The Transmission of Sunlight through Cloudy Skies: An Analysis Based on Standard Meteorological Information

This work examines the use of standard meteorological information to describe the attenuation of sunlight associated with cloudy skies. Datasets consisting of broadband ultraviolet and total spectrally integrated solar irradiance are available for Chicago over the period April–October 1993. A series of regression models seeks to explain variability in the measured irradiances using fractional cloudiness, cloud-ceiling altitude, and visibility as independent variables. Fractional cloud cover alone can explain 39%–51% of the variance in the radiation data. Regression models that combine all three independent variables can increase the explained variance to 61%–77%, depending on local time and spectral region. Comparison of broadband ultraviolet irradiances with simultaneous total solar irradiances indicates that cloudy skies provide less attenuation in the former dataset.