The distribution of fog is uneven. Over the earth as a whole, by far the greater number of fogs occurs over the oceans which are mostly of the advection type. The distribution of fogs over the oceans is closely related with the ocean currents and ocean temperatures.
These ocean fogs occur wherever air moves from a warmer region over a cold ocean current. In the tropical and sub-tropical regions, cool ocean currents flow along the west side of the continents. These regions, therefore, are among the foggiest regions of the world.
In the temperate zone, the advection of warm tropical air over the cool surface of oceans results in the formation of ocean fogs. Advection fogs form over land during the winter season in the middle latitudes.
Continental interiors have more fogs in winter and less in summer. On the contrary, coastal regions have more fogs in summer and less in winter because of the greater temperature contrast in the warmer part of the year.
Unlike advection fogs, radiation fogs are more frequent over land areas in winter. The anticyclonic conditions prevailing over land areas in winter offer the ideal conditions for the occurrence of radiation or ground fogs, because under such conditions long winter nights afford maximum radiational cooling of the lowermost layer of air.
Radiation fogs are infrequent over oceans. Over the oceans as well as the continental locations the tropical areas have less fog than those of the middle and high latitudes.