Geoscience Reference
In-Depth Information
VISUAL CONCEPT CHECK
4.2
The atmosphere contains a wide variety of
natural constant gases, variable gases, and
particulates. People also contribute to the
overall character of the atmosphere. What
potential impacts do you think this factory
complex has on the atmosphere?
invisible to the naked eye. Because this radiation exists in wave
form, it carries energy and thus can move from one place to
another without requiring an intervening medium. When this
radiation reaches you, part of the energy of the wave is convert-
ed back into heat, which is why you feel warm sitting beside a
campfire. Some of the radiation may exist in the form of visible
light that we can see, but a great deal of the radiation emitted is
infrared waves, whose longer wavelength is usually detectable
only with special infrared detectors.
Another form of heat transfer is
conduction
, which in-
volves the diffusion of energy through molecules that are
in contact with one another. This diffusion occurs because,
as the temperature of molecules increases, they begin to
vibrate more rapidly, causing collisions that produce simi-
lar motions in adjacent molecules. In this fashion, sensible
heat always moves from areas of relatively high tempera-
ture to zones of relatively low temperature. You can experi-
ence conduction yourself if you pick up a heated pot from
a stove or other fire.
The third form of heat transfer is
convection
, which in-
volves the upward movement of heat. The next time you boil
water on the stove, notice how a small circular current of water
arises within the pot where hot water moves up and is replaced
at the bottom by relatively cooler water. As you will see in later
chapters, convection is a very important mechanism of heat
transfer on Earth and is associated with atmospheric circulation
and precipitation.
Flow of solar Radiation
in the Atmosphere
With the mechanisms of heat transfer in mind, it is time to
explore the various ways that solar radiation interacts with
Earth. As you work your way through this discussion, refer to
Figure 4.16, which illustrates the flow of solar radiation on the
planet. Of all the incoming radiation that reaches Earth, about
25% flows uninterrupted to the surface as
direct radiation
.
However, this amount can vary greatly depending on local geo-
graphic variables such as cloud cover or density of atmospheric
dust. Figure 4.17 shows, for example, the impact that cloud
cover can have on direct solar radiation.
The remaining 75% of incoming insolation is either
absorbed or otherwise redirected in the atmosphere (see
Figure 4.16).
Absorption
takes place when variable gases
and particulates in the atmosphere interrupt the flow of so-
lar radiation by absorbing specific wavelengths. For exam-
ple, almost all UV wavelengths (those less than 0.3 µm or
micrometer in length) are absorbed by oxygen and ozone.
Similarly, radiation at the 1.3 µm and 1.9 µm wavelengths
is absorbed very strongly by water vapor and CO
2
. Overall,
approximately 24% of incoming solar radiation is absorbed,
with 18% absorbed by atmospheric water vapor and dust,
3% by ozone, and 3% by clouds. Remember, this absorption
of solar radiation is important because it helps to moderate
temperature in the atmosphere.
Conduction
The transfer of heat energy from one substance
to another by direct physical contact.
Direct radiation
Solar radiation that flows directly to the
surface of Earth and is absorbed.
Convection
A circular cell of moving matter that contains
warm material moving up and cooler matter moving down.
Absorption
The assimilation and conversion of solar radiation
into another form of energy by a medium such as water vapor. In
this process, the temperature of the absorbing medium is raised.
