Diurnal variation of cloud cover over the Baltic Sea

•The average daytime cloudiness to be by a few per cent higher than that produced by standard routines.•The results for the night being similar or lower.•Application of the algorithm results to environmental studies on a regional scale should consider the factors that have a potential to reduce the reliability of data.•The results can be successfully used to determine average metrics of cloudiness over the Baltic Sea.•The average values of daily changes in cloud cover over the sea depend on the season (it is higher in warm months, and lower in winter), which generally corresponds to the length of the day (which is short in winter, and long in summer) and contrasts with the amount of cloudiness (which is high in winter, and smaller in a warm season).