Geoscience Reference
In-Depth Information
Water Vapor
An important variable gas in the atmosphere
is water vapor. Water is found in three physical states: liquid,
solid (ice), and gas (water vapor). The amount of water vapor in
the atmosphere near Earth's surface is about 2% in most parts
of the planet, but can range from just less than 1% over deserts
and polar regions to about 4% in tropical zones. Atmospheric
water vapor is especially vital because it absorbs and stores heat
energy from the Sun and is thus an important component, along
with carbon dioxide, of the greenhouse effect. As airflow within
the atmosphere moves vapor around, the effect is to moderate
temperature and transport energy around Earth.
The amount of water vapor at any given place in the atmo-
sphere depends on many variables, such as the proximity to a
large body of water or the air temperature. Remember that a
direct relationship exists between the temperature of the air and
the amount of water vapor it can hold, with warmer air capable
of holding more vapor than cooler air. You have probably expe-
rienced this relationship yourself. Try to remember what it feels
like on a hot, muggy day in the summer when the air is sticky
and uncomfortable. It feels that way in large part because the air
holds a lot of water vapor, which is possible because the air is so
warm. This vapor/temperature relationship is an important prin-
ciple in physical geography because it directly influences the
process of precipitation. As we will discuss more thoroughly in
Chapter 7, precipitation happens when bodies of air cool and
water changes from vapor to liquid.
Figure 4.6 shows atmospheric water vapor over the North
American continent in February 2009. Notice that on this par-
ticular day, the highest concentration of water vapor was in
the central and eastern United States. If you happened to be in
those areas, you would have likely noticed numerous clouds
in the sky. The water vapor in those clouds flowed into the re-
gion in association with a body of warm air originating over the
tropical Pacific Ocean. Areas of relatively dry air appeared over
northwestern Mexico and the upper Midwest.
Figure 4.6 Water vapor over the north American conti-
nent.
This image was taken by the Geostationary Opera-
tional Environmental Satellite (GOES) on February 11, 2009.
Dark zones represent regions in the atmosphere about 6 km
to 10 km (~3.7 mi to 6.2 mi) above the Earth's surface con-
taining very little water vapor, whereas bright areas represent
relatively high concentrations of water vapor. In areas other
than the tropics and poles, the average water vapor content is
about 2% of the air.
concerns many scientists today. Although the greenhouse effect is
indeed linked to this issue, it can also be viewed in a positive way
because it has helped maintain a constant atmospheric tempera-
ture that is hospitable to life. The concem now is that CO
2
levels
are rapidly rising beyond recent historical norms due to human
industrial activity, specifically the consumption of fossil fuels. As
a result, Earth appears to be in the midst of a rapid warming trend
that may have major environmental consequences in the future.
You can examine the processes associated with the green-
house effect more closely in Figure 4.7. Although we will dis-
cuss the flow of solar radiation more thoroughly later in the
chapter, this diagram shows the significance of CO
2
in the atmo-
sphere and is relevant here. As illustrated in Figure 4.7, Earth re-
ceives shortwave radiation from the Sun, which passes through
the atmosphere and is absorbed by the surface. Subsequently, the
Earth releases energy as longwave radiation. Some of this radia-
tion flows directly back into space, but most is absorbed by the
atmosphere, where it is held in large part by CO
2
(water vapor
is another greenhouse gas). The atmosphere then redirects some
longwave energy back to the surface as
counterradiation
. Some
atmospheric longwave energy escapes to space.
The greenhouse effect is the process through which coun-
terradiation is returned to the surface. The really fascinating
aspect of the greenhouse effect, as far as life on Earth is con-
Carbon Dioxide
Another important variable gas is carbon
dioxide (CO
2
), which currently makes up about 0.040% of the at-
mosphere (or 400 parts CO
2
for every 1 million parts of the atmo-
sphere). Despite this very small percentage, CO
2
is a critical part
of the atmosphere for two very important reasons. First, plants
absorb CO
2
and release oxygen as a by-product. Second, atmo-
spheric CO
2
contributes significantly to the
greenhouse effect
,
which is the process through which the atmosphere traps long-
wave radiation. This process warms the atmosphere, which, in
turn, warms Earth. Another significant greenhouse gas is methane,
which is derived from natural gas and decaying organic matter.
Methane currently makes up about 0.00017% of the atmosphere.
You may associate the greenhouse effect most closely with the
concept of global warming, an environmental issue that greatly
greenhouse effect
The process through which the lower part
of the atmosphere is warmed because longwave radiation from
Earth is trapped by carbon dioxide (CO
2
) and other greenhouse
gases.
Counterradiation
Longwave radiation that is emitted toward
the Earth's surface from the atmosphere.
