THE ATTENUATION OF SUNLIGHT BY HIGH-LATITUDE CLOUDS - SPECTRAL DEPENDENCE AND ITS PHYSICAL-MECHANISMS

Measurements of the ground-level solar irradiance from Palmer Station, Antarctica, and Ushuaia, Argentina, reveal a systematic wavelength de pendence in the attenuation provided by cloudy skies. As wavelength in creases from 350 to 600 nm, the measured cloudy-sky irradiance, expres sed as a fraction of the clear-sky value, decreases. Results from Ushu aia for a solar zenith angle of 45 degrees show that a cloudy sky that reduces the spectral irradiance at 500 nm to 50% of that for clear sk ies is accompanied by irradiances at 350 and 600 nm, which are approxi mately 59% and 49%, respectively, of the clear sky value. A weaker wav elength dependence appears in the data for Palmer Station. The observe d behavior can arise from Rayleigh backscattering of sunlight beneath the cloud, followed by reflection of this upwelling radiation from the cloud base back to the ground. This sequence of events is most effect ive at short wavelengths and leads to cloudy skies providing less over all attenuation as wavelength decreases.