It seems that Beijing just claimed its title last week as the latest “*furnace”, alongside the more traditional cities like Wuhan, Nanjing and Chongqing, which are notorious for their hot summers. But a more scientific term to describe these cities is “urban heat island” (UHI), which refers to the tendency of a city to be warmer than its surrounding area. Temperature differences are usually in the range of 1 to 6 C, and can reach as much as 12 C at night.