Abstract: The tendency of heavy fog events has increased rapidly in recent years in the Beijing region.A better understanding of the vertical structure and physical formation mechanism of these heavy fog events is limited due to a lack of high temporal resolution data of fog microphysics and atmospheric profiling data.In this study, we investigated the type, vertical structure and physical formation mechanism of 13 fog events in autumn and winter in Beijing based on the atmospheric profiling data of Microwave Radiometer Profiles(MWRPs) with 35 channels produced by Radiometrics Corporation and routine meteorological and satellite data.We classified the 13 fog events into advection fog and evaporation fog from their basic formation processes.Radiation fog as part of the persistent process was not listed alone. The average top height of advection fog was less than 1.0 km, and that of precipitation evaporation fog was between 0.5 km and 1.5 km.Advection fog in the Beijing region was mainly caused by the cooling process of warm and wet flow advection from the southwest and southeast moving over a cold underlying surface.The strong inversion formed was favorable to the sustained development of the fog events.Evaporation fog was caused by the cooling process of water vapor evaporated from falling raindrops, or the cooling process due to cold air advection.We also categorized the 13 fog events into those with single fog structure and those with fog-cloud structure, based on the vertical structure observed by MWRP.Some typical cases were further verified by satellite and surface meteorological observation data.The results show that advection fog and evaporation fog accounted for 69% and 31% of the 13 fog episodes, respectively, indicating that the heavy long-lasting fog events in Beijing were mainly caused by advection and precipitation processes, which was closely related to the abnormal weather process.The proportion of single-structure fog was only about 15%, while that with a co-existing structure of low-level fog and high-level cloud reached about 85%.Severe fog events, lasting for more than three days, were almost all characterized by a fog-cloud structure.