Readers of the excellent Watts Up With That will be familiar with their diligent expose of the problems with the siting of weather stations that supply meterological data. (He has a special site to track these siting/placement/measurement issues).
This daily weather data is the basis of "Proof" of warming. A typical post is here. A photograph of an actual site is at right - in Tuscon. Note -it is sited in an asphalt parking lot.
The reason Tuscon is important? It is the Number one warming site in the US. And, during the time period between 1950 and today, when Tucson saw most of this measured temperature increase, the population of Tucson increased from under 200,000 to over 1,000,000. That's a lot of extra urban heat, in addition to the local effects of this parking lot.
Another blast from the past is the material supplied by a cold war warrior remembering their weather reporting activities on the DEW line in the early sixties. These guys, who had a real job watching for the first soviet bombers en route to nuke North America, were also required to make regular temperature measurements and forward them to relevant authorities. Problem is it was a manual system and the operators saw little difference between -44F and -55F being reported - especially if (in)accuracy meant avoiding a wandering and hungry polar bear or a major arctic storm. Near enough was good enough....
The big problem all this data is now used to feed the climatologists latest models... GIGO.
Compounding this is the strategy of the NCDC to fill in missing data by interpolating data from nearby stations....is it too high a hope that only one station had data inaccuracies?