Diurnal global irradiation was measured with precision pyranometer in Nsukka, Nigeria and compared with values generated from five different empirical models proposed by earlier investigators. Climatological parameters of sunshine hours, relative humidity, and maximum air temperature, as well as geographical declination, latitude and altitude factors are required as the model inputs. The calculated values obtained from the modified empirical formula of Swartman and Ogunlade gave the best agreement with the measured data. Agreement was within ±6 per cent. The analytical results derived from the regression equation of Angstrom agreed to within ±10 per cent with the measured solar radiation intensity. These formulas could be applied with the average best fitted coefficients developed in this investigation to estimate the solar radiation intensity fairly accurately in other parts of Nigeria lacking solar irradiation data.Results are also presented on the effect of dust haze attenuation which characterizes part of the environmental dry season. The period is seen to represent the season of maximum irradiance for the year primarily because of the absence of rain-bearing clouds. The prevailing winds however come north-easterly from the Sahara Desert and the associated dust-laden air results in reduced clearness index and increased diffuse irradiance ratio. The days overcast with dust haze give results that are in close agreement with Canadian cloudy days as presented by Orgill, but the overall dry season correlation follows that of Modi and Sukhatme in India.
Solar Energy 12/1981; 26(2):181-186. DOI:10.1016/0038-092X(81)90083-9