Global Transmissivity and Diffuse Fraction of Solar Radiation for Clear and Cloudy Skies as Measured and as Predicted by Bulk Transmissivity Models

Abstract--Measurements of cloudiness and of global, direct, and diffuse radiation taken over a 13 mo period at Davis, CA, are analyzed in terms of global transmission (K0 and diffuse fraction (Kd) for clear sky conditions and for various cloudiness conditions. A number of global transmission clear sky models are compared with observations for ranges of total water column and turbidity and some are found to give representative values for the global radiation at the ground. The dependence of the diffuse fraction on global transmission is found to be best represented by linear formulae--with different dependencies found for clear and cloudy conditions. Global transmission models are also compared with observations for cloudy conditions and found to give representative values of cloud transmissivities if climatological differences in the cloudiness at the measurement site and those sites used to calibrate the cloud models are considered. These results support the use of routine instantaneous surface meteorological data to calculate the most likely instantaneous global and diffuse radiation on a horizontal surface in the absence of any radiation measurements. These calculated irradiances are best used for solar energy system dynamic modeling in which system responses to typical sequences in meteorological conditions are being examined.