全球变化知识资源中心

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<0.001) in CC over the entire study period, more pronounced in spring and summer. Important changes in the prevalence frequency (PF) of certain cloud types over the last 60years were also detected. Low, middle, and high clouds exhibited significant long-term trends of opposite sign. A marked increase in the PF of low and high clouds was observed in the recent decades, accompanied with simultaneous decrease in the middle cloud occurrence. Convective clouds prevailed increasingly more frequently over time during winter, while low stratiform clouds declined, in agreement with other observations worldwide. Significant correlation was also found between the PF of stratiform clouds and precipitation frequency.