Abstract: The atmospheric boundary layer process is crucial to the formation, dissipation, and intensification of dense fog. However, studies of persistent, large-scale dense fog often overlook spatial differences in boundary layer structure among adjacent areas during fog events. To address this gap, the authors investigated a persistent dense fog event that occurred in Nanjing and its surrounding areas from 24 November to 3 December 2018. During this period, exceptionally widespread and persistent dense fog affected East China, while substantial spatial and temporal variations in fog formation and dissipation were observed among monitoring stations within a 60 km radius of Nanjing. This study used high-resolution, vertically integrated observations to capture abrupt changes in visibility and the evolution of vertical atmospheric structure during fog development and to establish predictive indicators for severe dense fog. The aim was to clarify the atmospheric boundary layer processes governing the life cycle of this 10-d continuous dense fog event in Nanjing. Results indicate that the synoptic conditions were characterized by a relatively zonal mid-to-high latitude circulation pattern, weak pressure gradients, stable atmospheric stratification, weak turbulent mixing, and limited convergence and divergence. During the event, three weak cold air incursions occurred and were accompanied by pronounced temperature inversions, often with multiple inversion layers. During the first stage, the fog was primarily radiation fog. Owing to topographic effects and the influence of upwind water bodies, Pukou Station experienced significantly shorter periods of low visibility than other regions. During the second stage, advection fog dominated, and several episodes of rapid fog intensification occurred. Weak cold air infiltration, southeasterly warm and moist advection, and a transient, ultra-low-level easterly jet with wind speeds exceeding 8 m s⁻¹ within the inversion layer produced a coupled dynamic–thermal effect that accelerated fog formation and vertical development. A pronounced change in water vapor density was observed near the fog top, and a fog-top height diagnostic index was established based on the vertical gradient of water vapor density. The fog-top height (200–500 m) consistently remained below the top of the inversion layer (400–800 m). Rapid intensification of severe fog was not necessarily accompanied by stronger temperature inversions, indicating that inversion strength alone has limited predictive value. In contrast, simultaneous sharp increases in liquid water content (＞0.1 g m⁻³) and water vapor density (＞8 g m⁻³) provided a 0.5–2.5 h lead time for predicting fog intensification, offering quantitative indicators for fine-scale dense fog forecasting.