Geoscience Reference
In-Depth Information
1
O
2
and O
3
1
CO
2
1
H
2
O
1
Atmosphere
0
0.2
0.3
0.4 0.5 0.6
0.8
1
1.5
2
3
4
5
6
7 8 9 10
20
30
(μm)
Figure 2.9
Absorptivity at different wavelengths by selected constituents of the atmosphere and by the atmosphere as a
whole.
Source: After Fleagle and Businger (1963)
is scattered, producing more colourful skies at sunrise and
sunset.
Absorption
of the radiant energy has more far-reaching
consequences than reflection or scattering. As an object
absorbs energy its temperature rises, because the radiant
energy is converted to heat (thermal energy). Re-radiation
of this energy tends to occur at a temperature different
from that of the initial, radiating object, and thus the
radiation emitted is at a different wavelength. Earth, for
example, is considerably cooler than the sun; thus the
energy it emits is characteristically of longer wavelengths
than the original solar inputs.
We can summarize the radiation laws as follows:
the surface of the object,
is a constant (the Stefan-
Boltzmann constant) with a value of 5·67
10
-8
W m
-2
K
-4
, and
T
is the absolute temperature.
4
As substances get hotter, the wavelength at which
radiation is emitted will become shorter (
Figure 2.7
).
This is called Wien's displacement law, which can be
represented as
m
=
m
is the wavelength at
which the peak occurs in the spectrum,
/T, where
is a constant
with a value of 2,898 if
m
is expressed in micrometres,
and
T
is the absolute temperature of the body.
5
The amount of radiation passing through a particular
unit area is inversely proportional to the square of the
distance of that area from the source (1/
d
2
), as shown
in
Figure 2.8
.
1
All substances emit radiation when their temperature
is above absolute zero (-273
C or 0K).
2
Some substances absorb and emit radiation at certain
wavelengths only. This is true mainly of gases.
Imagine Earth from 300,000 km into space. An isolated
sphere; predominantly blue, patched with brown and
green and wreathed in white. A world of water, dotted with
land, partly clothed in swirling cloud. This is a view of the
global system. Into this system pours the input of solar
energy; from it come reflected and reradiated energy,
3
If the substance is an ideal emitter (a black body) the
amount of radiation given off is proportional to
the fourth power of its absolute temperature. This
is known as the Stefan-Boltzmann law and can be
represented as
E
=
T
4
,where
E
equals the maximum
rate of radiation emitted by each square centimetre of
