What caused the foggy weather in China?

Fog is water vapor condensate composed of small water droplets or ice crystals floating in the air. Fog is produced in the near-surface layer of the atmosphere. Since fog is water vapor condensation, we should look for its reason from the conditions that cause water vapor condensation. Water vapor in the atmosphere is saturated for two reasons:

First, because of evaporation, the water vapor in the atmosphere increases;

The other is the cooling of the air itself. Cooling is more important for fog. When there are condensation nuclei in the air, if water vapor continues to increase or melt, condensation will occur in saturated air. Fog is formed when condensed water drops reduce the horizontal visibility to less than 1 km.

In addition, excessive wind speed and strong disturbance are not conducive to the formation of fog.

Therefore, in the area that is conducive to the cooling of the lower air layer, if the water vapor is sufficient, the wind is mild, the atmospheric stratification is stable, and there are a large number of condensation nuclei, it is most likely to produce fog. Generally speaking, there are more opportunities to form fog in industrial areas and urban centers, because there are abundant condensation nuclei there.