Using the high spatiotemporal meteorological observation data from 339 traffic weather stations along highway, and 72 conventional observation stations, four heavy fog events during 1-9 December 2013, on 23 October 2015, 22 December 2015 and 12 February 2016 are analyzed. Based on 194 cases, climatic characteristics of extremely heavy fog in initiation and disappearance, microphysical characteristics of explosive enhancement and the triggering factors of heavy fogs are analyzed. The results show that visibility dropped sharply within very short time (in 30 min) because fog droplet number concentration, water content and droplet size increased remarkably, and fog droplet spectrum became broader. In addition, the sharp drop of temperature resulted from radiation reinforcement at night, the weak cold advection of underlying layer, the increase of evaporation after sunrise, and the lakeland wind effect made fog reinforce eruptively.