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 mu m), ultraviolet-A (0.32-0.40 mu m), and photosynthetically active radiation (0.40-0.70 mu m) were made under obscured overcast sk ies in a rural area. Radiance distributions were modeled for seven mea surement scans with solar zenith angles varying from 19 degrees to 49 degrees. For the seven scans, the atmospheric transmittance of photosy nthetically active photon flux density varied from 0.16 to 0.25. The c orresponding fraction of cloud-free sky photosynthetically active phot on flux density ranged from 0.21 to 0.32. The corresponding fraction o f ultraviolet-B cloud-free sky irradiance was between 0.20 and 0.34, w ith typically lower fractions of cloud-free sky irradiance in the ultr aviolet-B than in the photosynthetically active photon flux density. T he sky radiance was modeled from the ensembled measurements according to the standard overcast sky radiance distribution for each of the wav elength bands. Although the ultraviolet wave bands had slightly greate r normalized radiance at the zenith and smaller radiance at the horizo n 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 n ot statistically significant. Therefore, the authors concluded that th e normalized obscured overcast sky radiance distribution for ail three wave bands could be satisfactorily described by the standard overcast sky model of the photosynthetically active radiation distribution.