A cloud of water droplets suspended in the air, limiting visibility to less than 1 000 m. Fog forms when a layer of air close to a surface becomes slightly supersaturated and produces a layer of cloud, that is, when vapour-laden air is cooled below dew point. In advection fog, this cooling is brought about as warm, moist air passes over cold sea currents. See Nakanishi and Niino (2006) Boundary-Layer Met. 119, 2, in a highly technical paper, on predicting advection fog.

Radiation fog forms during cloudless autumn nights when strong terrestrial radiation causes ground temperatures to fall. Moist air is chilled by contact with the ground surface. See Sachweh and Koepke (1995) Geophys. Res. Lett. 22, 9 on radiation fog and urban climate, and Underwood et al. (2004) J. Appl. Met. 43, 2 on radiation fog in California. Where cold air streams across warm waters, steam fog forms. This is common when relatively warm surface air over lakes in frost hollows convects into the cold katabatic airflow above it, and is also the mechanism behind Arctic sea smoke; see Walker (2003) Weather 58, 5 on radiation and steam fog. Frontal fog forms when fine rain falling at a warm front is chilled to dew point as it falls through cold air at ground level. See also Roach (1995) Weather 50 on land fog.