Recent photographic documentation of poor siting conditions at stations in the U.S. Historical Climatology Network (USHCN) has led to questions regarding the reliability of surface temperature trends over the conterminous U.S. (CONUS). To evaluate the potential impact of poor siting/instrument exposure on CONUS temperatures, trends derived from poor and well-sited USHCN stations were compared. Results indicate that there is a mean bias associated with poor exposure sites relative to good exposure sites; however, this bias is consistent with previously documented changes associated with the widespread conversion to electronic sensors in the USHCN during the last 25 years. Moreover, the sign of the bias is counterintuitive to photographic documentation of poor exposure because associated instrument changes have led to an artificial negative (“cool”) bias in maximum temperatures and only a slight positive (“warm”) bias in minimum temperatures. These results underscore the need to consider all changes in observation practice when determining the impacts of siting irregularities. Further, the influence of non-standard siting on temperature trends can only be quantified through an analysis of the data. Adjustments applied to USHCN Version 2 data largely account for the impact of instrument and siting changes, although a small overall residual negative (“cool”) bias appears to remain in the adjusted maximum temperature series. Nevertheless, the adjusted USHCN temperatures are extremely well aligned with recent measurements from instruments whose exposure characteristics meet the highest standards for climate monitoring. In summary, we find no evidence that the CONUS temperature trends are inflated due to poor station siting.

Given the now extensive documentation by surfacestations.org (Watts [2009]) that the exposure characteristics of many USHCN stations are far from ideal, it is reasonable to question the role that poor exposure may have played in biasing CONUS temperature trends. However, our analysis and the earlier study by Peterson [2006] illustrate the need for data analysis in establishing the role of station exposure characteristics on temperature trends no matter how compelling the circumstantial evidence of bias may be. In other words, photos and site surveys do not preclude the need for data analysis, and concerns over exposure must be evaluated in light of other changes in observation practice such as new instrumentation.

Indeed, our analysis does provide evidence of bias in poor exposure sites relative to good exposure sites; however, given the evidence provided by surfacestations.org that poor exposure sites are predominantly MMTS sites, this bias is consistent with previously documented changes associated with the widespread conversion to MMTS-type sensors in the USHCN. Moreover, the bias in unadjusted maximum temperature data from poor exposure sites relative to good exposure sites is, on average, negative while the bias in minimum temperatures is positive (though smaller in magnitude than the negative bias in maximum temperatures). The adjustments for instrument changes and station moves provided in version 2 of the USHCN monthly temperature data largely account for the impact of the MMTS transition, although an overall residual negative bias remains in the adjusted maximum temperature series. Still, the USHCN adjusted data averaged over the CONUS are well aligned with the averages derived from the USCRN for the past five years.

The reason for station exposure not playing an obvious role in temperature trends probably warrants further investigation. It is possible that, in general, once a changeover to bad exposure has occurred, the magnitude of background trend parallels that at well exposed sites albeit with an offset. Such a phenomenon has been observed at urban stations whereby once a site has become fully urbanized, its trend is similar to those at surrounding rural sites [e.g., Boehm, 1998; Easterling et al. 2005]. This is not to say that exposure is irrelevant in all contexts or that adherence to siting standards is unimportant. Apart from potentially altering the degree to which a station’s mean value is representative of a region, poor siting in the USHCN may have altered the nature of the impact of the MMTS transition from what it would have been had good siting been maintained at all stations. Moreover, there may be more subtle artifacts associated with siting characteristics such as alterations to the seasonal cycle. Classification of USHCN exposure characteristics as well as observations from the very well sited USCRN stations should prove valuable in such studies. Nevertheless, we find no evidence that the CONUS temperature trends are inflated due to poor siting.

The authors wish to thank Anthony Watts and the many volunteers at surfacestations.org for their considerable efforts in documenting the current site characteristics of USHCN stations. The authors also thank Anthony Arguez for helpful comments on this manuscript. Partial support for this work was provided by the Office of Biological and Environmental Research, U.S. Department of Energy (Interagency Agreement DE-AI02-96ER62276).