Historical observations of cloudiness (1882–2012) over a large urban area of the eastern Mediterranean (Athens)

Clouds play a major role in the radiative balance of our planet and largely control dynamic and thermal atmospheric processes that affect the climatic system. In addition to cloud cover (CC), different types of clouds are of special interest, as they contribute in different ways to the energy budget between the earth and the atmosphere. The study analyzes historical sub-daily reports from cloudiness observations conducted at the National Observatory of Athens, dating since the late nineteenth century. Long-term trends and diurnal, intra-annual, and inter-annual variability of both CC and frequency of prevailing cloud types were estimated. The analysis revealed statistically significant positive trends ( p