Rainbow at St. Paul's Cathedral, London

On Sunday I came across this rainbow, by St. Paul's Cathedral, with Blackfriars station in the foreground, with an arch of the bridge over the Thames.

I don't remember seeing a rainbow with a double bow, and a dark section in the centre. Evidently the darker section between the lower, clearer Primary bow and the higher, fainter Secondary bow is known as Alexander's dark band. That band was named after Alexander of Aphrodisias who first described the effect in around 200 AD.