This paper attempts to test the applicability of existing correlation models to the estimation of diffuse radiation with respect to measured values at a station. There are two types of model: The first type depends on the fraction of monthly average daily diffuse radiation to total solar radiation,$$K = {{bar H_d } mathord{left/ {vphantom {{bar H_d } {bar H}}} right. kern-nulldelimiterspace} {bar H}}$$, as a function of the “clearness index”,$$K_T = {{bar H} mathord{left/ {vphantom {{bar H} {bar H}}} right. kern-nulldelimiterspace} {bar H}}_O $$. The second type expresses the fraction$${{bar H_d } mathord{left/ {vphantom {{bar H_d } {bar H}}} right. kern-nulldelimiterspace} {bar H}}$$or$${{bar H_d } mathord{left/ {vphantom {{bar H_d } {bar H}}} right. kern-nulldelimiterspace} {bar H}}_O $$as a function of the sunshine fraction$${{bar n} mathord{left/ {vphantom {{bar n} {bar N}}} right. kern-nulldelimiterspace} {bar N}}$$Therefore, it presents statistically based correlations between global radiation and its diffuse component on a horizontal surface and suggests two equations to determine the ratio of diffuse radiation to total radiation received on a horizontal surface. The results of these correlation equations are compared with other accepted equati
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