Abstract
Data on daily global irradiance ( K ↓) and hours of bright sunshine ( n) for stations Sebele (24°34′S) in Botswana, and for Niamey (13°29′N), Chical (14°36′N), and Zinder (13°48′N) in Niger were analyzed. The corresponding daily values of extra-terrestrial irradiance ( K ↓↓) and daylength ( N) were calculated for each data pair using well-known astronomical relationships. The data were screened, inadmissible records were deleted, and the remaining data grouped by month. Linear regression was used to fit the Angstrom-Prescott equation namely, K ↓ K ↓↓ = a + b[ n N ] , to be grouped data for each month. The results of analysis of the 12-yr record of global irradiance andsunshine data for Sebele, Botswana gave an overall mean intercept value of 0.23 ± 0.022 and a mean slope of 0.49 ± 0.025. For Sebele, where skies are clear during most of the year, the daily and mean monthly global irradiance can be estimated with reasonable accuracy from measured hours of sunshine along with the calculated extra-terrestrial irradiance and daylength. Results of analysis of the much shorter and incomplete records for the three locations in Niger were more variable and showed a higher atmospheric attenuation than at Sebele. These findings could probably be attributed to the higher levels of suspended atmospheric dust over Niger compared to Botswana.
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