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Figure 2.12 Origin of longwave radiation at Earth's surface for a cloudless (left) and
cloudy (right) atmosphere. The absorption spectra from Figure 2.2c are reproduced
for the wavelength range 3-40 μm.
Question 2.11: Use Eq. ( 2.23 ) to explain why the albedo is nearly constant in time on
an overcast day (see Figure 2.10 )?
2.2.4 Downwelling Longwave Radiation
Because outer space, beyond the atmosphere, is too cold and too much of a vacuum
to emit any radiation in the wavelength range that we consider as longwave radiation,
the only source of downwelling longwave radiation at Earth's surface is the atmo-
sphere.
The atmosphere contains gases that have strong absorption bands in the wave-
length region of longwave radiation, notably CO 2 , O 3 , N 2 O, CO, O 2 , CH 4 and water
vapour (see Figure 2.2 ). This implies that these gases also emit longwave radiation at
the wavelength of these absorption bands, in all directions. The amount of longwave
radiation received by Earth's surface is hence dependent on the vertical distribution of
temperature (which determines the amount of black-body radiation), and the concen-
trations of the aforementioned absorbing gases (also indicated as greenhouse gases or
[GHGs]). Because the temperature of the atmosphere has only a small diurnal cycle
(relative to the absolute temperature), the incoming longwave radiation is present day
and night and only shows a limited diurnal variation.
The combination of the emission bands of the various gases leads to a broad range
of wavelengths with a high absorptivity ( Figure 2.2c ), with the exception of the
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