From AMS Glossary
A common type of fog, produced over a land area when radiational cooling reduces the air temperature to or below its dewpoint.
Thus, a strict radiation fog is a nighttime occurrence, although it may begin to form by evening twilight and often does not dissipate until after sunrise. Factors favoring the formation of radiation fog are 1) a shallow surface layer of relatively moist air beneath a dry layer and clear skies, and 2) light surface winds. It can be most confusing near sea coasts with cold coastal water. It can be difficult at times to differentiate between this and other types of fog, especially since nighttime cooling intensifies all fogs. Radiation fog is frequently and logically called ground fog, but in U.S. weather observing practice, the latter term is defined only with respect to the amount of sky that is obscured by the fog.