Measurements of Downward Scattered Solar Radiation from Isolated Cumulus Clouds

Abstract

Ground-based measurements of global and direct solar radiation were obtained in conditions when only isolated, approximately cubic cumulus clouds were present. The angular distribution of downward scattered radiation was determined as these clouds moved through the path of the sun's rays. Scattered energy is found to be negligible at scattering angles greater than 15°. As the scattering angle decreases, energy amounts increase exponentially, until at ?1° the energy scattered by the cloud is between 5 and 15% of the total diffuse radiation from a clear sky. The amount is directly proportional to cloud thickness. Further, direct transmission through the cloud is inversely proportional to cloud thickness. Hence, for the small, cubic, cumulus-type clouds observed, the total downward radiation emanating from a cloud is approximately constant, irrespective of thickness. These results are in general qualitative agreement with theoretical calculations for model cumulus clouds.