VARIABILITY OF SOLAR IRRADIANCE

The variation of solar irradiance resulting in additional difficulties for the utilization of the solar energy is assessed by changes in the transparency of atmosphere mainly due to the impact of stochastic clouds. The variation of solar irradiance can be described in the best way by the "alternating irradiance", which we define as a difference between the measured and calculated beam and diffuse irradiance for the clear-sky conditions which both are periodical variables, but determined for each instant. The alternating irradiance is investigated by correlation analysis methods, which allow to identify the significantly diurnal periodical character of diffuse irradiance. During the years 1955n2000 all components of solar irradiance decreased at a rate of 0.1% per year, showing a noticeable linear trend, but the time function may have a periodical component for long periods of time, exceeding 30 years. However, the ratio of diffuse and global irradiance has remained a constant value.