The Great Smog of London APR 19 2011
In December 1952, a thick smog settled over London for several days. This was a particularly bad episode of the London Fog, which was hardly a natural occurrence...the "fog" was mostly due to the burning of soft coal. It is now thought that the Great Smog resulted in around 12,000 deaths.
Here's a collection of photos of the smog, including this daytime shot.
That dim greyish-orange ball in the sky is the Sun.