It is the suspension of tiny water droplets or crystals and ice in an air layer near the Earth's surface.
By international convention, the term fog when the horizontal visibility on the ground is less than 1 km; when the horizontal visibility of the ground is greater than 1 km, the suspension is called mist.
Fog is a cloud based on contact with the ground. Fog can form when air becomes saturated through radiative cooling, advective cooling, expansion cooling (adiabatic), or the addition of water vapor.
Fog in Campinas - SP
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Fog - Low Visibility