Came across this interesting site:
http://www.norcalblogs.com/watts/weather_stations/
The author visits the various MMTS weather collection sites and has others do the same. They submit photos of the sites where the "warming" data has been collected. It seems that a very high number of the MMTS stations are located in close proximity to air conditioning units.
While the AC is keeping the interior of a building cool, it is blowing out warm and even hot air outside creating a micro-climate in the rear of the building or wherever the AC's exhaust is blown. In other cases the MMTS units are located next to parked and idling vehicles, on or near concrete and asphalt pavement, and often in direct sunlight.
The data is only going to be as good as the data stations. Having large numbers of data collecting stations installed improperly will skew the data badly.
Geophysicist Glenn Morton has found that using data from just the month of October will change the whole "global warming" paradigm. And why October? Because that's when the AC is turned off and furnaces have not yet kicked on. These devices' exhaust are frequently located right next to a MMTS weather station.