What conditions most favor the formation of radiation fog?

Radiation fog forms under specific atmospheric conditions, primarily when warm, moist air is present over low, flatland areas during clear, calm nights. The key aspects contributing to radiation fog include the cooling of the ground and the air directly above it, which can occur effectively when the nighttime sky is clear and winds are light.

As the ground cools due to the loss of heat through radiation, it causes the air in contact with it to cool as well, leading to condensation of moisture in the air when it reaches its dew point. This process creates a layer of fog that is particularly common in valleys and low-lying areas where cold air tends to settle.

The other conditions mentioned in the options refer to different types of fog. For example, moist tropical air moving over cold offshore waters may lead to the formation of sea fog (or advection fog) rather than radiation fog. By contrast, the movement of cold air over much warmer water generates fog due to the warm, moist air being lifted and condensing, which again is a different phenomenon. Lastly, high humidity in coastal regions during the summer months may contribute to various types of fog, but it doesn't specifically align with the clear, calm night scenario that favors radiation fog formation.