Fog
A warm frontal zone
Fog can form when warm rain falls through cool air near the surface.
Two conditions lead to the formation of fog
- Either the air is cooled to saturation
- Sufficient moisture is added to the air until saturation occurs
In either case, fog can form when the temperature/dew point spread is 5 degrees or less.
A cold frontal passage will normally clear out fog and other atmospheric visual obstructions.