Avection fog forms when moist air blown over cooler surfaces. Fog usually occurs when the dew point is 5C or below and the ambient temperature is close to the dew point. Also the air must be very humid for the water droplets to condense into fog.  Fog can be differentiated from mist by its density. If visibility is reduced below 2km by water droplets in the air then it is classed as mist. However is the visibility is reduced below 1km then it is classed as Fog. Fog forms in 3 different ways Radiation, Advection, and you also get freezing fog.

Originally posted 2008-11-04 15:34:21.