In humid places, in the early morning and towards evening, water vapour around the fine dust in the atmosphere and near the ground condenses to form fog. Fog is white in colour but in towns and cities it is grey in colour. In areas lacking wind, the fog stays near the ground, making it difficult for the people and traffic to get about.
When fog is formed in large industrial cities, it mixed with soot and smoke from the chimneys to form smog, which is very harmful to people and materials. Fog is a cloud, which remains close to the ground.
Below Fog info from Wikipedia (Read More):-
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 2 km. For aviation purposes in the UK, a visibility of less than 10 km but greater than 999 m is considered to be mist if the relative humidity is 95% or greater - below 95% haze is reported.
http://en.wikipedia.org/wiki/Fog
