Influence of Solar Elevation on Estimates of Cloud Cover

The increase of cloud shade with a decrease in solar elevation is explained by a simple cloud model. Analysis of long-term hourly data on point cloudiness and fraction of bright sunshine for 41 Canadian weather stations has led to empirical relationships between areal cloudiness, point cloudiness and cloud shade. These relationships can be used in solar energy calculations, where point cloudiness data alone are available, in the interpretations of satellite images of cloud cover where the satellite transfer function is unavailable, and in estimates of areal cloudiness from point cloudiness data for use in agricultural studies. Sufficient detail about the analysis procedure is given to permit similar relationships between cloud shade, and areal and point cloudiness to be derived for any region where hourly weather observations have been made.