Spectral Distribution of Solar and Thermal Radiation

10.7 Spectral Distribution of Solar and Thermal Radiation

The actual extraterrestrial solar spectrum is shown in Fig. 10.5. Here it is plotted using a linear wavelength scale. It has nearly the same shape as a blackbody source at 6000 K, and the wavelength at peak emission is between blue and green, as expected from the Wien's law calcula- tion. As solar radiation passes through the atmosphere of the earth, some wavelengths are almost completely absorbed. The ozone layer in the stratosphere absorbs much of the ultraviolet radiation. Water vapor is the main absorber in the infrared.

The strong absorption of short wavelength ultraviolet radiation by ozone is of particular importance to living organisms. Referring to Fig.

10.1, it can be seen that radiation at these wavelengths can cause skin cancer. It is actually capable of inducing mutation in any genetic material and of germicidal action. This is the reason for the recent concern over destruction of the ozone layer by release into the atmosphere of

Spectral Distribution of Solar and Thermal Radiation 161

Wavelength

F I GURE 10.5. Spectral irradiance of the sun just outside the atmosphere and at sea level through a 1.5

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 particle) 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, caus- ing the sun to appear red.

scattering is from dust, smoke, and other aerosols in the atmosphere. Conditions can exist which result in preferen- tial 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

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

and the wavelength at peak emission is 10

The emittance

Radiation Basics

288 K blackbody

Wavelength (urn)

10.6. Spectral distribution of thermal radiation from the earth and from the clear atmosphere. Emission bands below 8 and above 18

F I G URE

are mainly from water vapor. Bands between 13 and 18

are mainly The narrow band at

9.5 is from ozone (redrawn from Gates, 1962).

spectrum of most terrestrial objects is similar to Fig. 10.6, but the peak location and height shift somewhat depending on the surface temperature.

Thermal radiation is emitted and absorbed in a clear atmosphere mainly by water vapor and

with a narrow ozone absorption band around 9.5

Infrared radiation is absorbed or emitted as a result of changes in the vibrational and rotational energy levels of molecules. Wa- ter vapor,

and are the only common atmospheric constituents with energy levels that are excited by thermal radiation. An atmospheric emittance spectrum is shown in Fig. 10.6 along with the 288 K blackbody spectrum. It can be seen that the atmosphere acts almost like a blackbody in some wavebands where there is strong absorption and emission. In other wavebands the absorptivity and

are low. The "window" between 8 and 13

has particular importance. This coincides with the blackbody emission peak for the earth at 288 K. Much of the radi- ation emitted by the earth in these wavelengths is not absorbed by the atmosphere and is lost to space.