Abstract: Based on the data observed with visibility observers, wind profile radars, and from intensive automatic stations and conventional meteorological observations, the synoptic diagnosis of the two foggy weather processes and their responses to boundary layer thermodynamic conditions in Liaoning province in 2018 were investigated.The results show that the causes of the two heavy patches of fogs are quite similar.The fog experience two development phases under similar weather conditions.The first phase of the fog is radiation fog when the central part of Liaoning province lies in the weak convergence zone, the fog appears in the southerly wind area where the seawater vapor is transported to the Yingkou-Shenyang area.Then a weak ascending cooling cooperates with radiative cooling to form a near-surface temperature inversion.At the same time, the dew-point deficit decreases, and the relative humidity increases, leading to the explosive development of the fog.In the second development phase, the cold advection invades the near ground, causing the near-surface temperature to drop and forming inversion, and the fog develops again.So, the low-level cold advection intrusion is an important aspect that cannot be ignored in fog prediction, and its arrival time and location are the key influencing factors of fine fog prediction.