Analysis of daily average clearness index and relative possible number of sunshine hours n/N have been made using accumulated meteorological data obtained from Nigeria Meteorological Agency (NIMET) station, Oshodi, Nigeria from 1985 to 2014. The values of regression constants ( and ), sum of the regression constants ( ), Mean-Bias-Error (MBE), root-mean-square-error (RMSE) and correlation-coefficient (RC) are presented and an Angstrom model of monthly average clearness index with normalized sunshine duration is then developed. The statistical performance of the results show that the regression coefficients are higher than 68% implying a good agreement between and n/N; more than 50% of clearness index values have been observed indicating there are transparent sky throughout the year and there is a good agreement between the measured and calculated values of global radiation for the stations as attested by very low values of MBE with correlation coefficients ranging from 0.68 to 1.00.

Keywords: Analysis of daily average clearness index and relative possible number of sunshine hours n/N have been made using accumulated meteorological data obtained from Nigeria Meteorological Agency (NIMET) station, Oshodi, Nigeria from 1985 to 2014. The values of regression constants ( and ), sum of the regression constants ( ), Mean-Bias-Error (MBE), root-mean-square-error (RMSE) and correlation-coefficient (RC) are presented and an Angstrom model of monthly average clearness index with normalized sun