Geoscience Reference
In-Depth Information
blue wavelength is preferentially scattered, whereas the longer
wavelengths pass on through the atmosphere. Approximately
7% of this scattered radiation is redirected back into space.
You may also wonder why sunsets are so colorful, with
the orange and red parts of the spectrum visible (Figure 4.18c).
This change happens because solar radiation is streaming to-
ward your eye from a much lower angle at dawn or dusk than
when the Sun is high in the sky (Figure 4.18b). As a result,
the incoming radiation is passing through a thicker slice (when
viewed horizontally) of the atmosphere to reach your line of
sight. This thickened atmosphere causes the blue wavelengths
to be scattered out entirely before they reach you, leaving the
longer wavelengths (oranges and red) for you to observe.
As you can see, atmospheric reflection and scattering have
a strong influence on the way you perceive Earth. The bright-
ness of cloud tops and the colors of the sky are visible environ-
mental indicators of these processes. As a by-product of these
interactions, some reflected and scattered radiation is also redi-
rected downward toward the surface of Earth as
indirect radia-
tion
. Approximately 20% of solar radiation that reaches Earth
does so in this indirect fashion.
Stored energy can be lost from Earth in several ways. Sen-
sible heat can be transferred from the Earth's surface to the
atmosphere through the process of convection. In this case,
warm air rises upward from Earth and cooler air descends to
replace it. Heat can also be removed through the process of
evaporation
, which is the change of liquid water to water va-
por by absorption of heat. In addition, absorbed radiation can
simply be re-radiated back into space as longwave radiation.
When longwave radiation is released, it can either escape di-
rectly back into space, be absorbed by atmospheric CO
2
and
water vapor, or be backscattered by dust particles.
Reflected Radiation
Reflected radiation is energy that
bounces off the Earth's surface in such a way that it does not
provide heat. Overall, approximately 3% of incoming radiation
is reflected in this way. The proportion of incoming radiation
reflected by the surface is, just as for the atmosphere, dependent
on the albedo of an object. Although you may not be directly
aware of surface albedo, you are probably indirectly aware of
its effects. Almost everyone knows how uncomfortable it feels
to wear a dark-colored shirt on a very hot day with bright sun-
shine. The reason for this is that dark objects absorb radiation
and therefore increase in temperature. A dark asphalt highway,
for example, reflects only about 5% to 10% of incoming so-
lar energy. A snow-capped mountain peak, in contrast, reflects
80% to 95% of incoming solar radiation and thus absorbs little
heat. Figures 4.19 and 4.20 show how albedo can influence the
amount of energy reflection on different surfaces.
The amount of radiation reflected depends not only on al-
bedo, but also on the Sun angle in the sky. Remember from
Chapter 3 that the Sun is always at some angle in the sky rela-
tive to any location on the planet. (This concept was illustrated
in Figure 3.3, if you wish to review.) With respect to incom-
ing solar radiation, this angle is called the
angle of incidence
.
Think of the angle of incidence as being similar to the angle at
which you might throw a rock into the water. If you drop the
rock straight down into the water, it is immediately absorbed
by the water and sinks to the bottom. However, if you throw
the rock at a shallower angle (relative to the water), it might
skip across the water. The angle of incidence works in much the
same fashion in that solar radiation from a high-angle Sun inter-
acts directly with Earth; in addition, the Sun's rays are confined
to a relatively small area and are thus intense at that point. When
the Sun is lower in the sky, however, more radiation is reflected
when it strikes Earth; also, the radiation is spread over a larger
area. When this takes place, the radiation is not as intense com-
pared to higher-angle Sun locations.
Interaction of solar Radiation
and the Earth's surface
Let's now examine the approximately 45% of all solar energy
(direct and indirect) that strikes the Earth's surface. What hap-
pens to this portion of incoming radiation is important because
it influences variables such as temperature, atmospheric circu-
lation, the density and kind of vegetation in a region, soils, and
even where glaciers occur. Relationships such as these will be
discussed in later chapters. In general, either of two things hap-
pens when solar radiation strikes the ground: (1) it is absorbed,
or (2) it is reflected.
Absorbed Radiation
Of the amount of solar energy that
reaches the ground, 96% is absorbed by the various land and
water bodies on the surface, thus heating Earth. One way this
heat energy can be stored is in the form of
sensible heat,
which
is heat that can be sensed by touching or feeling and can be
measured by a thermometer. A second way it can be stored is
as latent heat, when water from land and ocean/lake surfaces
is transformed into water vapor within the atmosphere through
evaporation.
latent heat
is a form of heat that is hidden and
cannot be measured with a thermometer because it goes into
breaking bonds between molecules when a substance changes
physical state, such as from a liquid to a gas.
Indirect radiation
Radiation that reaches Earth after it has
been scattered or reflected.
Evaporation
The process by which atoms and molecules of
liquid water gain sufficient energy to enter the gaseous phase.
Heat that can be felt and measured with a
Angle of incidence
The angle at which the Sun strikes Earth
at any given place and time.
sensible heat
thermometer.
Heat stored in molecular bonds that cannot be
latent heat
measured.
