Geoscience Reference
In-Depth Information
(0.7-3 μm). The visible wavelength range is also the part of the spectrum that is used by
plants for photosynthesis (photosynthetically active radiation [PAR]).
Longwave radiation originates from either Earth's surface or the atmosphere (with or
•
without clouds). It is characterized by wavelengths in the range 3-100 μm, which cor-
responds to a black-body temperature of the order of 290 K. This range is also indicated
as thermal infrared radiation.
Net radiation is the sum of net shortwave radiation (
K*
) and net longwave radia-
tion (
L*
):
QK L
KKLL
***
=+
=−+−
↑
(2.2)
↓
↑
↓
where
K
↓
and
K
↑
are the downwelling and upwelling shortwave radiation at the sur-
face, whereas
L
↓
and
L
↑
are the respective longwave radiation luxes. Another term
for
K
↓
is global radiation.
Global radiation originates from the Sun and is modiied by the atmosphere (see
Section 2.2.2
). The upwelling shortwave lux is the relected portion of
K
↓
and hence
depends on the magnitude and characteristics of the global radiation and on the prop-
erties of Earth's surface (see
Section 2.2.3
). Downwelling longwave radiation origi-
nates from the atmosphere, both from the clear sky and from clouds (see
Section
2.2.4
). Upwelling longwave radiation is mainly emitted by Earth's surface (see
Sec-
tion 2.2.5
). Finally,
Section 2.2.6
summarizes the sum of all terms: the net radiation.
Question 2.1:
The distance from the centre of the Sun to Earth is 149.6 ·10
6
km (equal
to 1 astronomical unit [AU]), and the diameter of the Sun is 1.34·10
6
km (Zelik et al.,
1992
).
a) Verify that the value of the solar constant (see
Appendix A
) indeed corresponds to a
surface temperature of the Sun of roughly 5800 K.
b) Verify that the black-body radiation lux densities at 293 K and 5800 K differ by
nearly 7 orders of magnitude.
2.2.1 Interaction between Radiation and the Atmosphere
On its way through the atmosphere, radiation is affected in two ways: by scattering
and by absorption.
Scattering
If electromagnetic radiation hits a particle, part of the energy will be scattered in all
directions (but not necessarily equally in all directions). This scattered radiation is
called diffuse radiation. The case in which scattered light hits another particle and
is scattered again is called multiple scattering. The type of scattering depends on the
relationship between particle diameter,
d
p
, and the wavelength of the radiation,
λ
(see
also
Figure 2.1
):
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