In the old days London used to suffer from they called "pea-soupers" - a thick smog containing particles of soot coming off from coal fires, which at their peak killed thousands every year.
After the Clean Air Act of 1956 pollution went down and so we don't get smogs, but we still get the odd thick fog, as we did today, and which lasted most of the day.
So I took a photo.... not really a prize winner I'd admit.
