The United States Historical Climatology Network monthly temperature data – version 2

In support of its climate monitoring and assessment activities, NOAA's National Climatic Data Center has developed an improved version of the U.S. Historical Climatology Network temperature dataset (i.e., HCN version 2). In this paper, the HCN version 2 temperature data are described in detail, with a focus on the quality-assured dataset sources and the systematic bias adjustments. The bias adjustments are discussed in the context of their impact on U.S. temperature trends from 1895-2006 and in terms of the differences between HCN version 2 and its widely used predecessor (now referred to as HCN version 1).

Because the collective impact of changes in observation practice at U.S. HCN stations are of the same order of magnitude as the background climate signal, bias adjustments are essential to reducing the uncertainty in U.S. climate trends. The largest biases in the HCN are shown to be associated with changes to the time of observation and with the widespread changeover from liquid in glass thermometers to the maximum minimum temperature sensor (MMTS). With respect to the version 1 annual and seasonal temperature trends, HCN version 2 trends are generally somewhat smaller because of an apparent over correction in version 1 for the MMTS instrument change and because of the inclusion of adjustments for undocumented shifts in version 2