The U.S. Historical Climatology Network (USHCN) is a group of 1218 stations spread across the 48 contiguous states drawn from the U.S. Cooperative Observer Network aimed at producing a high-quality data set of daily and monthly values for maximum and minimum temperature, precipitation, snowfall, and snow depth.
While the U.S. Cooperative Observer Network has data reaching back to the late 1800s, the USHCN network has been chosen to represent the highest quality subset of this data using a number of criteria including length of record, percent of missing data, number of station moves and other station changes that may affect data homogeneity, and resulting network spatial coverage.
The USHCN project dates to the 1980s (Quinlan et al. 1987). At that time, in response to the need for an accurate, unbiased, modern historical climate record for the United States, the Global Change Research Program of the U.S. Department of Energy and National Climate Data Center (NCDC) chose a network of 1219 stations in the contiguous United States that would become a key baseline data set for monitoring U.S. climate. This initial USHCN data set contained monthly data and was made available free of charge from CDIAC. Since then it has been comprehensively updated several times [e.g., Karl et al. (1990) and Easterling et al. (1996)].
The USHCN version 2 serial monthly data release used in this study is the most recent update to the HCN datasets. Version 2 data were produced using a new set of quality control and homogeneity assessment algorithms. There are two papers (Menne and Williams, 2009 and Menne et al., 2009) which provide an overall description of the adjustment methodology as well as an assessment of the version 2 maximum and minimum temperature trends. The USHCN v2 website provides a brief summary of the processing steps.