The highest temperature that Jonathan Paul has ever recorded in a London Tube station is about 42 Celsius, or 107.6 Fahrenheit. Paul, a researcher at Royal Holloway, University of London, uses his thermometer-equipped smartphone to take such readings. 42C is the kind of heat that would send someone running to the nearest air-conditioned building. Underground, though, they can’t. There’s nothing but stifling tunnels and screeching trains down here.
The Tube network runs through thick clay, and that dense material has been soaking up heat generated by trains since the tunnels were first dug, in some cases more than 100 years ago. Fitting air-conditioning units
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