A new model for estimating diffuse solar radiation in Italy from clearness index and minimum air mass

The aim of this study is: (a) to propose a new simple model for estimating the monthly mean daily diffuse radiation D, MJ m⁻² day⁻¹, using as input only the clearness index K_t and minimum air mass and (b) to develop a map of diffuse solar radiation incident on a horizontal surface in Italy. The correlation herein suggested is D = 5.6 K_t^−0.55 (sin h_n)^1.58 where h_n is the noon altitude of the sun on the 15th of the month, in degrees, and K_t = G/H₀; G and H₀ are the monthly mean daily global and extraterrestrial solar radiation respectively. This equation is applied to Adrano, Palermo, Macerata and Genova stations, provided with measured data of global and diffuse radiation, and the relative computed values of D are compared both with the experimental data and with ones computed by the following correlation suggested by Coppolino in a previous work: D = 7 (s/S)^−0.25(sin h_n)^1.55 where s/S is the monthly mean daily value of the relative sunshine duration. The values of D computed by the above equations, respectively, result as nearly equal to each other for each month and each location. The validity of both considered correlations to fit the measured data of D for each tested station is shown by the standard per cent error of estimate φ (%). For both considered equations these errors are < 10 for Adrano, Palermo and Macerata and little higher than 10 for Genova. Therefore, both above correlations are independent of the latitude, the altitude above sea level and geographical situation (inland or coastal site) of the location; consequently, both of them can be considered valid and useful to predict (with a high respect both the above correlations are applied, with good reliability, to 30 more locations displaced at various latitudes, altitudes above sea level and geographical situations in Italy.