The conditions necessary for formation of radiation fog are: sufficiently high dewpoints, sufficient cooling during the night and slight turbulence. (If turbulence is a bit stronger, only low clouds (Stratus) are formed).

Advection fogs develop when air moves over a cooler land or sea surface, the surface temperature being below the dewpoint of the moving air. Over land, cooling by radiation frequently intensifies the effects of advection when warm moist air arrives from the sea. This  is described as advection-radiation fog. Advection is also responsible for steam fogs: in this case, it is the cold air which is advected over a warm water surface as this water evaporates into the air. Both are grouped as air-mass fogs. By contrast a frontal fog arises from the interaction of two air masses. This may occur in two ways: when a cloud extend to the ground (often over hills), or when the air becomes saturated by continuous rain. Latter conditions are likely to develop in the cold air ahead of a warm front.  

The picture shows an advection fog, more precisely an advection-radiation fog. Over the southern part of Belgium, a rather huge fog area was formed due to night cooling (radiation fog). Winds blew weak from the SSE and over the affected area some radiation fog patches were already present, but a bit later a wall of fog was advected giving very poor visibilities for about one hour.