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The secret of the fog
In the morning, the fog is thick, the sky and the earth are shrouded in white, the roads are gone, the woods are gone... How is the fog formed? Which places in my country have the most fog? Let’s explore the secrets of the fog together.
The formation of fog
Meteorological experts define fog as: A natural phenomenon in which water vapor in the air condenses into fine water droplets suspended in the air, making the ground level visibility less than 1 kilometer. When the fog is very thick and the horizontal visibility is less than 500 meters, it is called heavy fog; when the fog is light and the horizontal visibility is greater than 1 km but less than 10 km, it is called light fog.
How is fog formed? First, the water vapor in the air must reach a saturated state, that is, the relative humidity must reach or be close to 100%. Secondly, there must be cooling. When the temperature drops, water vapor in the air will condense. If the air is humid enough, more water vapor will condense. When enough water molecules combine with tiny dust particles in the air, they form tiny water droplets suspended in the air, forming fog.
Under normal circumstances, the temperature is relatively high during the day and the air can hold more water vapor, so it is not easy to form fog. At night, when the temperature drops, the air's ability to hold water vapor weakens, and the air near the ground becomes colder. The water vapor in the air quickly reaches a saturated state, and the excess water vapor condenses into fog. This is why there are so many early mornings in autumn and winter. The cause of heavy fog.
Fog family inventory
There are many types of fog, and we can use the fog family to describe them. Below, four main types of fog are introduced based on the frequency of appearance of fog family members:
1
Radiation fog
The boss of the fog family is called radiation fog. Just listen to the name and you will know that the formation of radiation fog is related to heat radiation. It is a fog formed when the air near the ground is cooled by radiation at night and the humid air cools to saturation. Radiation fog generally appears in the latter half of the night or early in the morning, and has a small scope. It will have a great impact on urban traffic, highways, airport flight takeoffs and landings, and land and water shipping.
2
Advection fog
The second child in the fog family is called advection fog. It is a fog formed by warm and humid air flowing to the cooler ground (or water surface) and gradually cooling down. Different from radiation fog, advection fog may appear at any time of the day, but it must be formed with the appropriate wind direction and speed. If the wind continues, it will persist for a long time, so sometimes the advection fog generated at sea can last for several days. Although it is only the second largest, advection fog is larger in size than its older brother, radiation fog—its range is hundreds of kilometers or more, and its intensity is much greater than that of radiation fog.
3
Evaporative fog
The third member of the fog family is called evaporative fog, and its "body shape" is thin. From the perspective of the production process, it is somewhat similar to "Second Brother" advection fog, which is formed by the parallel flow of air. However, it is formed by the flow of cold air onto warm water. When there is a big difference between the air temperature and the water temperature, the evaporated water vapor on the water surface is cooled and condensed, forming fog. Evaporative fog has a small range and weak intensity, and often occurs on lakes or rivers on cold mornings in late autumn.
4
uphill fog
The fourth is called uphill fog.As the name suggests, this kind of fog generally appears in mountainous areas. It is caused by moist air rising along the hillside. Uphill fog mostly begins to form at the foot of the mountain or on the mountainside. Like advection fog, its "body" is large and often covers a wide range. When it stays in the valley, it often forms a beautiful sea of clouds.
In addition to the four members introduced above, the fog family also includes frontal fog , valley fog , ice fog, smoke, etc.
Places where heavy fog frequently occurs
Now that we have clarified the causes and types of fog, let’s look for the places with the most fog. In the Tang Dynasty , the writer Han Yu wrote in an article titled "Discussing the Book of Taoism with Wei Zhongli": " In the middle of Shu, the mountains are high and foggy, and the sun is rarely visible. Every time the sun rises, "This sentence means that the mountains in Sichuan are high and foggy, and the sun is rarely seen. When the sun finally comes out, the dogs don't know what it is, so they go crazy. Barking, this is the origin of the idiom "Sichuan dog barking at the sun".
The reason why heavy fog occurs frequently in the Sichuan Basin is, first of all, because in spring, autumn and winter, after the cold air from the north invades Sichuan from the south, it cannot spread out to the surrounding areas, so it stays over the basin, forming a film of cold air. A strange phenomenon often occurs 1000 to 1500 meters above this layer of cold air film: As the altitude increases, the temperature not only does not drop, but increases, this layer of air is also called the inversion layer . It is like a big pot lid covering the Sichuan Basin, making it difficult for low-level water vapor to escape, laying the foundation for the formation of fog. Secondly, in winter, there is often no wind or very little wind near the ground in the Sichuan Basin, which is conducive to the formation of fog. In addition, the rivers here are crisscrossed and there is abundant water vapor. Therefore, the special terrain causes heavy fog to occur frequently in the Sichuan Basin.
The place with the most foggy days in my country is Mount Emei in the southwest of the Sichuan Basin. The average annual foggy days at the Golden Summit of the mountain reaches 322 days, and the most foggy days reach 338 days. Fog appears almost every day. Chongqing, also located in the Sichuan Basin, also has a lot of fog. There are 365 days in a year, with an average of 103 foggy days in Chongqing and a maximum of 206 foggy days. Therefore, Chongqing is known as the "Fog City of China".
Cleverly getting water from the fog
The appearance of heavy fog will bring inconvenience to people's travel and have a bad impact. But the fog that appears in desert areas has brought life to many animals and plants.
The Namib Desert is located in the southwest corner of Africa, close to the sea, but extremely dry. The strange thing is that it is not as desolate as we imagined: there are lush shrubs growing on some tall sand dunes, and there are elephants, ostrich, zebra, rhino, lion, hyena, jackal. and other animals have settled here for a long time.
It turns out that although rainfall in the Namib Desert is extremely rare, the airflow over the ocean is very humid. At night, cold air flows slowly northward along the coast. Once the saturated moist air encounters cold, the water vapor will condense to form fog. Therefore, there is often heavy fog there. On average, there will be a heavy fog every 10 days or so. These fogs sometimes carry more moisture than rain. Plants that have grown in arid environments for a long time have already learned how to absorb water from fog. They spread their wide leaves and let the water droplets condensed from the mist adhere to the leaves. When there are enough water droplets, they will slowly flow to the roots. Animals living in the desert also know how to get the water of life from the fog. For example, when a beetle in the desert wants to drink water, it will climb up the sand dunes in the early morning, stand with the wind behind it, cross its hind legs, and lower its head. In this way, the mist will condense on it, and finally turn into Drops of water dripped into his mouth.
Similarly, the Atacama Desert in Chile is also extremely arid. Its special geographical location causes cold currents to often form heavy fog there. In order to survive, local plants have learned how to use their leaves to capture water from the fog. Local residents were inspired by this and designed a fog-catching net specially used to collect fog water. People hang thin and dense nets vertically in the wild, and install a pipe at the lower end of the net. The other end of the water pipe is directly connected to the bucket. In the early morning, when I opened the bucket, I saw clear and bright water continuously flowing out of the water pipe and into the bucket... On foggy days, A village's fog-catching net can "capture" fog every day With 10,000 liters of "tap water", the villagers not only have enough to drink, they can also shower and bathe.
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Source: National Youth Work Committee
Editor: Zhao Caixia
