There was a lot of fog outside on my way to work today. For some reason, that made me think of the old London pea-souper† fogs, and how it must've been in those days. Come Watson, the game's afoot!
† Thick yellow fog. It had been common to heat dwellings by burning soft coal, sending soot up the multiple chimney stacks one still sees protruding from the roofs. The emitted smoke particles made ideal condensation nuclei in more than ample numbers — a perfect recipe for fog to form when the air was still and the ground was cold.