Page 105 - Intro to Space Sciences Spacecraft Applications

Intmduction to Space Sciences and Spacecraft Applications
9
I radiation curve for 5900 OK black body
.solar radiation outside atmosphere

.2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
WAVELENGTH (m)

Figure 4-7. Electromagnetic propagation. Much of the solar radiation inci-
dent on the earth is absorbed by the atmosphere.

Figure 4-7 shows the effects of this compatibility of certain elements with
certain frequencies (wavelengths) over the range of radiations incident on
the earth from the sun.
The topmost curves in Figure 4-7 represent the spectral energies that
arrive from the sun at the top of the earth’s atmosphere. The dashed line
indicates the theoretical energies given by the relationships discussed ear-
lier in this chapter for an idealized blackbody. As you can see, the corre-
lation of this theoretical curve with the measured irradiation outside the
atmosphere is quite good. The lower solid curve indicates the amount of
energy, by wavelength, that reaches the earth’s surface. Although there is
attenuation in most of the incident radiations, you can see that there are
some wavelengths which are drastically (and some completely) attenuat-
ed in their passage through the atmosphere. The associated elements
which absorb these energies are also indicated on the figure.

The Greenhouse Effect. Figure 4-7 shows that much of the energy of the
sun’s radiation does reach the earth’s surface. This incident radiation
warms the surface, which then reradiates this energy-due to this temper-
ature and Wien’s law-in the infrared frequencies. However, the C02 and
H20 molecules in the atmosphere are highly absorbent in the infrared and,
as a consequence, the atmosphere is heated, as was indicated in the earli-