Final answer:
Radiation fog forms on cool, clear, calm nights due to the ground rapidly losing heat through radiation, leading to air near the surface cooling and condensing into fog.
Step-by-step explanation:
The type of fog that forms on cool, clear, calm nights when the earth's surface cools rapidly by radiation is known as radiation fog. This phenomenon occurs due to the cooling of land after sunset by thermal radiation in calm conditions with clear skies. The cool surface of the earth lowers the air temperature near the ground, causing the air temperature to reach its dew point and water vapor to condense into fog.