Solar radiation surface spectrum

It is quite apparent from Fig. 8-63 that solar radiation which arrives at the surface of the earth does not behave like the radiation from an ideal gray body, while outside the atmosphere the distribution of energy follows more of an ideal pattern. To determine an equivalent blackbody temperature for the solar radiation, we might employ the wavelength at which the maximum in the spectrum occurs (about 0.5 /im, according to Fig. 8-63) and Wien s displacement law [Eq. (8-13)]. This estimate gives... 

We have already described the radiation spectrum of the sun and noted that the major portion of solar energy is concentrated in the short-wavelength region. It was also noted that as a consequence of this spectrum, real surfaces may exhibit substantially different absorption properties for solar radiation than for long-wavelength earthbound radiation. 

Using the spectrum of Fig. 8-6S, estimate the magnitude of the solar radiation flux at the earth s surface. How does this calculation compare with one based on Eq. (8-119) ... 

The central role of hydroxyl radicals in atmospheric chemistry is well illustrated by examining the atmospheric cycles of methane and carbon monoxide. A quantitative assessment of both of these species was carried out in the 1920s in Belgium by Marcell Migeotte, who detected their absorption lines in the spectrum of infrared solar radiation reaching Earth s surface. 

Figure 2.12. Spectrum of solar radiation, on top of the atmosphere, at the Earth s surface on a clear day, and examples of the scattered part on typical overcast and clear days (based on NASA, 1971 Gates, 1966).

The peak of the solar radiation, for example, occurs at A = 2897.8/ 5780 = 0.50 p,tn, which is near the middle of the visible range. The peak of the radiation emitted by a surface at room temperature T = 298 K) occurs at 9.72 p,m, which is well into the infrared region of the spectrum. 

The hemispherical total absorptivity is not only a property of the absorbing surface. Rather, it depends on the spectral distribution of the incident radiation energy. This is shown by the different values of a for the mainly short-wave solar radiation, in which the absorption properties at small wavelengths are decisive, and for the incident radiation from an earthly source, for which the long-wave portion of the absorption spectrum a (X,T) is of importance. 

It is well known that the spectral distribution and irradiance of the solar radiation at the Earth s surface depend on the location and is subjected to seasonal and diurnal variations. Therefore, a reference spectrum is needed as a basis for comparison with the spectral energy distribution of artificial light sources. Data from CIE No. 15 1971 (colorimetry official recommendations of the International Commission on Illumination) that recommend a standard illuminant D65 with a scheduled color temperature of approximately 6500 K have been used as a basis over the years. 

The sum of the direct and the diffuse radiation on a surface. Total solar radiation is sometimes used to indicate quantities integrated over all wavelengths of the solar spectrum. The most conunon measurement of solar radiation is total radiation on a horizontal surface, often referred to as global radiation. 

---