Fog is ever-changing and complicated, but it is nothing more than radiation fog and advection fog. Although there are various phenomena, the essence is the same: water vapor condenses when it meets cold. Sometimes fog indicates sunny days, sometimes fog indicates rainy days, and it looks chaotic. However, as long as the characteristics of radiation fog and advection fog are mastered, observed in many aspects and carefully analyzed, the laws of fog, sunny and rainy weather can be accurately grasped, and the weather can be predicted. This is very useful for agriculture, transportation, aerospace and navigation.