Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influenced by nearby bodies of water, topography, and wind conditions. In turn, fog affects many human activities, such as shipping, travel, and warfare.

Fog forms when the difference between air temperature and dew point is less than 2.5 °C (4.5 °F).

Two types of fog are:

Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snow-pack. It is most common at sea when moist air encounters cooler waters, including areas of cold water upwelling, such as along the California coast (see San Francisco fog).

Upslope fog forms when moist air is going up the slope of a mountain or hill (orographic lifting) which condenses into fog on account of adiabatic cooling, and to a lesser extent the drop in pressure with altitude.

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