Geoscience Reference
In-Depth Information
Atmospheric attenuation of solar radiation
In the previous section we considered the solar radiation that would be incident
on the ground if there were no atmosphere. However, the amount, spectrum and
directionality of solar radiation at the ground are all significantly altered by
interactions through the atmosphere, which occur in two main ways, by scattering
and by absorption. Some of the radiation is scattered by the gas molecules that
make up the air, and this is called
Rayleigh scattering
. Solar radiation is scattered
preferentially at lower wavelengths by gas molecules and it is the resulting scattered
blue radiation that we see and associate with clear sky above us. Additional
scattering of solar radiation occurs due to the atmospheric aerosols such as dust
and smoke in the air.
All scattering processes alter the direction of solar radiation and some radiant
energy is scattered back and does not reach the surface. In addition, some radiant
energy is absorbed from the solar beam as it passes through the atmosphere, giving
rise to atmospheric warming. Absorption occurs preferentially in wavelength
bands that correspond to excited states in important minority gases in the air, nota-
bly ozone in the stratosphere, which mainly absorbs ultraviolet radiation, but also
water vapor, carbon dioxide and other so-called
radiatively active
gases. Figure 5.8
shows how the incoming spectrum of solar radiation is progressively eroded
through the atmosphere by these several scattering and absorbing processes in
clear sky conditions.
If the sky is not clear, substantial energy in the solar beam is lost through the
atmosphere because of the presence of clouds. The ice particles and water droplets
in clouds interfere strongly. They scatter solar radiation mainly backward into
(b)
(a)
(c)
Figure 5.8
The progressive
loss of energy in the solar
beam by scattering and
absorption as it passes
through the atmosphere in
typical clear sky conditions
showing the representative
spectra (a) of extraterrestrial
radiation, (b) after absorption
by ozone, (c) after Rayleigh
scattering, (d) after aerosol
interactions, and (e) after
absorption by H
2
O, CO
2
, etc.
(d)
(e)
0.5
1.0
Wavelength (
μ
m)
1.5
