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(a)
1000
22 May, cloudy
23 May, sunny
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0
0
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Time (hour UTC)
(b)
1000
22 May, cloudy
23 May, sunny
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Time (hour UTC)
Figure 2.6
Observations of shortwave radiation at Haarweg Meteorological Station
for a clear day (May 23, 2007) and a cloudy day (May 22, 2007). (
a
) Global radia-
tion. (
b
) Diffuse radiation.
Not only the
amount
of radiation reaching Earth's surface is inluenced by atmo-
spheric composition, but the directional composition as well: how much radiation
comes from which part of the hemisphere.
Figure 2.6b
shows the diffuse radiation
(radiation not coming from the direction of the solar disc) on the same days as shown
in
Figure 2.6a
. On the cloud-free day roughly 80% of the radiation comes from the
direction of the Sun (i.e., 20% is diffuse radiation), whereas on the cloudy day this is
close to zero (all radiation is diffuse).
Figure 2.7
shows the spectral composition of the shortwave radiation at Earth's
surface for a clear day. In the total radiation at the surface, the absorptions by ozone
(short wavelengths) and water vapour (longer wavelengths) are clearly visible. Fur-
thermore, the spectral peak of the diffuse radiation has shifted to lower wavelengths
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