Advection is a term used to describe the horizontal movement of air – such as air moving from over the ocean to over the land or air moving from south to north.
When a warm, humid air mass moves over a cooler surface, the lower layers of the air mass will begin to cool and if dew point temperature is reached, fog will form. This is known as advection fog and, unlike radiation fog, advection fog can form at any time of the day or night.
Advection fog is quite common with a tropical maritime airmass where the warm, moist air from the south-west moves over the colder land as it travels north.
The fog will clear when the cold surface warms or if wind & turbulence lifts the air, forming low stratus cloud.