When the temperature of an air mass containing large quantity of water vapors falls all of a sudden, condensation takes place within itself on fine dust particles. So, the fog is a cloud with its base at or very near to the ground.
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Because of the fog and mist, the visibility becomes poor to zero. In urban and industrial centre smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke, is described as smog. The only difference between moisture than the fog. In mist each nuclei contains a thicker layer of moisture. Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface. Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents. Fogs are minim clouds in which condensation takes place around nuclei provided by the dust, smoke, and salt particles.