once the droplets of condensed water vapor are too heavy to remain in the atmosphere, they fall to Earth as precipitation. Rain, snow, sleet, fog and dew are all forms of precipitation.

After the precipitation reaches the surface of Earth, it does one of four things. It can either be absorbed by plants, percolate through the soil to become ground water, run off the surface into streams and rivers--becoming surface water and eventually flowing into the oceans, or evaporate.

Water is a major force in the sculpting of Earth's surface and is an important means of transporting the energy that drives atmospheric circulation. Although water covers three-fourths of the surface of Earth, water represents a relatively small percentage of Earth's total volume--a 3-foot diameter model of Earth would have only one cup of water. Thus, water is actually relatively very scarce.