The term “fog” is used when microscopic droplets reduce horizontal visibility at the Earth’s surface to less than 1 km, while the term “mist” is used when the droplets do not reduce horizontal visibility to less than 1 km. In practice, mist is considered synonymous with “light fog”. The term “smog” (from the words “smoke” and “fog”) is widely used for conditions where fog and heavy air pollution are present, with chemical reactions between the fog droplets and various pollutants.
Reduction in visibility depends on the structure of the fog, and especially on the number density and size distribution of the droplets. This structure may vary a great deal in time and space. The air in “fog” usually feels damp, moist or wet. When illuminated, individual fog droplets are frequently visible to the naked eye and are seen to be moving in a somewhat turbulent manner.
“Fog” forms a whitish veil that covers the landscape, while “mist” generally forms a fairly thin, greyish veil. When mixed with dust or smoke, fog may take on a faint coloration.