FAA Ground School Practice Test

Radiation fog typically forms under specific atmospheric conditions that promote cooling of the air near the surface. This phenomenon occurs when warm, moist air rests over a cooler surface, allowing the moisture in the air to condense into tiny droplets, creating fog. The combination of clear skies and calm winds at night is crucial, as it facilitates the cooling of the ground, leading to a decrease in air temperature close to the surface.

In this context, the situation described in the correct answer involves warm, moist air combined with a low, flatland area. During the night, the flat terrain allows for significant radiative cooling, particularly when there are clear skies. This combination creates ideal conditions for condensation, resulting in the development of radiation fog.

The other options do not provide the ideal conditions for radiation fog formation. For instance, cold, dry air would not typically hold enough moisture to condense into fog, while warm, moist air over mountainous areas may lead to other forms of fog due to differing topographical influences. Similarly, cool air over lakes may not still retain the moisture needed to form fog without the right combination of humidity and temperature variations typically found in radiation fog scenarios.