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London was once the world's foggiest city. At the height of the Industrial Revolution, during the 19th century, some fogs in London lasted a week and were so dense you could not read during the day, even by a window. The author of 'London Fog: The Biography' is Christine Corton. (Photo: The Big Ben Clock Tower seen through a gate during a foggy day in central London. Credit: Stefan Wermuth/Reuters)