The distance factor in the relationships between solar radiation and sunshine

The effect that distance ( d ) has upon the correlation coefficient ( r ) obtained between paired radiation ( R ) and sunshine ( S ) stations was determined for the North Central region of the United States. Distances between stations varied from 0 to 608 km. In the regression of r against d three features of interest were found. First, the slope of the annual r values with distance is essentially linear from about 60 to 550 km and equals about −0.12/100 km. Secondly, from d = 0 to 60 km, the value of r remains constant at about 0–88, creating a ‘zone of indifference’ due to inherent differences between radiation and sunshine recording instruments and the insensitivity of the latter. Thirdly, beyond about 550 km the slope appears to reach a constant value, which varies with season but is about r = 0–30 on an annual basis. Seasonal and geographical differences in the degree of correlation between r and d are noted and discussed briefly.