Two conditions will lead to the formation of fog: Either the air is cooled to saturation, or sufficient moisture is added to the air until saturation occurs. In either case, fog can form when the temperature/dew point spread is 3 degrees C (5 degrees F) or less and decreasing. Pilots planning to arrive at their destination near dusk with decreasing temperatures should be particularly concerned about the possibility of fog formation. There are various types of fog: Radiation Fog, Advection Fog, Upslope Fog, Precipitation-induced Fog, and Ice Fog. 

Radiation Fog typically forms when there is a clear sky, little or no wind, and a small temperature-dewpoint spread (high relative humidity). The fog forms almost exclusively at night or near daybreak. 

Advection Fog forms when moist air moves over colder ground or water. It is most common along coastal areas but often develops deep in continental areas. Unlike radiation fog, it may occur with winds, cloudy skies, over a wide geographic area, and at any time of the day or night. It deepens as wind speed increases—up to about 15 knots (wind much stronger than 15 knots lifts the fog into a layer of low stratus or stratocumulus). 

Upslope Fog forms as a result of moist, stable air being cooled adiabatically as it moves up sloping terrain. Once the upslope wind ceases, the fog dissipates. Unlike radiation fog, it can form under cloudy skies. It is common along the eastern slopes of the Rockies and somewhat less frequent east of the Appalachians; it can be quite dense and extend to high altitudes.

Precipitation-Induced Fog occurs when relatively warm rain or drizzle falls through cool air and saturates it by evaporation. This type of fog can be quite dense and continue for long periods of time. It may also extend over large areas, completely suspending air operations. It is most commonly associated with warm fronts, but can occur with slow-moving cold fronts and with stationary fronts.

Ice Fog occurs in cold weather when the temperature is well below freezing and water vapor sublimates directly as ice crystals. Conditions favorable for its formation are the same as for radiation fog—except for very cold temperatures (usually -25 degrees F or colder). It occurs mostly in the Arctic regions, but is not unknown in middle latitudes during the winter season. Ice fog can be quite blinding to someone flying into the sun.