Variability of Sunlight Duration in Tomsk in 1961–2018

Variations in the sunlight duration (SLD) in Tomsk are analyzed for the period from 1961 to 2018 and separately for 1961–1990 and 1981–2010. Data on clouds and total solar radiation obtained at the TOR-station of IAO SB RAS in 1996–2018 are used. The actual long-term monthly mean SLD ranges from 44 h in December to 317 h in June–July. The analysis of the long-term variation in SLD shows its increase from 1961 to 1989 and its decrease starting from 1999 due to an increase in the low cloud cover and high frequency of continuous clouds. The SLD in Tomsk in the modern period has increased relative to the historical period. Regression equations between SLD and the total solar radiation (Q) are derived.