Water
Occurrence
Wa
ter
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 vapour, it
occurs as fog, steam, and clouds. Atmospheric vapour is measured
in terms of relative humidity, which is the ratio of the quantity
of vapour actually present to the greatest amount possible at a
given temperature. 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 . 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.