We will shortly review here the most important studies on the statistical solar radiation behaviour, with special focus on those works that have produced important advances on solar radiation modelisation. Until relatively recently, most of the studies were focused on the daily distributions. Nevertheless, in the last years, the analysis of the instantaneous behaviour has given rise to a special consideration. This is because the need to use the instantaneous values in specific applications, as photovoltaic devices and estimation of erythemal doses, and the appreciable differences observed with regards to the monthly, daily and hourly distributions. Such differences must be specially taken in account in energy and biological applications, where the systems have a non-linear response and are very sensitive to the instantaneous values.
Several studies on these topics have been carried out. The first results go back to the famous works of Angstrom (1924, 1956), who derived regression expressions for the different components of the daily solar radiation based on the sunshine duration. Other works, which are worth to point out are those of Black et al. (1954) and Glover and McCulloch (1958). These works revealed certain bimodality or, at least, a strong asymmetry in respect to the mean, as confirmed afterwards by several authors: Bennet (1965, 1967), Klink (1974a), Andretta et al. (1982), Barbaro etal. (1983).
Among the solar irradiation measurements, the daily global values have been, for a long period, the most frequently studied. Nevertheless, they have not adequately been analysed using statistical or even graphical methods. A previously suggested normalisation by the corresponding extraterrestrial irradiation values has recently become a customary practice. The monthly frequency distributions for many U. S. stations have been presented by Bennet (1967), showing a relative bimodality and skewness in the distributions. The monthly frequency distributions have also been studied by Klink (1974b) and Baker and Klink (1975), showing a negative skewness (or to the left), platycurtic and a tendency to bimodality.