Fog is generally applied to vapours condensed on land, especially if those vapours are laden with smoke.
Q. What is the reason why condensed vapour sometimes forms into clouds, and sometimes into fog?
A. If the surface of the earth be hotter than the air, then the vapour of the earth (being chilled by the cold air) becomes fog: but if the air be hotter than the earth, the vapour rises through the air, and becomes cloud.
Q. If cold air produces fog, why is it not foggy on a frosty morning?
A. 1st—Because less vapour is formed on a frosty day; and
2ndly—The vapour is frozen upon the ground before it can rise from the earth, and becomes hoar-frost.
Q. Why are fogs more general in autumn than in spring?
A. In spring the earth is not so hot as it is in autumn. In autumn the earth is generally warmer than the air; and, therefore, the vapour (issuing from the earth) is condensed into fog by the chill air.
Q. Why are fogs more common in valleys than on hills?