Sky conditions at Ilorin as characterized by clearness index and relative sunshine

Clearness index ( K T) and relative sunshine ( S) are used to characterize the sky conditions at Ilorin, Nigeria (latitude: 8°32′N, longitude: 4°34′E, elevation: 375 m), a tropical location. Daily analysis shows that the majority of the days are relatively cloudy with hardly any very clear or overcast days. Based on monthly averaged K T, the monthly variations of cumulative frequency of K T, f, and prevalent climatic conditions, the annual sky conditions have been classified into six patterns: three patterns each for the two broad seasons, dry and rainy. The trend in the seasonal variation of S lends further credence to this six seasonal pattern. Determined cumulative frequency curves further confirm the assertion that Liu and Jordan’s generalized cumulative distribution curves (CDC) are not applicable to tropical locations hence available design procedures based on these curves should be reconsidered [Liu, B. Y. U and Jordan, R. C. (1960). The Inter-relationaship and characteristic distribution of direct, diffuse and total solar radiation. Solar Energy 4, 1–19]. An empirical expression relating monthly maximum clearness index ( K max) and the monthly average clearness index ( K T) is presented and recommended for tropical locations. Comparative analysis shows that a similar expression by Saunier et al. is also appropriate to this location while that of Hollands and Huget is quite inadequate. [Saunier G. Y., Reddy T. A. and Kumar S. (1987). On monthly probability distribution function of daily global irradiation values appropriate for both tropical and temperate locations. Solar Energy 38, 169–177; Hollands K. G. T. and Huget R. G. (1983) A probability density function for clearness index, with applications. Solar Energy 30, 195]. Furthermore, the values of the minimum clearness index (0.12–0.24) for Ilorin are noticed to be much higher than the generally accepted value of 0.05 for overcast sky conditions. The results are further compared, especially with those of Ideriah and Suleman [Ideriah, F. J. K. and Suleman, S. O. (1989). Sky conditions at Ibadan during 1975–1980. Solar Energy 43, 325–330.] and Akuffo and Brew-Hammond [Akuffo, F. O. and Brew-Hammond, A. (1993). The frequency distribution of global solar radiation at Kumasi. Solar Energy 50, 145–154.] for Ibadan and Kumasi, all in the West African sub-region. Although the locations are slightly different in agro-climatic conditions, results are found to be very comparable.