The Meteorological Observatory of Moscow State University has been developing the ideas of Professor K. Ya. Kondratyev since 1955 by regular measurements of solar radiation in different regions of the spectrum and under conditions of natural illuminance, and also by monitoring various optical characteristics of the atmosphere. Observing the temporal series of solar integral radiation according to average cloud conditions has revealed a weak tendency to a decrease in direct, diffuse, global and reflected solar radiation and an increasing trend in absorbed radiation and net radiation over the past 50 years. This is expressed most distinctly in winter. The main reason for the negative tendency in solar radiation is the increase in cloud amount due to an increase in the recurrence of cyclonic activity.

Regular measurements of solar radiation carried out in the network of actinometric stations of the former Soviet Union (FSU) have shown a decreasing tendency in the time series of direct and global solar radiation during last decades. A statistically significant decrease has been found in 60% of the time series of global radiation annual totals obtained from 160 actinometric stations. Three large areas, where this decrease exceeded 2% per 10 years, were found (in the European part of the FSU, in West Siberia, and in the Far East). As the main reason for changes in solar radiation, increased cloudiness and atmospheric turbidity are considered. A more detailed analysis of long-term changes in radiation budget components obtained from two stations situated in rather different conditions (Moscow Meteorological Observatory in a large town and Toravere Actinometric Station in a rural area) allows us to point out the influence of a large town as a source of air pollution on the radiation transfer in the atmosphere, as well as on cloudiness and air temperature.