Abstract
Measurements of global solar radiation ( R s) and global photosynthetically active radiation (PAR) were made for a period of 12 months at Ilorin, Nigeria in order to find the relationship between them. The average ratio of PAR to R s for the year was 2.08 E MJ −1 with the dry and rainy season values of 2.02 and 2.12 E MJ −1, respectively. The minimum monthly mean daily ratio of 1.92 E MJ −1 was in January, a dry (Harmattan) season month while the maximum was 2.15 E MJ −1 in May, a rainy season month. Generally, the ratio values in the rainy season months and even dry season months (except the Harmattan months) remain constant at about 2.1 E MJ −1. On daily basis, the maximum and minimum ratios were 1.86 and 2.31 E MJ −1, respectively. Hourly values of the ratio increased on average from 2.07 to 2.15 E MJ −1 as the sky conditions changed from ‘clear’ to ‘cloudy’. There was no daylength effect on the ratio.
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