Advection fog is fog produced when air that is warmer and more moist than the ground surface moves over the ground surface. The term advection means a horizontal movement of air. Unlike radiation fog, advection fog can occur even when it is windy. Also unlike radiation fog, advection fog can occur when the skies aloft are initially cloudy.
The setup for advection fog will often include an advection pattern bringing in warmer and more moist air from the south. The set-up for the ground surface will be a snow covered ground or a saturated ground that has been chilled by cold temperatures before the winds shift back from a southerly type direction.
Since the ground surface is very cold it will influence the temperature of the air adjacent to the ground surface. This air will be chilled more than it otherwise would be due to the very cold surface ground temperature. If there is snow or moisture on the ground then the air will be cold and moist. When winds shift to the south it will bring in warmer air. This warmer air will be cooled due to the influence of the cold land surface. As air cools the temperature drops closer to the dewpoint. If the mixing of the warmer air with the colder air produces a relative humidity of 100% then fog can form.
Considering air that is saturated, as the temperature increases the amount of moisture in the air increases at an increasing rate. Warm air that is near saturation will saturate quickly when it is mixed with cold saturated air. This is because the amount of moisture in the air from mixing the air is greater than the amount of moisture needed for saturation at the temperature of the mixed air. The air, instead of supersaturating, will produce condensation in the form of fog.
For example, suppose air that has a temperature and dewpoint of 0 C is mixed with air that has a temperature and dewpoint of 20 C. The temperature of mixing this air if it is mixed in equal proportions is 10 C. A saturation vapor pressure at 10 C is 12.3 mb. The air with a temperature of 0 C has a saturation vapor pressure of 6.1 mb and the air with a temperature of 20 C has a saturation vapor pressure of 23.4 mb. When this air is mixed the new saturation vapor pressure is (6.1 + 23.4) /2 = 14.8 mb. Since 14.8 is the amount of moisture in the air after mixing and mixed air at 10 C only needs 12.3 mb to be saturated, the air is supersaturated. Instead of the Relative Humidity increasing above 100% though, condensation and fog will form.