Geoscience Reference
In-Depth Information
(b)
(a)
34
110
32
MW = megawatt
= 10
6
watts
100
30
28
90
26
80
24
300K
22
70
20
6000K
60
18
16
50
14
250K
40
12
10
30
8
20
6
4
10
200K
2
0
0
0 1020304050607080
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Q
(
R
m)
Q
(
R
m)
Figure 2.7
Variation in the intensity of black-body radiation with wavelength: (a) T = 6,000K (approximately the emission
temperature of the sun); (b) T = 200K, 250K and 300K (range of Earth emission temperatures). Note the differences in scale. K
is degrees Kelvin, based on absolute zero (-273°C).
Source: After Neiburger et al. (1982)
Source
in their absorption and therefore emission wavelengths
(
Figure 2.9
).
This property is very important to Earth, as
it means that the atmosphere absorbs and emits only in
certain wavelengths. At other wavelengths, radiation is able
to pass right through the atmosphere with little modifica-
tion. The atmosphere is composed of gas molecules,
particles of matter such as dust, water droplets and ice
crystals. Light waves striking these obstacles are scattered
in all directions, so that radiant energy is scattered back
to space as well as down to the surface. There is no change
of wavelength in this process, known as
scattering
, simply
a change of direction for some of the radiant energy.
The nature of scattering depends upon the size of
particles relative to the wavelength of the incident radi-
ation. Gas molecules are most effective at scattering light
in the blue wavelength. Since gas molecules compose
much of the atmosphere, we see the sky as blue whether
we view it from the ground or from space. When the sun
is setting or rising the radiant energy passes at a lower
angle through the larger particles of dust in the lower
atmosphere. The result is that more of the red wavelength
d
A
2d
d
B
distance d from source is just one-fourth that at distance 2d;
energy passing through area A is spread over an area four
times as large at B.
