Extensive applicability of a new model for estimating diffuse solar radiation from clearness index and minimum air mass

The applicability of a correlation among the monthly mean daily diffuse solar radiation D, MJ m−2 day−1, the clearness index Kt and minimum air mass to locations of the European, African, Asian and North American areas is tested. This correlation is D = KKt−0.55(sin hn)1.58 where K is an empirical coefficient only depending on the climatic characteristics of the location, Kt is the ratio of the monthly mean daily value of global radiation G to extraterrestrial one Go and hn is the noon altitude of the sun on the 15th of the month. This equation, previously proposed by Coppolino for all Italian locations using K = 5.6, is applied to 14 stations of the above areas displaced at various latitudes, elevations above sea level E and geographical situations, using K = 5.6 for the stations for which 0.48⩽(s/S)m ⩽ 0.63, K = 7.0 for the stations for which 0.40⩽(s/S)m < 0.48 or (s/S)m > 0.63, and K = 9.0 for the stations for which (s/S)m < 0.40. (s/S)m is the yearly mean value of the relative sunshine s/S. K = 5.6 is chosen for five stations provided only with sunshine records and for two of the other nine stations provided also with diffuse radiation measurements, K = 7.0 for six of the above nine stations and K = 9.0 for the remaining one. Moreover the prediction validity of the tested correlation is compared with that of two equations: one of D = ƒ (s/S,hn) type and the other of D = ƒ (Kt, G) type. The test results point out a good agreement between the measured data of D and ones from the above D = ƒ (Kt, hn) equation for eight of the above nine stations (i.e. with the exception of one with E = 2210 m), and deviations generally low among D-values from the three analysed equations, respectively, for other five stations. Consequently the D = ƒ (Kt, hn) correlation, with K-value chosen by the above guidelines, can be considered valid and useful to predict the monthly mean daily diffuse solar radiation with good reliability at locations of the above areas lacking in diffuse radiation records provided their latitudes range from 55°N to 30°S and their elevations above sea level are less than 2000 m.