Implications For The Assessment of Long-Term Temperature Trends

T he U.S. Historical Climate Network (USHCN;Karl et al. 1990) consists of a subset of stationsfrom the National Weather Service (NWS) co-operative (COOP) station network. The USHCN sta-tions are used in the construction of homogeneous climate data reference series and in the detection and monitoring of long-term climatic trends. Stations are selected for the USHCN dataset based on the length of period of record, the percentage of missing data in the station’s record, the total number of station moves and other station changes that may affect the homogeneity of the site’s data, and, finally, how the site contributes to the spatial coverage of the USHCN network. By themselves, these criteria, however, are not ad-equate in addressing specific station characteristics such as how well the instrument siting adheres to in-ternationally accepted standards of exposures (de-scribed fully in WMO 1996). In particular, it is im-portant to know the site of stations relative to various structures and surfaces. Generally, near-surface air temperature observations should be representative of the free-air conditions over as much of the vicinity as possible, at a height approximately 1.5 m above the ground. The site should be level, without locally sig-nificant topographical variations or steep slopes or hollows, and should offer free exposure to both sun-shine and wind (not to close to trees, buildings, or other obstructions; WMO 1996). It thus becomes critical to conclusively determine how much of any potential regional change in ob-served air temperatures might be due to land-use changes at the site itself. These changes may include local-scale urban development around the site, changes in local vegetation characteristics, etc. Those who are monitoring long-term climate trends nee