What causes advection fog to form?

Advection fog forms specifically when warm, humid air moves over a cooler surface. As this warm, moist air comes in contact with a colder surface, such as water or land during cooler conditions, it cools down and reaches its dew point. This process results in condensation of the water vapor present in the air, leading to the formation of fog.

This phenomenon often occurs in coastal areas where warm air from the ocean interacts with cooler land masses or where sea breezes push warm air over cooler waters. The presence of moisture is crucial because without sufficient humidity, there wouldn't be enough vapor for condensation to occur. This understanding highlights why the scenario of warm humid air being transported over a cold surface is the correct explanation for how advection fog forms.