Final answer:
The most prevalent warning for the formation of radiation fog is cooling of air near the ground below the dew point temperature due to radiation cooling, along with high humidity and calm, clear conditions.
Step-by-step explanation:
The most prevalent warning that radiation fog will form is a condition where there is a shallow layer of radiation cooling, usually occurring at night or in the early morning when the ground loses heat to the sky, resulting in the cooling of the air near the surface to below its dew point temperature. This type of fog requires clear skies and calm winds, which facilitate the loss of heat from the ground, leading to the condensation of water vapor in the air close to the ground. The presence of high relative humidity and a small temperature difference between the air and the dew point can also serve as indicators that radiation fog may develop.