How does fog condense?

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.

Conditions of fog formation

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.

Types of fog

1. Radiation fog: It mostly appears at night and early morning when the sun is shining, the breeze is blowing gently, and the water vapor near the ground is rich and stable or the temperature is reversed.

2. advection fog: Warm and humid air moves horizontally, passes through the cold ground or water surface, and gradually cools to form fog, which is called advection fog in meteorology.

3. 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 large amount of water vapor will evaporate on 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.

4. uphill fog: this is the fog produced by the humid air rising along the hillside and adiabatic cooling to supersaturate the air. This humid air must be stable and the slope of the hillside must be small, otherwise convection will form and it is difficult to form fog.

5. frontal fog: It often appears near the front where cold and warm air meet. Before and after, but mostly near the warm front. The fog in front of the front is formed because the raindrops in the warm air cloud above the front fall into the ground cold air and evaporate, which makes the air supersaturated and condensed. The fog behind the front is formed by warm and humid air moving to the area originally occupied by cold air before the warm front and cooling to supersaturation. Because the fog near the front often moves with the front, the army often uses this frontal fog to cover the troops and make sudden attacks on the enemy.

5. Mixed fog: Sometimes the fog formed by two reasons is called mixed fog.

6. Smog: 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 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 indicates sunny days, sometimes fog indicates rainy days, and it looks chaotic. However, as long as the characteristics of radiation fog and advection fog are mastered, observed in many aspects and carefully analyzed, the laws of fog, sunny and rainy weather can be accurately grasped, and the weather can be predicted. This is very 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 it has been reflected in many folk proverbs. For example, "Huangmei is foggy, rocking the boat doesn't ask for directions." This means that the fog at the turn of spring and summer is a harbinger of rain, so people also have the saying of "summer fog and rain". Another example is: "I can't see anyone in the fog, so I dare to wash my clothes." This means that winter fog is sunny, and so is autumn fog.

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

Why is it always the same fog, some rainy and some sunny?

This should be explained from the knowledge of meteorology. As long as there is a lot of water vapor in the lower air, when the temperature drops suddenly at night, the water vapor will condense into fog. Fog has radiation fog, that is, fog formed under relatively clear 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 heavy in the morning, and it 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, it's often "ten fogs and nine sunny days". You came to these conclusions.

In autumn and winter, after the cold air in the north goes south, as the weather turns fine and sunny, 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 is formed when warm and humid air from the ocean invades the mainland and suddenly encounters cold. When these warm and humid air currents meet the dry and cold air currents in 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 through fog. However, the above explanation of radiation fog and advection fog is only for the general situation. The relationship between fog and weather is not so simple, there are many complicated contents, which cannot be copied mechanically, but should be analyzed in detail. In other words, if we want to know fog accurately, we need observation, analysis and comprehensive judgment in many aspects.