Why is there foggy weather?

Generally speaking, fog appears when the relative humidity is greater than 70%, and haze appears when the relative humidity is less than 70%. When smog occurs, visibility is less than10km, 5-8km is moderate smog, less than 3-5km is severe smog, and less than 3km is severe smog.

Formation of haze weather

Haze refers to the whole atmospheric turbidity phenomenon in the atmospheric boundary layer and even the lower troposphere, and the visibility is below 10 km. The atmospheric aerosols that form haze weather mainly come from natural emissions and human activities, but the total amount of aerosol particles emitted by natural emissions and human activities is basically stable for a period of time, so why sometimes severe haze weather occurs, and sometimes it is blue sky and white clouds? The decisive control factor is meteorological conditions. When haze weather appears, it is usually accompanied by still wind, strong sunshine and low relative humidity. Severe haze weather occurs under the condition of strong inversion of boundary layer without exception. The inversion layer is like a pot cover, which limits the diffusion and dilution of substances in it. On the other hand, the development of urbanization and industrialization has changed the attributes of the underlying surface and the physical structure of the urban atmospheric boundary layer. Generally speaking, the smog is caused by poor diffusion conditions in the city, weak cold air and weak wind speed. At this time, various pollutants in the city cannot spread in time and accumulate near the ground. If the weather is dry and rainy, the sunshine is strong, and the humidity is low, various photochemical reactions between pollutants are prone to form smog.

Automobile exhaust is an important factor in the formation of smog, so the smog situation will be particularly serious on the main roads with busy traffic, and the visibility is lower than that in other places.