Why is Dalian so foggy at night that it disappears during the day?

Because the appearance of fog and haze is very similar, many people confuse fog and haze, but fog and haze are really two completely different things. What is fog and what is haze?

Fog is the product of condensation (or sublimation) of water vapor in the air near the ground, and it is an aerosol system composed of a large number of tiny water droplets or ice crystals suspended in the air near the ground. The meteorological definition of fog is that a large number of tiny water droplets float in the air, often milky white, making the horizontal visibility less than 1.0 km. Haze refers to dust, sulfuric acid, nitric acid, organic hydrocarbons and other particles in the air that make the atmosphere turbid, blur the vision and worsen the visibility. If the horizontal visibility is less than 10 km, the visual field obstacle caused by this nonaqueous aerosol system is called haze or haze, and the Hong Kong Observatory calls it dust-haze.

So what is haze weather?

Haze weather is a new weather phenomenon since the development of human industry, and it is a mixture of fog and haze. When the relative humidity is high in the morning or at night, fog forms; During the day, the temperature rises, the humidity drops, and it gradually turns into smog. Fog and haze can reduce visibility. The difference is that the relative humidity is not large when the smog occurs, and it is close to saturation or saturation when the smog occurs. Haze is when the relative humidity is less than 60% and the visibility is less than 10 km, the atmosphere is turbid, which leads to blurred vision. Fog appears when the relative humidity is greater than 90% and the visibility is less than 1.0 km, which leads to blurred vision. Therefore, the mixture of haze and light fog causes atmospheric turbidity, blurred vision and poor visibility, which mostly occurs under the condition of relative humidity of 60% ~ 90%, but its main component is haze. 、

How is the smog formed?

Among them, the conditions of fog generation are: 1. The humidity at low altitude is high, and the air is close to saturation; Second, the atmospheric stratification is very stable (atmospheric stability is F), the wind speed is small, the wind force is only one or two, the air does not produce convection, and the low-level water vapor floats in this area and does not spread around; Third, there are cooling conditions. Every autumn and winter, the winds in the North China Plain, the middle and lower reaches of the Yangtze River and the Sichuan Basin in China are very small. Atmospheric stratification is stable, usually with inversion layer; In some areas, due to the influence of surface precipitation and water vapor evaporation, the relative humidity of near-surface air increases. The above areas are partly cloudy at night, the sky is clear, the radiation cooling rate is obvious, and the wet air is saturated and condensed, forming fog. Under the action of external factors, it can accelerate the formation of fog, such as dust, smoke, fine particle pollution, etc., and it is easy to make the fog thicker.

The conditions of smog are as follows: 1. Control the stability of local air mass; Second, there are a lot of fine smog particles such as dust, sulfuric acid and organic hydrocarbons in the air, which makes the atmosphere turbid. When there are a lot of smog particles in the air, the atmosphere is turbid, which leads to blurred vision and poor visibility. If the horizontal visibility is less than 10 km, there will be haze or haze weather. The appearance of smog shows that the atmosphere has been polluted. In practice, smog is often accompanied by atmospheric inversion (a stable atmospheric stratification phenomenon). Because the appearance of inversion layer will aggravate the environmental air pollution, which will lead to haze weather to a certain extent. This is because the inversion layer is a very stable gas layer, which hinders the upward and downward diffusion of airflow and forms a fan-shaped pollution zone in the air. Once the inversion layer subsides, it will produce smoke pollution for a short time, thus aggravating the degree of ground air pollution.

Judging from the conditions of smog, all three are affected by weather or climate, and it is difficult to control them artificially at present; Among the two conditions of haze, the stability of local air mass is influenced by weather or climate, so it is difficult to control it artificially. A large number of fine particles in the air, such as dust, sulfuric acid and organic hydrocarbons, mainly come from man-made air pollutants, with emphasis on motor vehicle exhaust, coal-fired flue gas, dust and other pollutants. In addition, it is also related to burning plant straws in rural fields in some areas. In the stable weather situation, pollutants in the air are not easy to spread horizontally and vertically, which makes pollutants accumulate in the near-surface layer of the atmosphere, which leads to more and more serious pollution, which is an important reason for the large-scale smog in central and eastern China.

Original title: What you can't help but know about smog

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