What type of fog forms on clear, calm, and cool nights?

Radiation fog forms under conditions where the skies are clear, and there is little to no wind, particularly on cool nights. This type of fog develops when the ground loses heat rapidly through radiation, cooling the air close to the surface. As the air temperature drops, it can reach the dew point—where moisture in the air condenses into tiny water droplets, creating fog.

The formation of radiation fog is closely linked to the cooling effects of radiation from the Earth's surface at night. Because the air is typically calm during these conditions, there is little mixing, allowing the cooler, moist air to remain at ground level and form a dense fog.

This explanation highlights why radiation fog is the correct answer, demonstrating its dependence on specific atmospheric conditions that align perfectly with those described in the question.