Final answer:
Radiation fog forms on a clear night with light winds, as the ground cools rapidly and the air near the surface reaches dew point, leading to condensation and fog.
Step-by-step explanation:
The type of fog that requires a clear night sky and light winds is commonly referred to as radiation fog. This phenomenon occurs when the earth's surface cools rapidly during the night through the process of radiation (heat being emitted back into space), especially under a clear sky. Without the insulation of clouds, the ground loses heat more quickly and the air in contact with the ground becomes cooler. Light winds are necessary to mix the air near the ground with the slightly warmer air above, creating a saturation point where condensation can occur and fog forms. However, if the winds are too strong, they can disperse the fog, preventing it from settling. Therefore, the conditions for radiation fog to form are optimal on a night with a clear sky, to facilitate cooling, and gentle breezes, to aid in the development of condensation.