Pea-soupers or for that was so thick you would be lucky to be able to see 10 feet in front of you, apparently they could be so thick that people walked into the Thames. They had a greenish tinge to them that the nick name pea-souper.
The River Thames tended to cause fog but mixed smoke from the coal fires (there being no gas or electricity) that every Victorian house had.
From a handbook of the time:
Not only does a strange and worse Cimmerian darkness hide familiar landmarks from sight, but the taste and smell are offended by an unhallowed compound of flavours, and all things become greasy and clammy to touch.
During the continuance of a real London fog-which may be black, or grey or more probably orange-coloured-the happiest man is the one who can stay at home.
And lets face it a Victorian film without fog…that would be no good would it!