I read something a few months ago where a meteorologist explained that when weather stations were first set up they were placed in open countryside. Gradually over time urban sprawl had replace countryside by roads and buildings which absorb and re-radiate previous heat.
He managed to find some weather stations that were still in open countryside (Green belts have their uses) and compare them with others not far way but now in urban heat islands, and the difference in maximum temperature reading was considerable.