Why Fog Come in

Fog results from the cooling of moist air below its saturation (dew) point. Droplets form on hygroscopic nuclei originating from ocean spray, combustion, or reactions involving trace chemicals in the atmosphere. Visibility is reduced even more when such nuclei are present in high concentrations and faster cooling rates activate a larger fraction of such nuclei. Thus, polluted fog, with more numerous smaller droplets, results in lower visibility for a given water content. See also Dew point. Haze, the precursor to fog and mist, forms at relative humidity below 100% to about 80%. It is composed of hygroscopic aerosol particles grown by absorption of water vapor to a diameter of about 0.5 micrometer, concentration 1000 to 10,000 per cubic centimeter. Fog and mist form as the relative humidity increases just beyond saturation (100%), so that larger haze particles grow into cloud droplets with a diameter of 10 ?m and a concentration of several hundred per cubic centimeter. Fog and mist are a mix of lower-concentration cloud droplets and higher-concentration haze particles. By contrast, smog is formed of particles of 0.5–1-?m diameter, produced by photochemical reactions with organic vapors from automobile exhaust. Source: http://www.answers.com/fog?initiator=IE7:SearchBox