Fog is a mixture because it is an aerosol of water droplets or ice crystals in air. Fog can also contain molecules of acids or ammonia.
Fog is identical to other clouds in composition. By definition, it is low lying and usually draws its moisture from a local source. To be called fog instead of mist, it must reduce visibility to 5/8 of a mile or less. Fog forms when the difference between the temperature and the dew point is less than 4 degrees Fahrenheit. Fog usually requires humidity levels of near 100 percent to form. However, it can form when relative humidity is as low as 70 percent if there are high concentrations of airborne salt particles in the air to serve as nuclei for water droplets.