[1] Fog is a severe weather hazard that greatly influences traffic and daily life with potentially heavy economic loss. An increasing number of traffic accidents caused by fog have been reported in China in recent years. In this study, we show that the frequencies of fog events in wintertime over eastern-central China have doubled over the past three decades. For the same period, surface wind speeds have dropped from 3.7 m/s to about 3 m/s and the mean number of cold air outbreaks has decreased from 7 to around 5 times per winter; relative humidity and the frequency of light wind events have also increased significantly. Weakening of the East Asian winter monsoon system appears to be responsible for these changes. The weakened East Asian winter monsoon circulation brings less cold and dry air to the region, reduces wind speed, and favors the formation of fog. The regional increase in atmospheric aerosol loading may also change the regional circulation pattern, creating favorable conditions for fog. This hypothesis is tested using the National Centers for Environmental Prediction (NCEP) reanalysis data and model simulations with the National Center for Atmospheric Research Community Climate Model (NCAR/CCM3). The analyses show that the 500hPa trough in East Asia has shallowed over the past three decades. Meanwhile, the surface Siberian high has weakened which is likely the cause for the diminishment in speed of the prevailing northwesterly winds and the reduction in intrusions of dry and cold air from the northwest. The increase in atmospheric aerosols was shown to heat the atmosphere and generates a cyclonic circulation anomaly over eastern-central China. This circulation anomaly also leads to a reduction in the influx of dry and cold air over eastern-central China. These effects are responsible for the increased convergence of water vapor therein. All these changes favor the formation and maintenance of fog over this region.

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