Applicability of a simple model for computing diffuse solar radiation to locations of the European, African, Asian and North American areas

It is tested if the correlation type D = K(s/S)−0.25 (sin hn)1.55, 0.2 ⩽ s/S ⩽ 0.9 previously proposed by Coppolino for Italian locations using K = 7.0, allows a reliable estimation of the monthly mean daily diffuse solar radiation D (MJ m−2 day−1), from only the monthly mean daily relative sunshine s/S and the noon altitude of the sun on the 15th of the month hn (degrees) at locations of the European, African, Asian and North American areas. The test is performed at 14 stations in the above areas displaced at various latitudes L, elevations above sea level E and geographical situations, using K = 7.0 for the stations where 0.48 ⩽ (s/S)m, ⩽ 0.63, K = 8.5 for those where 0.40 ⩽ (s/S)m < 0.48 or (s/S)m > 0.63, and K = 10.0 for those where (s/S)m < 0.40; (s/S)m is the yearly mean value of s/S. K = 7.0 is chosen for five stations provided only with sunshine measurements and for three of the other nine stations provided with diffuse radiation measured data as well. K = 8.5 is chosen for five of the above nine stations and K = 10.0 for the remaining one. Moreover the prediction validity of the tested correlation is compared with that of two equations commonly quoted in literature: one of D = f(Kt, G) type and the other of D = f (s/S, G) type, where Kt = G/G0. G and G0 are the global and extraterrestrial monthly mean daily solar radiation, respectively. The test results point to a good agreement between the measured data of D and values from the above (D = f(s/S, hn)) equation for eight of the above nine stations (i.e. with the exception of one with E = 2210 m), and deviations are generally low among D-values from the three considered equations, respectively, for the other five stations. Consequently the tested D = f(s/S, hn) equation, with K-value chosen by the above guidelines, can be considered applicable, with good reliability, to all locations of the above areas provided 30°S < L < 55°N and E < 2000 m.