How does frontal fog form?

Frontal fog forms specifically when rain from a warm air mass falls into a colder air mass beneath it. As the rain descends, it evaporates into the colder air, which increases the moisture content of that air. This process leads to saturation of the cold air, resulting in the formation of fog.

A key aspect of this phenomenon is that it occurs at frontal boundaries, where warm and cold air masses meet. The warmer air can hold more moisture, and when it cools upon contact with the colder air, the excess moisture condenses, leading to fog. This process is crucial in understanding weather patterns and determines visibility conditions for aviation.

Other options address different meteorological phenomena or conditions but do not accurately describe the specific process that leads to the formation of frontal fog. For example, fog forming on clear nights is associated with radiation cooling rather than frontal activity, and while high humidity can contribute to fog in general, it does not specify the unique mechanism of frontal fog formation. Similarly, cooling temperatures at night may lead to other types of fog, such as radiation fog, rather than frontal fog.