The concentration of the invisible gas water vapor (H 2O), however, varies greatly from place
to place, and from time to time. Close to the surface in warm, steamy, tropical locations, water
vapor may account for up to 4 percent of the atmospheric gases, whereas in colder arctic areas,
its concentration may dwindle to a mere fraction of a percent (see Table 1.1). Water vapor
molecules are, of course, invisible. They become visible only when they transform into larger
liquid or solid particles, such as cloud droplets and ice crystals, which may grow in size and
eventually fall to Earth as rain or snow. The changing of water vapor into liquid water is called
condensation, whereas the process of liquid water becoming water vapor is called evaporation.
The falling rain and snow is called precipitation. In the lower atmosphere, water is present
everywhere. It is the only substance that exists as a gas, a liquid, and a solid at those
temperatures and pressures normally found near Earth’s surface (see Fig. 1.4).
Water vapor is an extremely important gas in our atmosphere. Not only does it form into
both liquid and solid cloud particles that grow in size and fall to Earth as precipitation, but it also
releases large amounts of heat—called latent heat—when it changes from vapor into liquid
water or ice. Latent heat is an important source of atmospheric energy, especially for storms,
such as thunderstorms and hurricanes. Moreover, water vapor is a potent green house gas
because it strongly absorbs a portion of Earth’s outgoing radiant energy (somewhat like the
glass of a green house prevents the heat inside from escaping and mixing with the outside air).
This trapping of heat energy close to Earth’s surface—called the green house effect—keeps the
average air temperature near the surface much warmer than it would be otherwise. Thus, water
vapor plays a significant role in Earth’s heat-energy balance.
Carbon dioxide (CO2), a natural component of the atmosphere, occupies a small (but
important) percent of a volume of air, just over 0.04 percent. Carbon dioxide enters the
atmosphere mainly from the decay of vegetation, but it also comes from volcanic eruptions, the
exhalations of animal life, from the burning of fossil fuels (such as coal, oil, and natural gas), and
from deforestation. The removal of CO2 from the atmosphere takes place during
photosynthesis, as plants consume CO2 to produce green matter. The CO2 is then stored in roots,
branches, and leaves. Rain and snow can react with silicate minerals in rocks and remove CO 2
from the atmosphere through a process known as chemical weathering. The oceans act as a
huge reservoir for CO2, as phytoplankton (tiny drifting plants) in surface water fix CO 2 into
organic tissues. Carbon dioxide that dissolves directly into surface water mixes downward and
circulates through greater depths. Estimates are that the oceans hold more than 50 times the
total atmospheric CO2 content. Figure 1.5 illustrates important ways carbon dioxide enters and
leaves the atmosphere.
