The very wonderful people at the Socioeconomic Data and Applications Center (SEDAC) have produced an urban heat-island data set. Despite sceptical voices to the contrary, the UHI effect has been known about for a long time and is a standard instrumental adjustment, or a potential consideration in public policy from SEDAC's point of view.

Their definition of London is pretty generous when plotted out, but early morning dog walkers will begin to notice the difference about now ...