Geoscience Reference
In-Depth Information
much lower concentrations. Their role will be outlined
later in this chapter. See also additional case study
'Atmospheric composition' on the support website at
shorter waves are scattered more than the longer waves
and so we see these scattered waves as blue sky. If the
reverse were true the sky would be permanently red, and
if there were no atmosphere, as on the moon, the
sky would be black. Dust and haze in the atmosphere
produce further scattering, but not all of this is lost. Some
of the scattered radiation is returned to space, but much
is directed downwards towards the surface as diffuse
radiation. This is the type of radiation which we also
experience during cloudy conditions with no direct
sunlight when the solar beam is 'diffused' by the water
droplets or ice particles of the clouds.
Another type of short-wave energy loss is absorp-
tion. Some gases in the atmosphere absorb certain
wavelengths (
Figure 2.9
),
as do clouds, dust and haze. On
absorption, the short-wave radiation is converted to long-
wave radiation. In this way we have a warming of the
atmosphere, though the amounts involved are small. The
most important loss of short-wave radiation in its path
through the atmosphere is by reflection. The water
droplets or ice crystals in clouds are very effective in
reflecting insolation. Satellite evidence shows that, for
Short-wave radiation in the atmosphere
As our beam of sunlight enters the atmosphere it first
passes through the thermosphere and the mesosphere
with little change. In the
stratosphere
the density of
atmospheric gases increases. There is more oxygen
available which reacts with the shortest or ultra-violet
wavelengths and effectively removes them, warming the
atmosphere and producing ozone in the process (
Figure
to heat at this stage (
Figure 3.2
).
As we descend into the
troposphere
the atmosphere
becomes rapidly denser and so there is greater interaction
between the sunlight and the atmospheric gases. The size
of the gas molecules of the air is such that they interact
with the insolation, causing some of it to be scattered in
many directions. This process depends on wavelength. The
Short-wave
Long-wave and heat fluxes
6
63
100 K
30 K
Surface
emission
lost to
space
Radiation
from sun
Atmospheric
emission
lost to
space
3
Stratospheric
absorption
19
Reflected
from clouds
107
Atmospheric
absorption
of
160
Emission by
atmosphere and
clouds
3
Absorbed
15
Absorbed by
atmosphere
6
Back
scattered
surface
emission
Latent
heat
Sensible
heat
L
Atmospheric
emission
to
surface
Direct
Diffuse
4
Reflected
from surface
Q
E
Q
H
Surface
emission
L
27
21
113
97
22
10
48
Absorbed by surface
Figure 3.2
Modification of short- and long-wave radiation by the atmosphere and the surface. Figures are expressed as a
percentage of incoming short-wave radiation at the top of the atmosphere based on a global mean.
