Fog:

• When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden (mostly due to temperature inversion), condensation takes place within itself on fine dust particles.
• Fogs are a mini clouds in which condensation takes place around nuclei provided by the dust, smoke and salt particles, with their base at or very near to the ground.
• Fogs formed by the condensation of warm air when it moves horizontally over a cold surface, are known as advection fog. These fogs are thick and persistent.
• Fog areas are positively related to cool ocean currents. The eastern portions of the oceans and western continental margins are characterised by most fogs because these areas are frequented by cool ocean currents. For example, California current, Peru current, Benguela current, Canary current, Western Australia current etc.
• Frontal fogs are formed in the high latitudes due to the convergence of cold polar air mass and warm westerlies.
• Radiation fogs are formed in low latitudes on continental areas during the winter season when warm and moist air lies over cold ground surface.
• Dense fogs are formed at the places where cold and warm ocean currents converge. For example, the Japanese coast due to the convergence of cold Kurile current and warm Kuroshio current. In urban and industrial centres 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.