Why is there so much fog in the mountains at night and in the morning?

There are three definitions of fog: 1. When the visibility is lower than 1 km due to the condensation of suspended water vapor in the atmosphere, meteorology calls this weather phenomenon fog. 2. Fog is a cloud close to the ground. 3. Fog is an aerosol composed of tiny droplets suspended in the atmosphere. (Description in "Environmental Monitoring") When the amount of water vapor the air holds reaches the maximum, it reaches saturation. The higher the temperature, the more water vapor the air can hold. When the temperature is 4°C, 1 cubic meter of air can hold up to 6.36 grams of water vapor; when the temperature is 20°C, the maximum amount of water vapor that can be contained in 1 cubic meter of air is 17.30 grams. If the air contains more water vapor than the saturated amount of water vapor at a certain temperature, the excess water vapor will condense out. When enough water molecules combine with the tiny dust particles in the air, the water molecules themselves will also interact with each other. When bonded, they become small water droplets or ice crystals. The water vapor in the air exceeds the saturation level and condenses into water droplets. This is mainly caused by the drop in temperature. This is also the reason why mornings in autumn and winter are foggy. If the ground loses heat, the temperature drops, and the air is quite humid, then when it cools to a certain level, part of the water vapor in the air will condense out and turn into many small water droplets, suspended in the air layer near the ground, forming Fog. Both it and clouds are caused by a drop in temperature, and fog can actually be said to be clouds close to the ground. The temperature is higher during the day and the air can hold more water vapor. But at night, the temperature drops and the air's ability to hold water vapor is reduced. Therefore, some of the water vapor will condense into fog. Especially in autumn and winter, because the nights are long and there are more opportunities for cloudless winds, the ground dissipates heat more quickly than in summer, causing the ground temperature to drop sharply. This makes the water vapor in the air near the ground easy to evaporate from late night to morning. It reaches saturation and condenses into small water droplets, forming fog. The early morning temperatures in autumn and winter are at their lowest, which is when the fog is thickest. [Edit this paragraph] Formation conditions The conditions for fog formation include cooling, humidification, and condensation nuclei. Increase water vapor content. This is formed by radiative cooling. It mostly occurs at night and in the early morning when it is sunny, gentle, and the water vapor near the ground is relatively abundant and stable, or there is a temperature inversion. Meteorologically, it is called radiation fog; the other is warm and moist air. The fog formed by moving through the cold ground or water surface and gradually cooling is called advection fog in meteorology; sometimes the fog formed by two reasons is called mixed fog. It can be seen that those conditions that meet these conditions are late autumn and early winter, especially the mornings in late autumn and early winter. We can also see an evaporative fog. That is, cold air flows through warm water. If the air temperature is very different from 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 cause the fog to dissipate. Therefore, evaporative fog has a small range and weak intensity, and generally occurs around ponds in the second half of the year. Smog in cities is caused by another reason: human activities. Morning and evening are the peak periods for heating boilers. Pollutants such as smoke and dust suspended in large quantities and vehicle exhaust are not easy to diffuse under conditions of low air pressure and light wind. Combined with water vapor in the lower air, it is easier to form smoke ( Fog), and this kind of smoke (fog) often lasts longer. The reasons for the dissipation of fog are: first, due to the heating of the underlying surface, the fog droplets evaporate; second, the wind speed increases, blowing the fog or lifting it into clouds; third, the turbulent mixing, water vapor upwards, heat transfer downwards, near-surface fog Drops evaporate. The duration of fog is mainly related to the dryness and humidity of the local climate: generally speaking, short fogs are common in arid areas and dissipate within an hour, while long fogs are most common in humid areas and can last for about 6 hours. In addition, there must be no wind when there is fog. Otherwise, the small water droplets in the air will be blown away by the wind, and the fog will not gather.