Fog was a vapour. It proved troublesome to humans, whose vision it obscured.
When combined with with smoke from coal fires, lingering fog created smog. (PROSE: Amorality Tale)
In the 1900s, when a Rutan scout arrived on Fang Rock, fog pervaded the air. The Fourth Doctor hypothesised that the Rutan was using it to isolate the lighthouse keepers and their visitors. Reuben's assertion that, despite thirty years experience, he had not been able to see the onset of fog, gave credence to this. The Doctor had Leela sound the lighthouse's foghorn to warn any boats of the navigational hazards. (TV: Horror of Fang Rock)
In 1963, Barbara Wright informed Susan Foreman that there would likely be fog while she walked home from Coal Hill School one evening. (TV: An Unearthly Child)
A time energy barrier appeared as a dense fog. Those who entered the fog, such as Jackie and Joe Hargreaves, would always exit the same way from which they had come. (PROSE: Bay of the Dead)