Why is there fog?

At night or in the morning, when the ground temperature is low, the water vapor in the air liquefies into small water droplets and forms fog. The conditions for fog formation are cooling, humidification and concretion. Fog is not "down", but the water vapor of the air itself.

If the air humidity is too high, when the temperature drops, the water vapor in the air will condense into fog. So fog usually appears in winter, and cities and basins near rivers are the easiest to form. Because of the high humidity in those places, the water vapor is not easy to disperse.