Frontal fog forms when warm precipitation falls from warm air above a frontal surface and evaporates into cooler air close to the Earth's surface. This happens in frontal zones, where warm and cold air masses meet. As the warm rain falls into the colder air, it cools and condenses, forming fog.
Frontal fog often occurs ahead of warm fronts, a type of front where warm air replaces cold air. This type of fog can be widespread and is often associated with light winds.
In summary, frontal fog is a result of the cooling of warm precipitation as it falls into colder air in a frontal system, causing condensation and fog formation.
