Various radiation and sunshine correlations

Abstract

Abstract Let I d , I and ( S / S o ) h be the symbols used for the monthly average hourly diffuse solar radiation, the monthly average hourly global solar radiation, and the monthly average hourly sunshine fraction respectively. Experimental values of I d , I and ( S / S o ) h have been given by Dogniaux for Uccle (Belgium) using data obtained from 1951 to 1980 for different sky conditions. Condition A, clear skies (no clouds) and minimum turbidity, corresponding to the maximum values of radiation received when the sunshine fraction is 1 during the period considered; condition B, clear skies (no clouds) and average turbidity, corresponding to all the values of radiation measured when the sunshine fraction is 1 during the period considered; condition C, corresponding to all the values measured during the period considered, independently of the state of the sky. In this work we study the correlations of I d / I with ( S / S o ) h and I / I o . The following conclusions are obtained: (1) To determine I d / I (condition C) the correlation with I / I o is preferable to the correlation with ( S / S o ) h due to the larger scattering of experimental values for the last one. (2) The linear correlation of I d / I with I / I o obtained using data for both conditions, B (clear skies, average turbidity) and C, is similar for the ranges of values of I / I o corresponding to these conditions, to the correlations for condition B and for condition C, and, as a consequence, can be taken in practice as valid for clear and cloudy skies. (3) The following linear correlation, with r > 0.98 is obtained when values of I d / I for conditions B and C are considered: ( I d / I ) C = 0.388 + 1.052 (1 − ( S / S o ) h )( I d / I ) B The last two conclusions are obtained using data for all months, when the experimental values for the first and last hours for which data are available for each month are not used to perform the correlation.