In the south of Eastern Siberia (Russia), there are three optical observatories of the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences. The observatories are equipped with instruments to observe the Sun, near and deep space. The first one, Sayan Solar Observatory, is located in the mountains at the height of 2,000 m (51.67 N, 100.99 E). The second, Baikal Astrophysical Observatory, is on the southern bank of Lake Baikal near the settlement of Listvyanka (51.86 N, 104.86 E). These two observatories were built in the 60s — 70s of the last century. The third observatory is now being built in settl. Tory, Buryatia (51.78 N, 103.00 E).

During space observations in the optical range, atmospheric conditions are of great importance. The purpose of this work is to study the long-term dynamics of cloud cover within the region of the optical observatories. We used the following data: Monthly mean High cloud cover, Low cloud cover, Medium cloud cover and Total cloud cover (TCC) values derived from ERA-Interim 1979–2018 with spatial resolution 1º×1º and Monthly mean TCC values derived from NCEP Reanalysis Gaussian Grid 1948–2019. We calculated cloud cover values averaged along 50,475º N (NCEP) and along 51º N (ERA) over 101º–105º E (5 grid points for ERA and 3 grid points for NCEP).

We found that since about the mid-1990s, the amount of high, medium, and low cloud cover has been decreasing in summer. While only positive cloud cover anomalies were detected during construction of the observatories and the first observations at them, since 2005, the cloud cover anomalies are predominantly negative.