Climatology of fog occurrence over a wide flat area in Serbia based on visibility observations

Long‐term analysis of occurrence of fog events at 14 stations across part of fairly flat terrain of Serbia, which includes the area of the Pannonian Plain, is conducted. For the majority of stations (Palić, Novi Sad, Kikinda, Vršac, Belgrade‐Surčin, Smederevska Palanka, Valjevo, Loznica, Negotin) data were available for the 33‐year period, 1973–2005. For the rest of stations, data were available for shorter periods: for Sombor and Veliko Gradište 1982–2005, for Sremska Mitrovica and Belgrade‐Vračar 1983–2005, and for Zrenjanin 1989–2005. Fog event is defined only based on visibility observations. Almost 75% of analysed data sets showed a negative annual trend of low‐visibility events throughout the analysed time periods. This negative trend is increasing as it goes towards the north of Serbia and most of the stations with decreasing trend are rural. Fog is most frequent in the period between October and February and it is likely mostly due to radiative cooling and especially in October when an anticyclonic synoptic situation is prevailing. Stations with higher elevations have more lasting fog events and fog events with more variable mean relative humidity compared to their counterparts, which ranges from 92.1–97.4%. For most of stations December is the month with the highest frequency of occurrence of fog in the presence of a low cloud. Frequency of fog onset during low wind speed (below 2 m/s) is analysed along with its correlation with a frequency of fog onset when the cloud base height is equal to or above 2,000 m. In ~71% of the stations correlations are higher than 0.6, and stations with a lower elevations show a generally better correlation than those with higher elevations.