Updated population metadata for United States historical climatology network stations

The United States Historical Climatology Network (HCN) serial temperature d ataset is comprised of 1211 high-quality, long-term climate observing stati ons. The HCN dataset is available in several versions, one of which include s population-based temperature modifications to adjust urban temperatures f or the "heat-island" effect. Unfortunately, the decennial population metada ta file is not complete as missing values are present for 17.6% of the 12 2 10 population values associated with the 1221 individual stations during th e 1900-90 interval. Retrospective grid-based populations; within a fixed di stance of an HCN station, were estimated through the use of a gridded popul ation density dataset and historically available U.S. Census county data. T he grid-based populations for the HCN stations provide values derived from a consistent methodology compared to the current HCN populations that can v ary as definitions of the area associated with a city change over time. The use of grid-based populations may minimally be appropriate to augment popu lations for HCN climate stations that lack any population data, and are rec ommended when consistent and complete population data are required. The rec ommended urban temperature adjustments based on the HCN and grid-based meth ods of estimating station population can be significantly different for ind ividual stations within the HCN dataset.