Regional characteristics of cloudiness in Serbia during the period 1991-2017

Monthly climate data of days with clear and cloudy skies at 38 stations (1991-2017) and daily visual observations of total cloud amounts (TCA) available at six principal stations (1992-2017) were used to investigate regional, subregional, and temporal characteristics of cloudiness in Serbia. Annual, seasonal, and monthly means were used in the analysis. Results showed differences due to influences from topography, predominant synoptic situations, and possible teleconnection with the North Atlantic Oscillation (NAO). According to our criteria, the annual number of cloudy-sky days (similar to 80-120) was higher than that of selected clear-sky days (similar to 50-90), except for 3 years of prevailing anticyclonic circulations. A maximum cloudiness frequency was in winter (December), while a minimum frequency was in summer (August). TCA variability was the highest in an area of complex orography. Southeast Serbia was characterized by the smallest daily temporal variation of TCA based on 7, 14, and 21 h observations as well as by a high number of clear-sky days during all seasons, while the western mountainous part was much cloudier. From October to January, morning TCA was higher than afternoon TCA, ranging from 70% (January) to 78% (December). Statistically significant positive trends in the number of cloudy-sky days were found for three stations. A statistically significant moderate positive/negative correlation coefficient was found between the monthly cumulative frequencies of clear-sky/cloudy-sky days and the NAO for the winter seasons, respectively. During the 18 winters with the negative NAO index (i.e., frequent westerly warm and moist advection), there were approximately six times more cloudy-sky days (588) than clear-sky days (88).