Abstract: Based on meteorological observational data from 34 stations in Chongqing area from 1973 to 2015,the spatiotemporal characteristics and causes of fog events in Chongqing were analyzed using methods of conventional statistics and trend analysis.The results showed that the occurrence frequency of fog events during winter in Chongqing has distinct decadal variability,with an increasing trend from 1973 to 1989,turning to a decreasing trend around the 1990s,and then decreasing gradually.The spatial distribution of fog days number varies largely with the topography and landforms in Chongqing,with a larger number of fog days in the middle and western regions and smaller in the southeastern and northeastern regions.During the early 1990s and under the background of global climate warming,the decreasing trend of fog events during winter in Chongqing is mainly due to the transition phase of Pacific Decadal Oscillation (PDO) from warm to cold in preceding autumn (from September to November),increasing intensity of the Siberian High,increasing temperature and saturation specific humidity and decreasing relative humidity in the near-surface layer.In addition,the rapid development of urbanization in Chongqing and the increase of Aerosol Optical Depth (AOD) during winter also have an impact on the decrease in fog event.

Key words: Fog, Pacific Decadal Oscillation (PDO), Siberian High, Aerosol Optical Depth (AOD)