radiation fog A condensation effect that occurs over land surfaces on clear nights with light breezes, caused by surface radiation cooling. Favoured initial conditions are very humid air, with wet and cold surfaces (e.g. marshes). The fog, most common in winter, is generally cleared by the Sun's warmth in the morning, but thick fog over wet surfaces in winter may persist much longer, particularly if an upper cloud layer screens the Sun.

radiation fog Condensation effect over land surfaces on clear nights with light breezes, due to surface radiation cooling. Favoured initial conditions are very humid air, with wet and cold surfaces, e.g. marshes. The fog, most common in winter, is generally cleared by the Sun's warmth in the morning, but thick fog over wet surfaces in winter may persist much longer, particularly if an upper cloud layer screens the Sun.