Why is there fog?

There are generally three kinds of fog. First, the humidity at low altitude is high and the air is close to saturation. Second, the atmospheric stratification is very stable, the wind speed is small, the wind is only one or two levels, and the air does not produce convection. Low-level water vapor floats in this area and will not spread around. Third, it needs to be carried out under the cooling effect. The content of water vapor in the air is related to temperature. In order to reach saturation, the amount of water vapor contained in air per cubic meter is a function of temperature. That is to say, when 1 m3 of air contains 10g of water vapor, you can't see it when the temperature is 20℃ because it can't reach saturation, but when the temperature drops to 10℃, the water vapor in the air reaches saturation, and when the relative humidity reaches100. When the warm and humid air flow and the rising motion of the air continue to strengthen, the fog also changes into clouds with the rising motion, and snow is produced when it meets cold at high altitude. For example, the fog in Beijing on June 5438+February 3, 2002 was mainly caused by the special atmospheric circulation, that is, because there was almost no strong cold air activity for a period of time, North China was continuously affected by the weak convergence zone, resulting in a weak updraft in the air below 200 meters in the sky, which was beneficial to the evaporation of the surface and the increase of low-altitude humidity. At the same time, above 200 meters in the air, due to the high pressure, there is a downdraft, where the updraft meets the downdraft, forming a layered and stable inversion layer. According to normal atmospheric activities, it should be warm from the bottom and cold from the top, but that year was just the opposite, the bottom was cold and the top was warm. The appearance of inversion layer effectively prevents the diffusion of water vapor from the lower layer to the upper layer, and makes the air humidity in the near-surface layer below 200 meters increase to saturation, especially in partially cloudy weather, and the cooling is particularly obvious. When the relative humidity of air reaches above 90%, fog will appear. In addition, under the influence of external factors, the formation of fog will also accelerate. Fog needs some small condensation nuclei, and dust, smoke and polluted fine particles in big cities act as accomplices, which makes the fog thicker. Fu Jianguo, a senior engineer, said that fog often appears in Shahe area of Beijing-Changzhou Expressway, mainly because there is a reservoir on the east side of the expressway and there are two rivers nearby, and the air humidity is relatively high.