Steve McIntyre, in a timely post for this site given the recent project on Phoenix urban heat islands, has a post on the Phoenix adjustment in the GISS database and Hansen’s dicussion of Phoenix UHI in his 1999 paper.
One is left to wonder whether a station that has a 2.5C error-corection adjustment tacked on should even be included in a data set that is attempting to measure a warming signal on the order of magnitude of 0.5C, particularly since any reasonable person would argue that the 2.5C adjustment likely has an error bar of at least plus or minus 0.5C. I stand by my point that the signal to noise ratio in surface temperature measurement is terrible.
However, many GISS adjustments for site location and urbanization are negative, meaning urbanization has been reduced at the location since 1900, certainly an odd proposition. In fact, if memory serves, the total net adjustment of all stations in the GISS system is negative for site location and urbanization. I know, from here, the net USHCN adjustment for combined site location and urbanization is negative, adding 0.15F to current temperatures as compared to those in 1900, implying that site location quality has improved over time. Anyway, McIntyre promises to tackle this issue tomorrow, which I look forward to.