A good expla
Most thermometers measure temperature where people live, and people tend to build stuff that warms the local environment around the thermometer.
Called the urban heat island (UHI) effect, most of the warming occurs long before the thermometer site actually becomes “urbanâ€. For instance, if you compare neighboring thermometers around the world, and also compare their population densities (as a rough indication of UHI influence), it can be easily demonstrated that substantial average UHI warming occurs even at low population densities, about ~1 deg. F at only 10 persons per sq. km!
This effect, which has been studied and published for many decades, has not been adequately addressed in the global temperature datasets, partly because there is no good way to apply it to individual thermometer sites.