High Quality Content by WIKIPEDIA articles! Fog is a cloud that is in contact with the ground. A cloud may be considered partly fog; for example, the part of a cloud that is suspended in the air above the ground is not considered fog, whereas the part of the cloud that comes in contact with higher ground is considered fog. Fog is distinguished from mist only by its density, as expressed in the resulting decrease in visibility: Fog reduces visibility to less than 1 km, whereas mist reduces visibility to no less than 1 km but less than 2 km. For aviation purposes in the UK, a visibility of less than 2 km but greater than 999 m is considered to be mist if the relative humidity is 95% or greater - below 95% haze is reported.