The formation of fog and mist is basically the same as that of clouds. In cloud the resulting condensation into visible particles takes place high above the ground, but for fog or mist, the condensation takes place on the surface of the sea or ground. The only difference between fog and mist is the degree of visibility, ie visibility under 1 000 metres is classified as fog, whilst mist is where the visibility is greater than 1 000 metres but very reduced by the presence of visible moisture droplets.

There are two basic types of fog affecting mariners. They are as follows:

1. Advection or sea fog. On the open sea most fog is caused by warm, moist air blowing over a relatively cold sea surface and the lower layers of the air become chilled. They are very prevalent in the vicinity of cold water currents, ie on the west coast of South Africa and Namibia where the cold Benguela current flows. They are also more prevalent in the late spring and early summer when the sea surface temperature is most likely to be below that of the air blowing over it.
2. Radiation fog. Over the land, on a clear night with little or no wind, the air cools rapidly just after sunset as a result of the earth radiating its heat into space. The air in contact with the ground is cooled, condensation takes place and fog is formed. These fogs are called radiation fogs and occur mostly in winter with its long nights. They are densest in the latter part of the night and early morning. These fogs frequently affect harbours when the fog rolls out to sea. When there is cloud cover, fogs do not occur because the former acts as a blanket trapping the radiated heat from the earth.

Fog.