Which fog may form in advance of a warm front when warm rain falls into a layer of cold and moist air?

• A. Advection fog
• B. Radiation fog
• C. Frontal fog
• D. Ice fog

Answer: (C)

Frontal fog forms ahead of a warm front when warm rain falls into a cooler, moist layer near the surface. The rain evaporates as it moves through that cool air, adding water vapor to the layer. Since the air is already close to its dew point, the added moisture brings it to saturation and condensation occurs at ground level, creating fog. This precipitation fog is typical of warm-front situations, unlike radiation fog (nighttime cooling of the ground), advection fog (humid air moving over a cooler surface), or ice fog (extremely cold air causing water vapor to crystallize).