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Over 98 percent of all the water on Earth exists in the liquid
Polar easterlies
N state, mostly in the ocean, and only a small, variable amount of
60° water vapor is in the atmosphere at any given time. Since so much
Prevailing
Low westerlies water seems to fall as rain or snow at times, it may be a surprise that
30° the overall atmosphere really does not contain very much water
Horse latitudes vapor. If the average amount of water vapor in Earth’s atmosphere
High were condensed to liquid form, the vapor and all the droplets pres-
Northeast
trade winds ent in clouds would form a uniform layer around Earth only 3 cm
Low 0° Doldrums (about 1 in) thick. Nonetheless, it is this small amount of water va-
por that is eventually responsible for (1) contributing to the green-
Southeast
trade winds house effect, which helps make Earth a warmer planet, (2) serving
High as one of the principal agents in the weathering and erosion of
Horse latitudes the land, which creates soils and sculptures the landscape, and
30° (3) maintaining life, for almost all plants and animals cannot sur-
Low Prevailing vive without water. It is the ongoing cycling of water vapor into
60° westerlies and out of the atmo sphere that makes all this possible. Under-
Polar easterlies standing this cycling process and the energy exchanges involved
is also closely related to understanding Earth’s weather patterns.
FIGURE 22.13 Simplified pattern of horizontal and vertical
circulation in the actual atmosphere. Regions of high and low EVAPORATION AND CONDENSATION
pressure are indicated.
Water tends to undergo a liquid-to-gas or a gas-to-liquid phase
change at any temperature. The phase change can occur in
generally from the west in both hemispheres at speeds of 160 km/h either direction at any temperature. The temperature of liquid
(about 100 mi/h) or more. A jet stream may occur as a single belt, water and the temperature of water vapor are associated with
or loop, of wind, but sometimes it divides into two or more parts. the average kinetic energy of the water molecules. The word
The jet stream develops north and south loops of waves much like average implies that some of the molecules have a greater ki-
the waves you might make on a very long rope. These waves vary netic energy and others have less. If a molecule of water that
in size, sometimes beginning as a small ripple but then growing has an exceptionally high kinetic energy is near the surface and
slowly as the wave moves eastward. Waves that form on the jet is headed in the right direction, it may overcome the attractive
stream bulge toward the poles (called a crest) or toward the equa- forces of the other water molecules and escape the liquid to
tor (called a trough). Warm air masses move toward the poles become a gas. This is the process of evaporation. A supply of
ahead of a trough, and cool air masses move toward the equator energy must be present to maintain the process of evaporation,
behind a trough as it moves eastward. The development of a wave and the water robs this energy from the surroundings. This ex-
in the jet stream is understood to be one of the factors that influ- plains why water at a higher temperature evaporates more rap-
ences the movement of warm and cool air masses, a movement idly than water at a lower temperature. More energy is available
that results in weather changes on the surface. at higher temperatures to maintain the process at a faster rate.
The intertropical convergence zone, the 30° belt of high Water molecules that evaporate move about in all directions,
pressure, and the northward and southward migration of a and some will strike the liquid surface. The same forces that it
meandering jet stream all shift toward or away from the equa- escaped from earlier now capture the molecule, returning it
tor during the different seasons of the year. The troughs of the to the liquid state. This is called the process of condensation.
jet stream influence the movement of alternating cool and warm Condensation is the opposite of evaporation. In evaporation,
air masses over the belt of the prevailing westerlies, resulting in more molecules are leaving the liquid state than are returning.
frequent shifts of fair weather to stormy weather, then back again. In condensation, more molecules are returning to the liquid
The average shift during the year is about 6° of latitude, which is state than are leaving. This is a dynamic, ongoing process with
sufficient to control the overall climate in some locations. The in- molecules leaving and returning continuously (Figure 22.14).
fluence of this shift of the global circulation of Earth’s atmosphere If the air were perfectly dry and still, more molecules would
will be considered as a climatic influence after we consider the leave (evaporate) the liquid state than would return (condense).
roles of water and air masses in frequent weather changes. Eventually, however, an equilibrium would be reached with as
many molecules returning to the liquid state per unit of time as
are leaving. An equilibrium condition between evaporation and
22.3 WATER AND THE ATMOSPHERE
condensation occurs at saturation. Saturation occurs when the
Water exists on Earth in all three states: (1) as a liquid when the processes of evaporation and condensation are in balance.
temperature is generally above the freezing point of 0°C (32°F); Air will remain saturated as long as (1) the temperature
(2) as a solid in the form of ice, snow, or hail when the temperature is remains constant and (2) the processes of evaporation and
generally below the freezing point; and (3) as the invisible, molec- condensation remain balanced. Temperature influences the
ular form of water in the gaseous state, which is called water vapor. equilibrium condition of saturated air because increases or

22-13 CHAPTER 22 The Atmosphere of Earth 553