Comparison of monthly mean hourly sunshine fraction estimation techniques from calculated diffuse radiation values

Mean monthly hourly values of global I and diffuse radiation I-d, along wit h mean monthly daily values of the sunshine fraction sigma(d) available for four locations in the United Kingdom, are used to develop six models relat ing I-d/I with the monthly mean hourly clearness index K-t, the estimated m onthly mean hourly sunshine fraction sigma(h) and the monthly mean solar el evation at mid hour alpha. Two available methods are used to predict the va lues of sigma(h) from sigma(d) and the calculated I-d data are compared. St atistical tests performed for a total of six locations, including those use d to develop the models, show that the best results are obtained when sigma (h) predicted with the method developed by Page is employed in the estimati on correlation. (C) 1999 Elsevier Science Ltd. All rights reserved.