Why is there dense fog on a sunny day?

Generally speaking, there is a lot of fog in autumn and winter mornings. 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. 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. Look at this situation, it should be formed like this. 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. This phenomenon is very common in many high-altitude valley areas, such as Jinsha River Basin, where the mountains are steep and there are very low and warm valleys.