Seasonal variations of diffuse solar radiation at Ilorin (8°32′N,4°46′E), Nigeria

Abstract

Hourly diffuse solar irradiance (H_d) and the corresponding global irradiance (H) falling on a horizontal surface at Ilorin (8°32′N,4°46′E), Nigeria has been measured for a period of fifteen months commencing from February, 1993. As expected, each monthly mean diurnal variation of H_d exhibits a mid-day maximum, while that of the ratio H_d/H exhibits a minimum. The month to month variations of these parameters are consequent on the dominant atmospheric conditions observed in the months concerned. The monthly average daily total of diffuse radiation (H_dm) was found to be highest in February, 1994 with a value of (3347 ± 45) Wm⁻²day⁻¹, mainly due to the effects of Harmattan haze and thin clouds, while it was found to be lowest in November, 1993 and January, 1994 with a value of about 2500 Wm⁻²day⁻¹. Similarly, the ratio of the monthly average daily total of diffuse radiation to the monthly average daily total of global radiation (H_dm/H_m) had the lowest value of 0.49 in the relatively clear sky month of November, 1993 and the highest value of 0.74 in the mainly cloudy month of August, 1993. On an annual average, the diffuse component constituted about 60% of the global solar radiation arriving on the ground at Ilorin, Nigeria primarily due to clouds and Harmattan haze which are seasonal, in addition to the constant background of molecular scattering.