Which statement about radiation fog is true?

• A. It forms when warm air moves over cold water.
• B. It forms only during the day.
• C. It forms when the ground cools rapidly at night, and dissipates after sunrise.
• D. It requires strong winds to form.

Answer: (C)

Radiation fog forms when the ground cools by radiational cooling after sunset, chilling the air near the surface to its dew point so water vapor condenses into tiny droplets right at the surface. It tends to develop on clear, calm nights and then fades after sunrise as the sun heats the ground, warms the air, and mixing disperses the droplets.

That makes the statement about forming when the ground cools rapidly at night and dissipating after sunrise the correct description. The other scenarios describe different fog processes or timing: warm air moving over cold water creates a different type of fog (steam or advection fog), fog does not form only during the day, and radiation fog does not require strong winds—in fact, light winds or calm conditions favor its formation because strong winds mix and inhibit cooling to the dew point.