Advection fog is a type of fog that forms as cool and moist air moves over cooler surfaces. Surfaces can be either land or water, each cooler than the warm and more humid air moving horizontally above it. These fogs will often appear to roll into an area and have a forward movement.
As an air mass moves over a cool surface, the characteristics of that air mass in its lowest layers, can change. In the spring, warm air masses will undergo advective cooling as it moves over the recently thawed, or still frozen, ground from a long winter. The land surfaces, not yet warmed by solar radiation, will cause the air mass to reach saturation close to the ground. Once saturation is reached, a low cloud or fog, will form.
Because water has a high heat capacity, air will also move over water surfaces well into the summer before the water has a chance to heat up. As the air moves over the cooler water, a type of advection fog, known as sea fogs, will form. In areas near the shores of large lakes or the ocean, summer-time fogs can creep in from these large bodies of water.