Why is it often foggy in winter morning?

There is a limit to the water vapor that can be contained in the air. When it reaches the maximum limit, it is called water vapor saturation. The higher the temperature, the more water vapor is contained in the air. For example, in the air of 1 m3, the maximum water vapor that can be accommodated is 6.36g at 4℃; At the temperature of 20℃, 1 m3 of air can contain at most17.3g of water vapor. If the air contains more water vapor than water vapor at a certain temperature, the excess water vapor will condense out and become small water droplets or ice crystals.

If at 4℃, 1 m3 of air contains 7.36 grams of water vapor, then the excess 1 gram of water vapor will condense into water droplets. So if the water vapor in the air exceeds saturation, it will condense into water droplets, which is mainly caused by the temperature drop.

The loss of ground heat will reduce the ground temperature, and at the same time, it will affect the air layer near the ground, so that the air temperature will also decrease. If the air layer near the ground is quite humid, then when it is cold to a certain extent, some water vapor in the air will condense out and become many small water droplets suspended in the air layer near the ground. If there are too many small water droplets in the air layer near the ground, which hinders people's sight, fog will form.

The temperature is generally higher during the day, and more water vapor can be contained in the air. But at night, when the temperature drops, the amount of water vapor that can be contained in the air decreases. If there was more water vapor in the air at that time, some water vapor would condense into fog. Especially in winter, due to the long nights, sunny days and low winds, the heat dissipation on the ground is faster than that in summer, and the temperature near the ground drops sharply, so that the water vapor in the air layer near the ground can easily reach saturation, supersaturation and condense into small water droplets from midnight to morning, and float in the air near the ground to form fog. Therefore, it is often foggy in sunny mornings in winter. This fog meteorology is called "radiation fog".