U.S. Department of Agriculture scientists grew weeds in three sites: an organic farm in western Maryland, a park in a suburb of Baltimore, and in downtown Baltimore, which is choked with smog and about 3 to 4 degrees warmer
than the surrounding countryside because of the urban heat island effect.
A new study of the effect of land use changes on surface temperatures demonstrates what anyone who lives in a city intuitively knows: Urban areas are
hotter than the surrounding countryside — the heat island effect has been
Even in the early 19th century, it was already noticed that city temperatures were artificially warmer
than the surrounding countryside:
If you are not a climate scientist (or a realclimate reader), you would almost certainly believe, from your own experience, that cities are warmer
than the surrounding countryside — the «urban heat island».
St Louis is often 7 - 10 degrees warmer
than the surrounding countryside, it has a massive UHI.
There was the case of the paper the IPCC relied upon to show that urban heat islands (the fact that cities are generally warmer
than the surrounding countryside, so urbanisation causes local, but not global, warming) had not exaggerated recent warming.