Final answer:
Radiation fog forms in valleys at night due to the cooling of the air near the ground to the dew point after the land loses heat through radiation, which makes it the correct answer. This type of fog is common in the colder months and typically dissipates after sunrise when temperatures rise again.
Step-by-step explanation:
The correct option is C:
The type of fog that typically forms in valleys at night is C) Radiation fog. This occurs due to the cooling of the land after sunset which in turn cools the air close to the surface. As the air temperature drops to the dew point temperature, the water vapor condenses into tiny droplets, creating fog. Valleys are especially prone to radiation fog because they trap cooler air at the surface.
During the day, the sun warms the Earth's surface, leading to evaporation of moisture. At night, especially under clear skies with calm winds, the ground loses its heat to the atmosphere through radiation. This drop in temperature causes the air near the ground to cool. Subsequently, if the humidity is high enough and the temperature drops to the dew point, condensation takes place and fog is formed.
Radiation fog is most common in the late fall and winter months when nights are longer, providing more time for the land to cool. This type of fog tends to dissipate after sunrise as the sun warms the ground and the air temperature begins to rise again. Understanding this phenomenon also underscores the role of thermodynamics in weather patterns and climate changes. Since this type of fog is not related to the movement of air masses or warm air over water, the other options like Frontal, Advection, or Steam fog are not correct in this context.