Spectral distribution of global and diffuse solar radiation in Ny-Ålesund, Spitsbergen

Spectral distributions of solar radiation were measured over the wavelength range 300 to 800 nm in Ny-Alesund (78"55'N, 1 I"56'E). The apparatus consisted of a grating monochromator with an automatic scanning system, an integrating sphere supposed to give a good picture of the radiation received on a horizontal surface from the whole celestial hemisphere, a photomultiplier tube with S-20 response, and a strip chart recorder. Great importance was attached to the calibration procedure, which was carried out by means of a 1000 W quartz-iodine lamp with known spectral irradiance (calibration traceable to NBS, delivered by Eppley Laboratory). The main part of the discussion is based on relative spectral distributions. Spectra in absolute units were obtained by comparison with simultaneous pyranometer recordings of total solar energy, which by means of filter measurements could be reduced to the same wavelength range as for the spectral scans. In addition, the pyranometer recordings served as a means of judging whether the radiation conditions were sufficiently stable to give reliable spectral curves. Spectral measurements were made of radiation from: (I) clear sky and sun, (2) clear sky alone, a sky totally covered by (3) clouds or (4) fog, ( 5 ) zenith clear and (6) zenith overcast sky. The spectral curves of (I), (3), (4) and (6) do not differ very much, but deviate markedly from those of (2) and (5). An overcast sky has relatively higher spectral irradiances towards the short-wave end of the spectrum than the corresponding spectrum for a clear sky. Three groups of solar altitudes are considered, with mean altitudes of about 30" (noon), 18" (evening), and 10" (night). Comparison of the spectra of these groups clearly reveals the effect of increasing absorption by atmospheric gases with decreasing solar altitude. At the same time, due to growing importance of selective scattering processes, the proportion of long-wave to short-wave irradiances increases. The Ny-Alesund noon spectrum for clear-sky global radiation seems to agree quite well with spectra from lower latitudes referring to a correlated colour temperature close to 6500" I(. This also applies to the CIE standard daylight source De5. Considering the spectra in absolute units, it turns out that while the spectral irradiances of global radiation on an overcast day are about 40% of those for a clear day in the ultraviolet part, the corresponding relation decreases to about 30% in the near infrared. Sky radiation on a clear day is responsible for more than half of the ultraviolet content of global radiation, but does not contain more than about 5% of the near infrared radiation. Comparable spectra from Nottingham and Pretoria seem to be considerably more influenced by aerosol scattering. Nevertheless Pretoria has as high irradiances in the ultraviolet as Ny-Alesund, in all probability due to a smaller ozone absorption at the former station. A Few characteristic deductions of the spectral distributions are discussed. For situations with a clear sky daylight illuminances calculated on the basis of absolute spectral irradiances and the spectral sensitivity of the human eye agree well with directly observed illuminances for the same solar altitudes. The chromaticity co-ordinates representing different phases of daylight in Ny-Alesund are slightly on the green side of the black body locus. Somewhat unexpected is the fact that daylight at midnight has a slightly higher correlated colour temperature (is more bluish) than that observed during the middle of the day. Estimates demonstrate the significant part played in this connection by the ozone absorption in the Chappuis bands.