Which conditions are necessary for the formation of upslope fog?

For upslope fog to form, one key condition is the presence of moist, stable air being lifted over a surface that gradually rises, such as hills or mountains. As this warm, moist air rises, it cools due to the decrease in atmospheric pressure with altitude. When the air cools to its dew point, the moisture condenses, leading to the formation of fog. This process is primarily influenced by the stability of the air and the terrain over which it is moving.

Other conditions provided do not directly contribute to upslope fog formation. For instance, while rain falling through stratus clouds might create low visibility conditions, it does not specifically relate to the phenomenon of upslope fog. A clear sky with minimal wind and high humidity could lead to radiation fog instead, as it involves cooling of the surface during the night, not the lifting of moist air. Finally, cool air moving over warm land can lead to different types of fog, such as evaporation fog, but does not describe the mechanism necessary for upslope fog.

Thus, the correct choice highlights the importance of moist, stable air being forced upward over rising terrain, which is essential for upslope fog formation.