radiation fog

Radiation fog is a type of fog that forms over land on calm, clear nights. It occurs when the ground loses heat through radiation, cooling the air directly above it to below its dew-point temperature. This cooling causes water vapor in the air to condense into tiny water droplets, forming fog. This phenomenon is most common in valleys or damp areas where the cooling effect is more pronounced.12

Fog, in general, is a cloud of water droplets near ground level that reduces visibility to less than 1,000 meters (3,281 feet). It forms when air becomes saturated with moisture, often due to cooling processes like radiation, adiabatic expansion, or mixing of air masses with different temperatures.342