Movement of Water Through the Atmosphere
Water is everywhere on Earth—in the oceans, glaciers, rivers, lakes, the air, soil,
and in living tissue. All of these “reservoirs” constitute Earth’s hydrosphere.
The increasing demands on this finite resource have led scientists to focus on the
continuous exchange of water among the oceans, the atmosphere, and the
continents. This unending circulation of Earth’s water supply has come to be
called the hydrologic cycle (or water cycle).
The hydrologic cycle is a gigantic system powered by energy from the Sun in
which the atmosphere provides the vital link between the oceans and continents.
Water from the oceans and, to a much lesser extent, from the continents,
evaporates into the atmosphere. Winds transport this moisture-laden air,
often over great distances.
Complex processes of loud formation eventually result in precipitation. The
precipitation that falls into the ocean has ended its cycle and is ready to begin
another by evaporating again.
Once precipitation has fallen on land, a portion of the water soaks into the ground,
some of it moving downward, then laterally, and finally seeping into lakes and
streams or directly into the ocean.
Transpiration is the release of water vapor to the atmosphere by plants.
The Earth’s water balance is a quantitative view of the hydrologic cycle.
Because the total amount of water vapor in the entire global atmosphere remains
about the same, the average annual precipitation over Earth must be equal to the
quantity of water evaporated.
The hydrologic cycle depicts the continuous movement of water from the oceans
to the atmosphere, from the atmosphere to the land, and from the land back to the
sea. The movement of water through the cycle holds the key to the distribution of
moisture over the surface of our planet and is intricately related to all atmospheric
Water’s Changes of State
Water is the only substance that exists in the atmosphere as a solid, liquid, and
gas. In all three states of matter these molecules are in constant motion—the
higher the temperature, the more vigorous the movement. The chief difference
among liquid water, ice, and water vapor is the arrangement of the water
Ice is composed of water molecules that have low kinetic energies and are held
together by mutual molecular attractions. Here the molecules form a tight, orderly
network. As a consequence, the water molecules in ice are not free to move
relative to each other but rather vibrate about fixes sites. When ice is heated, the
molecules oscillate more rapidly. When the rate of molecular movement increases
sufficiently, the bonds between some of the water molecules are broken, resulting
In the liquid state. Water molecules are still tightly packed but are moving fast