According to a study published at
Home, (surfacstationsreport_spring09.pdf) only about 10% of the US weather stations record temperatures reliably (within 1F), and most record temperatures higher than the actual ambient.
There are two reasons for this - probably the most important is the encroachment of civilisation on the stations, particularly artificial readily heated surfaces such as black car parks close to them, or airconditioning heat exchangers or other cooling systems blowing hot air on them.
The second reason is changes in the Stevenson screens which affect the temperature at the thermometer. There was a systematic change in the type of paint used in 1979 (measured effect + 0.3-0.8F), but in addition, many stations , when converted to remote reading, had heat generating electronics installed in the screen.
I have no reason to suppose weather stations anywhere else are on average better than US ones. Which raises some awkward questions about temperature trends as actually observed, particularly in the last thirty or forty years.
(Sorry about the use of Fahrenheit - the yanks haven't caught up yet!)
John
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