Two high-impact processes of advection fog on 31 December 2021 and radiation fog on 19 January 2022 occurred in Hunan. Using conventional weather observations, highway disaster data, and NCEP reanalysis data, the weather background, meteorological elements, physical quantity fields, their effects on highways, and the water vapour source of HYSPLIT backward trajectory simulation of the two processes are compared and analysed. The results show that during the two fog processes, the middle and high latitudes of 500 hPa show a circulation situation of two troughs and one ridge, there are multi-layer inversions, the atmospheric stratification is stable, and the relative humidity of the ground is above 98%. In terms of water vapour transport, there is a rapid sinking transport of water vapour in the north. The main and strongest water vapour transport in the near-surface channel is a short-distance and slow-moving channel entering Hunan through the mountainous area of western Hubei, and the contribution rate of water vapour flux accounts for more than 50%, indicating that the continuous and slow infiltration of cold and wet air near the ground layer is conducive to the cooling and condensation of the gas block to form droplets. There are obvious differences in the low-level, ground situation and meteorological elements during the two heavy fog processes. The first advection fog process has a strong warm advection development at 700-600 hPa, and the ground is controlled by weak cold high pressure, accompanied by the thickening of the inversion layer. The second radiation fog process is controlled by the northerly airflow behind the trough at 500 hPa, and the ground is a weak pressure field. During the fog, the whole layer turns to cold advection. The temperature during the advection fog process is lower than that during the radiation fog process. There are obvious differences in water vapour sources and the height of water vapour channels between the two processes. The radiation fog transports cold air in the northern channel, two in the upper and two in the lower layers. The dry cold air is quickly transported to the fog area with two high-level water vapour channels, and the wet cold air is slowly transported to the fog area with two low-level channels. The water vapour channels of advection fog input half from the north and half from the south. The southern channel transports warm and humid air, and the northern channel transports cold air. The analysis and research results are helpful for forecasters to improve the accuracy of forecasting and early warning of two types of fogs. At the same time, it provides a reference for the precise control of fog weather in the transportation department.