Two heavy fog processes occurred in March and June 2008 in Dalian and the coastal areas are analyzed by using the sounding data from GTS1 digital radiosondes, conventional observation data, and automatically observed visibility data. The structural features of ABL temperature and wind fields are analyzed. The results show that both radiative fog and advective fog usually form under the conditions of zonal circulation, abundant water vapor and appropriate thermal state supplied by the middlelevel southeast wind. However, there is obvious difference between them. The advection cooling near the surface by the northeasterly wind is an important condition for the formation of radiation fog, leading to the dispersion of advection fog. The surface southeast wind is a condition for the generation of advection fog, but it contributes to the disappearance of radiation fogs. The inversion in the radiation fog is thick and strong, but weaker or no inversion in advection fogs. Radiation fogs evolve from occurrence to dispersion while the stratification varies from stable to unstable, but from unstable to stable for advection fogs.