The amount of daily irradiation received for a particular area is one of the most important meteorological parameters for many application fields. In this paper, the frequency distributions of global radiations are investigated using four kinds of probability density functions,
i.e. the Weibull function, logistic function, normal function and lognormal function. The radiations observed at six meteorological stations in Taiwan are selected as sample data to be analyzed.To evaluate the performance of the probability functions both the Kolmogorov-Smirnov
test and the root mean square errors are considered as judgment criteria. The results show that all the four probability functions are applicable for stations where weather conditions are relatively steady throughout the year as in Taichung and Tainan. While for stations revealing more dispersive distribution as in Hualien and Taitung, the lognormal function describes the frequency distribution quite better than other three functions. On the whole the lognormal function performs best followed by the normal function; the Weibull function widely used in other fields seems to be not appropriate in this case.