Correlation between normal direct radiation and global radiation depending on cloudiness

Abstract Measured intensities of the solar direct radiation at normal incidence I bn have been correlated with the coefficients K hi = I hi I oi , where I hi and I oi are respectively the intensity of the global solar radiation at ground level and the extraterrestrial radiation, both on a horizontal surface. Four correlation lines have been drawn, I bn = F ( K hi ); each line is linked to a particular degree of cloudiness. The result shows that the relationship existing between I bn and K hi is influenced by the degree of cloudiness, irrespective of the value of the air mass m . This allows a relationship to be found between I bn and cloudiness degree, having K hi as parameter.