Statistical correlation between hourly and daily values of solar radiation on horizontal surface at sea level in the Italian climate

The knowledge of hourly data of solar radiation is required for many practical applications. Since hourly data are measured only in a few stations, it is quite useful to have methods to calculate hourly values starting from daily data. In this paper we analyse the hourly data measured in Italian stations and propose a method to estimate hourly solar radiation. The approximation introduced by our method in estimating mean values is comparable with its statistical error. The curves we obtain are independent of season and station and depend only on the ratio K of experimental global solar radiation to extraatmospheric radiation. They can be fitted by paraboles whose coefficients can be written as a simple analytical expression of the K ratio. The variance of experimental data around means has been analysed. The statistical method we propose allows to reproduce the distribution of hourly values of solar radiation in a satisfactory way starting from the daily values of solar radiation.