Introduction - The Global Climate System: Patterns, Processes, and Teleconnections - page 6

Advection to continents

Figure 1.4 Estimated
amounts of water involved in
the global hydrologic cycle.

77 84 7 23 16

(84 - 77) = 7

Most water
exchange
occurs over
oceans

Precipitation
to
continents

Evaporation
from
continents

Evaporation
from oceans

Precipitation to
oceans

Runoff to oceans

77 84 7 23 16

(23 - 16) = 7

Oceans Continents

100 units = mean annual global precipitation = 85.7 cm (33.8 in)

have high Bowen Ratios, can attain much higher temperatures than those in a
maritime environment.

Water and its changes of state
The significance of water as an atmospheric variable is a result of its unique
physical properties. Water is the only substance that exists as a gas, liquid, and
solid at temperatures found at the Earth's surface. This special property enables
water to cycle over the Earth's surface. Figure 1.4 illustrates the relative parti-
tioning of water in the hydrologic cycle. As can be seen, a large proportion of the
exchanges occur over the world oceans. While changing from one form to
another, water in its various forms acts as an important vehicle for the transfer
of energy in the atmosphere.
The chemical symbol of water, H 2 O, is probably the best known of all
chemical symbols. Water in all of its states has the same atomic content, the
only difference is the arrangement of the molecules. At low temperatures the
bonds binding the water molecules are firm and pack tightly in a fixed geometric
pattern in the solid phase. As temperature increases, the available energy causes
bonds to form, break, and form again. This permits flow to occur and represents
the liquid phase of water. At higher temperatures and with more energy, the
bonding between the water molecules breaks down and the molecules move in a
disorganized manner. This is the gas phase. If the temperature decreases, the
molecules will revert to a less energetic phase and reverse the processes. Gas will
change to liquid and liquid to solid.
The processes of melting, evaporation, and sublimation from solid to liquid to
gas phase result in absorbed energy. This added energy causes the molecules to
change their bonding pattern. The amount of energy incorporated is large for
the changes to the water vapor stage, and much lower for the change from ice
to water.
The energy absorbed is latent energy and goes back to the environment when
the phase changes reverse. When water vapor changes to liquid, it releases the