Q: What causes advection fog?

Arab Weather - Advection fog (or radiation fog) is formed due to the cooling of the Earth's surface at night as a result of heat loss by radiation. This type of fog occurs when the sky is clear and the winds are calm or light, allowing heat to radiate from the Earth's surface into outer space.

When the ground cools, the air in contact with it cools, causing the air temperature to drop to the dew point (the temperature at which the air is saturated with water vapor). The water vapor condenses and forms tiny droplets of water suspended in the air, which results in fog.

Advection fog is common on clear, cool nights, especially in low-lying areas such as valleys.