Why is it easy to have foggy weather on sunny days in winter in Sichuan Basin?

Sichuan basin is located in the subtropical monsoon climate zone, and the climate should be warm and humid. In addition, the special topography of the basin: the southeast of the closed basin is relatively low, which is conducive to the entry of water vapor, while the northwest mountain is relatively high, which is not conducive to the loss of water vapor, resulting in high air humidity; As well as the closed terrain, the wind speed in Sichuan Basin is low all the year round, which is one of the areas with the lowest annual average wind speed in China. Therefore, the Sichuan Basin is prone to foggy weather due to its high water vapor and low wind.