What phenomenon is described as Radiation Fog?

Radiation fog is specifically the result of the cooling of the earth's surface, which typically occurs during the night when temperatures drop, especially under clear skies. As the ground loses heat through radiation, the air in contact with the surface cools down, and if this air cools to its dew point, moisture condenses into tiny water droplets, creating fog.

This phenomenon is prominently seen in valleys or low-lying areas where cooler temperatures can settle. The cooling process occurs effectively when there is minimal wind, allowing the air near the surface to become saturated and form fog. Therefore, the characteristic nature of radiation fog is closely tied to the surface cooling, which makes it distinct from other types of fog formed through different mechanisms.

In contrast, the other options describe various conditions that lead to different types of fog. For example, evaporation fog occurs when warm moist air meets cooler air, resulting in water vapor condensing, while the transition of moist air over cold land describes advection fog. Warm and humid conditions often lead to other forms of fog, such as steam fog, not radiation fog, which relies fundamentally on surface cooling.