Direct solar radiation: spectrum and irradiance derived from sun-photometer measurements.
The continuous spectrum of the direct solar radiation from lambda= 330 to 2690 nm, penetrating a cloudless atmosphere and arriving on the earth surface, is determined by measuring the solar irradiance in ten selected discrete spectral ranges defined by interference filters. Heretofore knowledge of the extraterrestrial solar spectrum has been required as well as of the transmittance functions to describe the spectral optical properties of the atmosphere. A set of appropriate and simple functions is given and discussed, which allows calculation of the molecular, aerosol, oxygen, and ozone optical thicknesses. The influence of atmospheric water vapor is considered through line by line calculations. The dominant and most fluctuating extinction parameters are the aerosol optical thickness and the content of precipitable water vapor. These are obtained by measurements with two sun photometers, developed according to the WMO recommendation. To test the derived solar spectrum at ground level the photometers are also run with nine broadband filters. The values observed differ little from those obtained by integration of the deduced spectral irradiance. Furthermore, the integral value of the resulting entire spectrum agrees reasonably well with the total direct irradiance gained from actinometer measurements.