Why is it foggy?

Fog is the condensation phenomenon of water vapor in the air near the ground. There are two basic conditions for the formation of fog. One is that the air near the ground is rich in water vapor, and the other is that the ground temperature is low. There is a lot of water on the ground after the rain, which is evaporated, and the temperature is still low after the rain, so it constitutes a condition. The following are the classification and formation reasons of various fogs: the most common fog on land is radiation fog: this kind of fog is formed because the air is supersaturated by radiation cooling, which mainly appears in sunny, breezy, near the ground and in the morning with abundant water vapor. At this time, there is no cloud cover in the sky, the ground heat radiates rapidly, and the temperature near the ground layer drops rapidly. If there is too much water vapor in the air, it will soon reach supersaturation and condense into fog. In addition, the wind speed also has a certain influence on the formation of radiation fog. If there is no wind, there will be no exchange between the upper and lower air, and the radiation cooling effect only occurs in the air layer near the ground, which can only produce thin shallow fog. If the wind is too strong, the upper and lower air exchange is fast and the flow is large, and the temperature is not easy to drop a lot, it is difficult to achieve supersaturation. Only when there is a breeze of L-3m/s, communication with appropriate intensity can not only extend the cooling effect to a certain height, but also not affect the full cooling of the lower air, so it is most conducive to the formation of radiation fog. Radiation fog appears on a clear and cloudless night or morning. As soon as the sun rises, the air will return to unsaturated state with the increase of ground temperature, and the fog drops will evaporate and dissipate immediately. Therefore, the appearance of radiation fog in the morning often indicates good weather that day. "Morning fog covers the ground, although it dries rice" and "ten fogs and nine sunny days" refer to this kind of radiation fog. The second kind of fog is advection fog: when warm and humid air flows through the cold ocean or land, the fog condensed in the lower layer of the air due to contact cooling is advection fog. As long as there is a suitable wind direction and speed, once the fog is formed, it often lasts for a long time. If there is no wind, or the wind direction changes, the source of warm and humid air flow is interrupted, and the fog will dissipate immediately. The third kind of fog is steam fog: if the water surface is warm and the air is cold, when the temperature difference between them is large, the water vapor will continuously evaporate from the water surface, break into the cold air, and then condense out of the cold air to become steam fog. Generally, the warm ocean currents in the south enter the polar regions, and the cold air in the polar regions will cover the warm water surface and form steam fog. For example, there is a strong Gulf Stream warm ocean current in the North Atlantic Ocean, which often rushes into the Arctic Ocean, causing a large area of steam fog on the surface of the Arctic Ocean. Sometimes, the cold air in the Arctic stays on the ice, where the ice cracks, the warmer water under the ice is exposed, forming a local steam fog, which mostly appears in the Arctic region at high latitudes, so people often call it "Arctic smoke". In addition to the polar regions, cold air often covers warm water in inland lakeside areas. At night, the lake is warmer than the land surface. When the land breeze blows to the warm lake at night, relatively shallow steam fog will form on the lake. In autumn and winter, whenever the cold air goes south, in the clear Wan Li and windy morning, when the warm water surface has not had time to cool down, this steam fog will diffuse. The fourth kind of fog is uphill fog: this is the fog produced by 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. The fifth kind of fog is frontal fog: it often occurs 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.