Spectral Distribution of Solar and Thermal Radiation
161
0.0
0.5
1 . 0
1 . 5
2.0
2.5
3.0
Wavelength (urn)
FIGURE 10.5. Spectral irradiance of the sun just outside the atmosphere and at sea
level through a 1.5 airmass atmospheric path. Atmospheric absorption at short
wavelengths is mainly from ozone. At long wavelengths it is mainly from water
vapor (redrawn from Gates, 1980).
rofluorocarbons. These compounds destroy ozone and could increase the
flux of harmful ultraviolet radiation at the surface of the earth.
Energy over the entire solar spectrum is reduced by Rayleigh (smallparticle) and Mie (large-particle) scattering. Rayleigh scattering is from
the molecules of air and is most pronounced at short wavelengths so the
scattered radiation is blue. This is the source of the blue color of the sky.
Blue wavelengths are preferentially scattered out of the solar beam, causing the sun to appear red. Mie scattering is from dust, smoke, and other
aerosols in the atmosphere. Conditions can exist which result in preferential scattering of long wavelengths by Mie scatterers, but generally there
is little wavelength dependence.
About half of the energy in the solar spectrum is at wavelengths shorter
than 0.7 pm and half at longer wavelengths (actually about 45 percent is
in the visible and 55 percent in the near-infrared). The spectrum changes
with solar zenith angle, cloudiness, and atmospheric composition, but
the distribution between visible and infrared remains almost unchanged.
Many of our computations require that the energy content of these
two wavebands are known. Nature made it easy for us by consistently
partitioning approximately half to each.
The mean emission of the earth approximates that of a blackbody with
a temperature of 288 K. The spectral emittance for such a blackbody is
shown in Fig. 10.6. Almost all of the radiation is at wavelengths longer
than 4 p m and the wavelength at peak emission is 10 pm. The emittance
161
0.0
0.5
1 . 0
1 . 5
2.0
2.5
3.0
Wavelength (urn)
FIGURE 10.5. Spectral irradiance of the sun just outside the atmosphere and at sea
level through a 1.5 airmass atmospheric path. Atmospheric absorption at short
wavelengths is mainly from ozone. At long wavelengths it is mainly from water
vapor (redrawn from Gates, 1980).
rofluorocarbons. These compounds destroy ozone and could increase the
flux of harmful ultraviolet radiation at the surface of the earth.
Energy over the entire solar spectrum is reduced by Rayleigh (smallparticle) and Mie (large-particle) scattering. Rayleigh scattering is from
the molecules of air and is most pronounced at short wavelengths so the
scattered radiation is blue. This is the source of the blue color of the sky.
Blue wavelengths are preferentially scattered out of the solar beam, causing the sun to appear red. Mie scattering is from dust, smoke, and other
aerosols in the atmosphere. Conditions can exist which result in preferential scattering of long wavelengths by Mie scatterers, but generally there
is little wavelength dependence.
About half of the energy in the solar spectrum is at wavelengths shorter
than 0.7 pm and half at longer wavelengths (actually about 45 percent is
in the visible and 55 percent in the near-infrared). The spectrum changes
with solar zenith angle, cloudiness, and atmospheric composition, but
the distribution between visible and infrared remains almost unchanged.
Many of our computations require that the energy content of these
two wavebands are known. Nature made it easy for us by consistently
partitioning approximately half to each.
The mean emission of the earth approximates that of a blackbody with
a temperature of 288 K. The spectral emittance for such a blackbody is
shown in Fig. 10.6. Almost all of the radiation is at wavelengths longer
than 4 p m and the wavelength at peak emission is 10 pm. The emittance