A country fog is white, without smell (unless perhaps a slight odour of ozone), and not disagreeable to breathe. It seldom thickens after the first hour after sunrise. It is pure condensed vapour, and therefore clean. The sun appears perfectly white when seen through it.
A London fog is brown, reddish-yellow, or greenish, darkens more than a white fog, has a smoky, or sulphurous smell, is often somewhat dryer than a country fog, and produces, when thick, a choking sensation. Instead of diminishing while the sun rises higher, it often increases in density, and some of the most lowering London fogs occur about midday or late in the afternoon. Sometimes the brown masses rise and interpose a thick curtain at a considerable elevation between earth and sky. A white cloth spread out on the ground rapidly turns dirty, and particles of soot attach themselves to every exposed object.
R. Russell 1880