I've been posting on the subject of Urban Heat Island for years; this is a good overview. There has been a dismissiveness from the scientific establishment about this subject, but this peer-reviewed paper seems to be getting it right. And the conclusion is inescapable: UHI raises daily low temperature readings and the "homogenization" computer programs use this anomaly to adjust rural weather station temperatures upwards.
Do this for hundreds of cities world wide and you do indeed get Global Warming.
The data sets are a frightful mess. That's about the only thing "settled" about Climate Science.
More old Borepatch posts on this topic here, here (this one is excellent), and here.