Read here. It's been well documented that urban areas experience much higher temperatures than rural areas. Since most of the climate network's global thermometers are located in urban/airport areas, the major temperature datasets include a significant urban heat bias that has not been properly corrected for. An example of this UHI phenomenon is the research data produced from the Mexicali City urban area.
"Garcia Cueto et al. state that Mexicali City "changed from being a cold island (1960-1980) to a heat island....from the "more updated information (2000-2005)," they found that "the greatest intensity of the urban heat island was in winter with a value of 5.7°C, and the lowest intensity in autumn with 5.0°C [editor's note: 9.0°F].""...."The results of this study clearly demonstrate that population growth and the clustering of people in cities can lead to localized warming (in areas where temperatures are routinely measured) that is both more rapid and much greater (by as much as an order of magnitude, in fact) than what climate alarmists typically attempt to characterize as the "unprecedented" warming of the 20th century. And that population-growth-induced warming -- spread across the world -- has likely contributed, in large part, to what they wrongly construe to be CO2-induced global warming."