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.