Evaluation of fractional clear sky over potential astronomical sites

The estimation of the night-time cloud fraction (CF) is found to be one of the key parameters for evaluating the number of useful nights at an astronomical site. This work evaluates useful astronomical night-time observation over eight sites using a minimum threshold of CF from 21 years (2000–2020) of ground-based hourly visual and daily satellite data along with 41 years (1980–2020) of long-term hourly reanalysis data. The estimated number of photometric nights is underestimated by 8–24 per cent using the reanalysis data at Indian Astronomical Observatory-Hanle in comparison with the visual observations, while the estimated number of spectroscopic nights is 70–75 per cent per year and in good agreement with the visual observations. Among the astronomical sites, Paranal is found to be the best for astronomical observations, with 87 per cent spectroscopic nights per year. On the other hand, Hanle, Ali and Devasthal, located in the Himalayan region, exhibit an average of 68–78 per cent spectroscopic nights per year based on long-term reanalysis data, while Merak exhibits 61–68 per cent spectroscopic nights per year. Vertical profiles and global horizontal distributions for CF and related variable parameters are further compared among the sites. Global CF trends based on 41 years of reanalysis data show a decreasing tendency over most land regions and an increasing tendency over oceanic regions as well as over the Sahara desert, Middle East, and Indian subcontinent along the adjacent Tibetan Plateau. Such different CF trends between the ocean and land regions are thought to be the result of differential surface warming and water vapour changes associated with climate change.