A procedure for estimating global solar radiation from sky cover is developed from the records of 47 stations in the United States with long periods of radiation observations during the 10‐year period, March 1961 through February 1971. It fits a general parabolic equation of the formY=B+ (1 −B)(1 −N)pto the observations, whereYis the observed global solar radiation divided by clear sky radiation andNis the sky cover. The variablesB(the point at which the parabola crosses theyaxis) andP(a variable parameter less than 1.0) are selected to minimize the sum of the errors (Y−Ycalc)2, whereYcalcis the calculated value ofY. The equationY=B+ (1 −B)(1 −N)0.61is selected as most representative, and theBvalues in this equation that minimize the sum of the errors squared for the individual stations are shown in Figure 1. The average absolute error of the 5306 data points is 1.18 MJ m−2, or 7% of the average observed radiation. Because of the uncertainties of the observed global solar radiation and observed sky cover, the procedure should be used with caution, particularly for periods of less
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