In which season does dense fog appear?

Dense fog: In spring and autumn and rainy season, under the influence of high-pressure backflow before the front, large-scale and long-lasting dense fog often appears. Dense fog will hinder visibility. If the visibility is less than 200 meters, it will affect land or sea traffic. The Meteorological Bureau will issue a "special report on dense fog" in areas or sea areas where dense fog appears.

cause

When the visibility in the atmosphere is lower than 1 km due to the condensation of suspended water vapor, meteorology calls this weather phenomenon fog. Generally speaking, there is a lot of fog in autumn and winter mornings. Why? We know that when the water vapor contained in the air reaches the maximum, it reaches saturation. The higher the temperature, the more water vapor is contained in the air. 1 m3 air, the maximum water vapor capacity is 6.36g at 4℃; When the temperature is 20℃, the maximum water vapor content in 1 m3 air is17.30g. If there is more water vapor in the air than saturated water vapor at a certain temperature, the excess water vapor will condense out. When there is enough water, it will combine with tiny dust particles in the air, and at the same time, water molecules themselves will combine with each other and become small water droplets or ice crystals. The water vapor in the air exceeds saturation and condenses into water droplets, which is mainly caused by the temperature drop. If the ground heat is lost, the temperature drops, and the air is quite humid, then when it cools to a certain extent, part of the water vapor in the air will condense out and become many small water droplets suspended in the air layer near the ground. This is fog. Both it and clouds are caused by temperature drop, and fog can actually be said to be a cloud near the ground. The temperature is higher during the day, so the air can hold more water vapor. But at night, the temperature drops and the capacity of water vapor in the air decreases, so some water vapor condenses into fog. Especially in autumn and winter, due to the long nights, there are not many opportunities without Feng Yun, and the ground heat dissipation is faster than that in summer, which makes the ground temperature drop sharply, so that the water vapor in the air near the ground can easily reach saturation in the middle of the night, condense into small drops and form fog. The morning temperature in autumn and winter is the lowest and the fog is the thickest.

Formation condition

The conditions for fog formation are cooling and humidification to increase the water vapor content. This is caused by radiation cooling, which often occurs at night and early morning when the water vapor near the ground is abundant and stable or there is inversion, which is called radiation fog in meteorology; The other is the fog formed by the horizontal movement of warm and humid air and the gradual cooling after passing through the cold ground or water surface, which is called advection fog in meteorology; Sometimes the fog formed by two reasons is called mixed fog. It can be seen that it is late autumn and early winter that meet these conditions, especially in the morning of late autumn and early winter. We can also see evaporation fog. That is, cold air flows through warm water. If there is a big difference between the air temperature and the water temperature, a lot of water vapor will evaporate from the water surface, and the cold air near the water surface will condense into fog. At this time, there is often an inversion layer on the fog layer, otherwise convection will make the fog dissipate. Therefore, the evaporation fog has a small range and weak intensity, and generally appears around the pond in the second half of the year.

The smog in the city is caused by another reason, and that is human activities. Morning and evening are the peak hours of heating boilers. Under the condition of low air pressure and low wind speed, a large number of pollutants, such as smoke, suspended solids and automobile exhaust, are difficult to diffuse, and it is easier to form smoke (fog) when combined with water vapor in the lower air, and this smoke (fog) often lasts for a long time. The fog dissipates because, firstly, the fog drops evaporate due to the warming of the underlying surface; Second, the increase of wind speed will blow the fog away or lift it into clouds; Then turbulent mixing, water vapor upload, heat transfer, and evaporation of droplets near the ground. The duration of fog is mainly related to the humidity of the local climate: generally speaking, the short fog in arid areas dissipates within 1 hour, while the long fog in humid areas is the most common and lasts for about 6 hours.