In the present article, interrelations between hourly and daily global and diffuse solar illuminance have been developed. By examining a large database, 56 primary locations, of TMY2 [1], it has been found that the values of ratio of hourly global or diffuse illuminance to corresponding daily values, (rvt = Iv/Hv ; rvd = 1vd/Hvd), are very close to the corresponding values for global and diffuse solar radiation, rt = I/H; rd = Id/Hd). Thus, it has been proposed that the existing correlations for rt and rd be employed to calculate rvt and rvd. The performance of the existing correlations for rt and rd to predict rvt and rvd has been evaluated employing TMY2 data and data of 6 IDMP (International Daylight Measurement Programme) [2] locations. It has been found that the correlation for rt due to Collares-Pereira and Rabl [3] predicts rvt satisfactorily. Correlation due to Satyamurty and Lahiri [4] for rd predicts rvd satisfactorily after suitable modification.

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