Turkish Journal of Physics






Downward infrared radiation (L) and some easy-to-measure meteorological variables (relative humidity, clearness index and vapour pressure) were collected for a period of two years at Ilorin. Downward infrared radiation data were obtained using a pyrgeometer (Eppley-PIR), while data for relative humidity and vapour pressure for Ilorin were obtained from the Nigerian Meteorological Services, Lagos. Based on these data, using statistical regression analysis, empirical models (seven one-variable and nine two-variable) for predicting daily mean infrared radiation, from both clear and cloudy skies, were tested. Their advantage is that they only make use of easily measured meteorological parameters. The best one-variable model is the one based on vapour pressure, with correlation coefficient R = 0.973 and standard error of estimate \sigma = 0.3354 MJm^{ - 2}. The best two-variable model is the one involving vapour pressure and clearness index, with R = 0.979 and \sigma = 0.3017 MJm^{ - 2}. Hence, from this result, the use of the multiple-variable model is only slightly justified, as the one-variable model already gives reasonable estimations. The applicability of some existing clear sky models to the Ilorin environment was also tested. While some of them were adequate for estimation of infrared radiation under clear sky conditions, others were not. Models that include both temperature and vapour pressure terms performed better.

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