Why is it foggy in autumn and winter?

when the visibility is less than 1km due to the condensation of suspended water vapor in the atmosphere, meteorology calls this weather phenomenon fog, and when the visibility is between 1km and 1km, it is called light 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 can be contained in the air. 1 cubic meter of air, when the temperature is 4℃, the maximum amount of water vapor can be 6.36 grams; When the temperature is 2℃, the maximum water vapor content in 1 cubic meter of air is 17.3 grams. If the water vapor contained in the air is more than the saturated water vapor at a certain temperature, the excess water vapor will condense out. When enough water molecules are combined with the tiny dust particles in the air, the water molecules themselves will bond 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, which will be suspended in the air layer near the ground and form 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 relatively high during the day, and more water vapor can be contained in the air. But at night, the temperature drops, and the capacity of water vapor in the air decreases, so some water vapor will condense into fog. Especially in autumn and winter, because the nights are long, and there are more chances that there is no Yun Feng, the ground heat dissipation is faster than that in summer, so that the ground temperature drops sharply, which makes the water vapor in the air near the ground easily reach saturation in the middle of the night and condense into small water droplets and form fog. The lowest temperature in the morning in autumn and winter is the thickest moment of fog.

conditions for fog formation

the conditions for fog formation are cooling, humidification and condensation nuclei. Increase the water vapor content. This is formed by radiation cooling, which mostly appears in the 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 warm and wet air moving horizontally and gradually 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 what meets these conditions is late autumn and early winter, especially in the morning of late autumn and early winter.

we can also see an evaporation fog. That is, cold air flows through the warm water surface. If there is a big difference between the air temperature and the water temperature, a large amount 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, which generally occurs around the pond in the second half of the year.

smog in cities is caused by another reason, that is, human activities. Morning and evening are the peak periods of heating boilers. Under the conditions of low air pressure and low wind, a large number of pollutants, such as smoke, suspended solids and automobile exhaust, are not easy 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 away or lift the fog into clouds; Then there are turbulent mixing, water vapor upload, heat transfer, and evaporation of fog droplets near the surface. The duration of fog is mainly related to the local climate: generally speaking, the short fog in arid areas dissipates within 1 hour, while the long fog is the most common in humid areas and lasts for about 6 hours.

The relationship between fog and weather

Fog is ever-changing and complicated, but it is nothing more than radiation fog and advection fog. Although there are various phenomena, the essence is the same: water vapor condenses when it meets cold. Sometimes fog predicts sunny weather, and sometimes fog predicts rainy weather, which seems chaotic. However, as long as we master the characteristics of radiation fog and advection fog, observe it in many ways and analyze it carefully, we can accurately grasp the laws of fog, sunny weather and rainy weather, so as to predict the weather. This is useful for agriculture, transportation, aerospace and navigation.

Fog is closely related to future weather changes. Since ancient times, the working people in our country have known this truth and reflected it in many folk proverbs. For example, "Huangmei is foggy, so you don't ask for directions when you rock the boat." This means that the fog at the turn of spring and summer is a harbinger of rain, so there is also a saying of "summer fog and rain" among the people. Another example is: "No one is seen in the fog, so wash clothes boldly." This means that winter fog is sunny, so is autumn fog.

to know the sky by fog accurately, we must also look at the duration of fog. Radiation fog is caused by cold weather and condensation of water vapor, so when the temperature rises during the day, it disappears and the weather is fine; On the other hand, "the fog is rain." If the fog doesn't disperse during the day, it may be rainy the next day, so the folk proverb says, "The fog doesn't last until noon, and the rain will ring after noon."

why is it also foggy, with some signs of rain and some signs of sunshine?

this should be explained from the knowledge of meteorology. As long as there is a lot of water vapor in the lower air, the water vapor will condense into fog when the temperature drops suddenly at night. Fog has radiation fog, that is, fog formed under relatively sunny and stable conditions. As long as the sun comes out and the temperature rises, the fog will naturally disappear. In this regard, the folk saying is: "The fog is thick in the morning, and the weather will be fine for a long time." "The sun is shining in the fog, and the stone is broken." "It's foggy in the morning, even though you are drying rice." When people see radiation fog, they are often "ten fogs and nine sunny days". You come to these statements.

In autumn and winter, after the cold air in the north moves south, with the weather turning fine and the sun shining, the moisture content in the air gradually increases, which is easy to form radiation fog, so the fog in autumn and winter can often predict the good weather tomorrow.

The fog in spring and summer is different. Most of it comes from the warm and humid air flow at sea. When it hits the cold ground, the air in the lower layer becomes cold, and the water vapor condenses into fog. This fog is called advection fog. It was formed when warm and humid air from the sea invaded the mainland and suddenly met with cold. When these warm and humid air currents meet the dry and cold air of the mainland, it will naturally be rainy. So the fog in spring and summer indicates rainy weather.

Fog is closely related to the weather, so we can know the change of the weather by the fog. However, the above explanation about radiation fog and advection fog is only about the general situation. The relationship between fog and weather is not so simple, and there are many complicated contents, so we can't copy it mechanically, but analyze it in detail. That is to say, to accurately understand the fog, but also to make various observations,