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For various applications it is necessary to know not only global solar radiation values\ but also the di}use and beam components[ Because often only global values are available\ there have been several models developed to establish correlations between the di}use fraction and various predictors[ These typically include the clearness index\ but also may include the solar angle\ temperature and humidity[ The clearness index is the proportion of extraterrestrial radiation reaching a location\ where the extraterrestrial value used in the calculation varies with latitude and time of year[ These correlations have been developed using data principally from latitudes greater than 39>\ often using only data from a few locations and with few exceptions have not used solar altitude as a predictor[ Generally the data consist of hourly integrated values[ A model has been developed using hourly data from a weather station set up at Deakin University\ Geelong[ Another model has also been developed for 04 minute data values in order to ascertain if the smoothing generated by using hourly data makes a signi_cant di}erence to overall results[ The construction of such models has been investigated\ enabling an extension to the research\ inclusive of other stations\ to be performed systematically[ A _nal investigation was carried out\ using data from other Australian locations\ to explain some of the considerable scatter by adding apparent solar time as a predictor\ which proved to be signi_cantly better than solar altitude[ Copyright Þ 1990 John Wiley + Sons\ Ltd[ KEY WORDS] di}use solar radiation^multivariate regression^mathematical modelling 0[ INTRODUCTION The evaluation of the performance of a solar collector such as a solar hot water heater or photovoltaic cell requires knowledge of the amount of solar radiation incident upon it[ Solar radiation measurements are usually for global radiation on a horizontal surface "Reindl et al[\ 0889^Skartveit and Olseth\ 0876#[ These global values comprise two components\ the direct and the di}use[ {I DN \ the direct normal irradiance\ is the energy of the direct solar beam falling on a unit area perpendicular to the beam at the Earth|s surface[ To obtain the global irradiance the Correspondence to] J[ Boland\

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