Obscured Overcast Sky Radiance Distributions for Ultraviolet and Photosynthetically Active Radiation

Sky radiance measurements in the wavelength bands of ultraviolet-B (0.28–0.32 μm), ultraviolet-A (0.32–0.40 μm), and photosynthetically active radiation (0.40–0.70 μm) were made under obscured overcast skies in a rural area. Radiance distributions were modeled for seven measurement scans with solar zenith angles varying from 19° to 49°. For the seven scans, the atmospheric transmittance of photosynthetically active photon flux density varied from 0.16 to 0.25. The corresponding fraction of cloud-free sky photosynthetically active photon flux density ranged from 0.21 to 0.32. The corresponding fraction of ultraviolet-B cloud-free sky irradiance was between 0.20 and 0.34, with typically lower fractions of cloud-free sky irradiance in the ultraviolet-B than in the photosynthetically active photon flux density. The sky radiance was modeled from the ensembled measurements according to the standard overcast sky radiance distribution for each of the wavelength bands. Although the ultraviolet wave bands had slightly greater normalized radiance at the zenith and smaller radiance at the horizon than has been found for the photosynthetically active radiation wave band, the differences between the photosynthetically active radiation and the ultraviolet-A and ultraviolet-B radiance distributions were not statistically significant. Therefore, the authors concluded that the normalized obscured overcast sky radiance distribution for all three wave bands could be satisfactorily described by the standard overcast sky model of the photosynthetically active radiation distribution.