Once in awhile, we get a foggy day in London.  This used to be far worse in the 1950s apparently, days and weeks of fog and smog, also known as ‘peasoup’ caused by the dirty coal fires and uncontrolled industrial emissions. London air is now a lot cleaner with European guidelines enforced strictly. With vehicle CO2 emissions falling year on year, and Congestion Charging to limit daytime vehicular access in the city.