How is the fog caused?

The reason of fog formation is that when there are condensation nuclei in the air, saturated air will condense if water vapor continues to increase or melt away. Fog is formed when condensed water drops reduce the horizontal visibility to less than 1 km.

Fog refers to water vapor condensate composed of water droplets or ice crystals suspended in the near-surface atmosphere, which is a common weather phenomenon. When the temperature reaches the dew point temperature (or near the dew point temperature), the water vapor in the air condenses to form fog. When the temperature is above freezing point, water vapor condenses into water droplets. When the temperature is below freezing point, water vapor condenses directly into solid ice crystals, such as ice fog.

Because the dew point is only affected by air temperature and humidity, there are two main reasons for the formation of fog: first, the water vapor in the air increases greatly, which makes the dew point rise to the air temperature, thus forming fog, such as steam fog and frontal fog; Second, when the temperature drops below the dew point, fog is generated, such as advection fog and radiation fog.

When the temperature is high during the day, the air can hold more water vapor. But at night, the temperature drops, and the tolerance for water vapor in the air drops, so some water vapor will condense into fog.

Especially in autumn and winter, due to the long nights, the ground heat dissipation is faster than that in summer, which makes the ground temperature drop sharply, making it easy for the water vapor in the air near the ground to reach saturation from midnight to morning, condense into small water droplets and form fog. The temperature is the lowest in the early morning of autumn and winter, and it is the thickest moment of fog.