The heart of the question is: how are weather stations located in Urban areas, which can often be subject to the heat island and other micro site problems associated with the artificial city environment, worked into the broader United States Historical Climatological Network (USHCN) so that the network might deliver accurate regional and national temperature figures? The answer is twofold. Firstly, the urban stations are "adjusted" via reference to the nearest rural station (ie a station presumably unaffected by these various problems). Secondly, the information from them is not used to determine the overall temperature numbers; only data from rural, "unlit"stations is used for this end.
But, McIntyre asks, is this really the case? He suspects that in at least one instance--the Grand Canyon (rural) vs. Tuscon (urban) station-- it is not just the former station that is being used to adjust the latter, but that the information from both is being "blended", so that the rural station is no longer quite as rural as it once was:
It appears to me that the total adjustment process (including USHCN adjustments incorporated by GISS) result in “adjusted” stations becoming a type of blend of urban and rural stations, so that it is not actually correct to say that the overall trend results only from rural stations.
It looks to me like the USHCN adjustment process feeds back trends from urban sites to adjust rural sites and this contradicts the claim that only rural sites are used in GISS trends.
In other words, his argument is that the purity of the data produced by the rural station is compromised by its proximity to/interaction with data from the urban site.
...the Grand Canyon values are being blended up and the Tucson values are being blended down.
The net impact of this is that, if Grand Canyon neighbors have urban trends, then the USHCN adjusted Grand Canyon version will also incorporate some amount of urban trend.
This theme, of the dirty urban stations introducing an "urban bias", and therefore corrupting the "pure" rural stations, is then taken up in Steve's comments sections. For example, Sam Urbinto comments:
I’d say that taking a rural station and adjusting it with urban adjustments rather turns it into an urban station.
My God, the stations have been "turned" by their proximity to urban data! I'm surprised they haven't gone gay. In fact, I'm surprised Steve doesn't refer to these urban stations as "too damn Liberal", insinuating that they are in some sense "black weather stations". The symbolism of rural=good, urban=bad, permeates the entire thread...
...as it does in Anthony Watts analysis of station data, incidentally. In his world, uncompromised rural stations inevitably show a downward temperature trend, whereas the lying urban stations show evidence of global warming.
But the oddest pattern is surely the degree to which red and blue states on these maps match their political counterparts. There are a few exceptions - Arizona, Montana, Utah, but it looks to me like voting patterns would be a better proxy for the existence of a 20th century temperature trend (by state) than tree rings.
As to why you can see crude Conservative symbolism percolating up through what is supposed to by a discussion of pure science, well I think this has something to do with the audience McIntyre is trying to cultivate. His "argumentation" often resembles a random data dump of charts and graphs, and the only real movement in it is therefore less logical than symbolical. I think it is also significant that Anthony Watts has considered using his appearances on Hannity and the Rush Limbaugh show to scare of volunteers for the Surface Stations project.
In both cases, they're throwing raw meat to the Bubbas.