III.

Occurrence

Water is the only substance that occurs at ordinary temperatures in all three states of matter, that is, as a solid, a liquid, and a gas. As a solid, or ice, it is found as glaciers and ice caps, on water surfaces in winter, as snow, hail, and frost, and as clouds formed of ice crystals. It occurs in the liquid state as rain clouds formed of water droplets, and on vegetation as dew; in addition, it covers three-quarters of the surface of the earth in the form of swamps, lakes, rivers, and oceans. As gas, or water vapor, it occurs as fog, steam, and clouds. Atmospheric vapor is measured in terms of relative humidity, which is the ratio of the quantity of vapor actually present to the greatest amount possible at a given temperature. See Atmosphere; Cloud; Fog; Humidity; Rain.

Water occurs as moisture in the upper portion of the soil profile, in which it is held by capillary action to the particles of soil. In this state, it is called bound water and has different characteristics from free water See Soil; Soil Management. Under the influence of gravity, water accumulates in rock interstices beneath the surface of the earth as a vast groundwater reservoir supplying wells and springs and sustaining the flow of some streams during periods of drought.