Refer to figure.
FRONTAL FOG. Occurs ahead of a warm front or occlusion. The main causes are:

1. The gradual lowering of the cloud base before a warm front (or some occlusions); and
2. the precipitation that has already fallen to the ground.
   • As the precipitation evaporates, it produces high humidity (still within the cold sector), and this allows the cloud base to keep lowering down to the ground in the form of frontal fog up until the front passes.

Further information:

• The fog can form along a belt up to 200 NM wide which then travels with the front. - unless the front is a very slow moving one, the fog is typically present for only about 2 to 3 hours.
• Orographic lifting (mountainous terrain) usually increases the fog intensity.
• Dissipation of frontal fog happens naturally by the passing of the front.

Note: Frontal fog does not form ahead of cold fronts, because the rain and showers connected with a cold front are made up of drops that are too large, they have a high falling speed and at the same time the temperature of the drops is low.