A new correlation between clearness index and relative sunshine

A new correlation between the clearness index Kt and the relative sunshine s/S is proposed for computing the monthly mean daily global radiation G(MJ m−2 day−1) at Italian locations. This equation is: Kt = 0.67(s/S)0.45 (sin hn)0.05 0.15 ⩽ s/S ⩽ 0.90, where Kt = G/Go; Go(MJ m−2 day−1) and hn (degrees) are the monthly mean daily extraterrestrial radiation and the noon altitude of the sun on the 15th of the month, respectively; s and S (hrs) are the monthly mean daily values of the real and theoretical sunshine duration, respectively. This formula is tested using data from 34 Italian stations displaced at various latitudes or elevations above sea level or climatic and geographical situations (inland or coastal site of the Mediterranean, Tyrrhenian, Adriatic or Ligurian Sea). The test results generally show low deviations between the measured and computed values of G with the exception of Pian Rosà, whose elevation is 3488 m. For 32 locations the maximum per cent standard error of estimate, Φ(%), is less than 10. Moreover, a comparison among the values of G computed for each tested location by the above formula, and two other correlations with measured data, points out that the per cent standard error of estimate Φ(%) referring to the present equation is better than ones obtained for the other considered correlations for 16 analysed locations, whereas it is almost equal for eight other locations. Consequently the correlation herein proposed can be considered suitable and useful to predict, with a high degree of accuracy, the monthly mean daily global radiation, even at Italian locations lacking in actinometric stations but provided with measured data of sunshine duration, provided the elevation of the place is less than approximately 2500 m.