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  • NCDC Frequently Asked Questions

    How reliable is the U.S. surface temperature record?

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    A recent study (Menne et. al., 2010) conducted by scientists at NOAA's National Climatic Data Center found no evidence that the U.S. temperature trend is inflated by poor siting of stations that comprise the U.S. Historical Climate Network (USHCN).  NCDC scientists conducted this study to determe the reliability of surface temperature trends over the conterminous United States (CONUS) following photographic documentation of poor siting conditions at USHCN stations.

    A comparison of trends derived from poorly and well–sited USHCN stations indicates that a bias associated with poor exposuire sites in the unadjusted USHCN version 2 data is consistent with previously documented changes of a widespread conversion to electronic sensors in the USHCN during the last 25 years.  Of significant note, the sign of the bias is counterintuitive to photographic documentation of poor exposure because the associated instrument changes led to an artificial negative ("cool") bias in maximum temperatures and only a slight positive ("warm") bias in minimum temperatures.

    Adjustments largely account for the impact of instrument and siting changes but appear to leave a small overall residual negative ("cool") bias in the adjusted USHCN version 2 CONUS average maximum temperature.  The adjusted USHCN CONUS temperatures are well aligned with recent measurements from NOAA's U.S. Climate Reference Network (designed with the highest climate monitoring standards for siting and instrument exposure), thus providing independent evidence that the USHCN provides an accurate measure of the U.S. temperature.

    The results of this study underscore the need to consider all changes in observation practice when determining the impacts of siting irregularities.  Information on the siting characteristics of USHCN stations and additional details on this study are available at


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