Geoscience Reference
In-Depth Information
uv
Visible
Infrared
Stefan-Boltzmann law:
Energy of radiation from a body is proportional
to the 4th power of absolute temperature.
Wein´s displacement law:
Wavelength of maximum energy from a body is
inversely proportional to absolute temperature.
5.0
Suns blackbody radiation at 6,000 K
2.0
1.0
Extraterrestrial solar radiation
0.5
Diffuse solar radiation
at Earth surface
0.2
Direct beam normal incidence solar radiation at Earth's surface
0.1
Earth's blackbody radiation at 300 K
0.05
0.02
Infrared radiation lost to space
0.01
The serrated nature of the grayscale radiation curves
is due to selective absorption of certain wavelengths
by particular atmospheric and stratospheric gases.
0.005
0.002
0.001
0.1
0.2
0.5
1.0
2.0
5.0
10
20
50
Radiation wavelength: microns, µ m
Chief absorption bands by greenhouse gases
O 2
O 3
H 2 O
CO 2
H 2 O
O 3
CO 2
H 2 O
uv radiation filter
Fig. 4.150 The great energy transfer from solar short wave to reradiated long wave radiation.
flux is of the same order as that received from the Sun at
the Earth's surface. Some of this energy is lost into space
for ever but a significant proportion is absorbed and
trapped by the gases of the atmosphere and emitted back
to Earth as counter radiation where together with
absorbed shortwave radiation it does work on the atmos-
phere by heating and cooling it. During this process
water vapor may condense to water, or vice versa, and the
effects of differential heating give rise to density differ-
ences, which drive the general atmospheric circulation.
The insulating nature of Earth's atmosphere, like that of
the glass in a greenhouse, is nowadays referred to as the
“greenhouse” effect. The general concept was originally
demonstrated by the geologist de Saussure who exposed
a black insulated box with a glass lid to sunlight, then
comparing the elevated internal temperature of the
closed box with that of the box when open. Thus it is the
absorption spectra of our atmospheric gases that ulti-
mately drives the atmospheric circulation (Fig. 4.150).
Water vapor is the most important of these gases,
strongly absorbing at 5.5-8 and greater than 20
m
wavelengths. Carbon dioxide is another strong absorber,
but this time in the narrow 14-16
m range. The 10 per-
cent or so of infrared radiation from the ground surface
that escapes directly to space is mainly in the 3-5 and
8-13
m wavelength ranges.
4.20
Heat transport by convection
Convection is the chief heat transfer process above, on and
within Earth. We see its effects most obviously in the
atmosphere, for example, in the majestic cumulonimbus
clouds of a developing thundercloud or more indirectly in
the phenomena of land and sea breezes. It is fairly obvious
in these cases that convection is occurring, but what about
within Earth? It is now widely thought that Earth's silicate
mantle also convects, witnesses the slow upwelling of man-
tle plumes and motion of lithospheric plates. But exactly
how do these motions relate to convection? We shall
return to the question below and in later chapters
(Sections 5.1 and 5.2).
4.20.1
Convection as energy transfer by bulk motion
We have seen previously that the heat transfer processes of
radiation and conduction cause the temperature and internal
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