How are fog and rain formed?

Clouds composed of liquid water droplets (including supercooled water droplets) are called water clouds. If the conditions for cloud droplets to grow into raindrops are met in the water cloud, and the raindrops have a certain falling speed, then it is rain or Mao Mao rain that falls. Clouds composed of ice crystals are called ice cloud, and clouds composed of water droplets (mainly supercooled water droplets) and ice crystals are called mixed clouds. Ice crystals or snowflakes falling from ice cloud or mixed clouds fall into the gas layer above 0℃, and after melting, they also fall to the ground as raindrops, forming rainfall.

In the process of rain formation, large water droplets play an important role. When the radius of water droplets increases to 2-3 mm, it is difficult to maintain such a large water droplet by the attraction between water molecules, and it is easy to be split by the impact of airflow on the way down, through a "chain reaction." Make big water droplets fall, and small water droplets continue to exist, forming new big water droplets. This is an important reason for the formation of rain in water clouds and mixed clouds with strong updraft.

Fog formation

Fog and clouds are water vapor condensates composed of small water droplets or ice crystals floating in the air, but fog is generated in the near layer of the atmosphere, while clouds are generated in the higher layer of the atmosphere. Since fog is water vapor condensation, we should look for its reason from the conditions that cause water vapor condensation. There are two reasons for water vapor saturation in the atmosphere: first, evaporation increases water vapor in the atmosphere; 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.