NOAA’s National Weather Service (NWS) manages the approximately 11,000 weather-monitoring stations across the country that are part of the Cooperative Observer Program. Volunteer observers at the stations generally record daily maximum and minimum temperatures and 24-hour precipitation totals and submit the data to NWS over the telephone, by Internet, or by mail. The records for stations in the Cooperative Observer Program can stretch back well over a century, with some records predating the establishment of NWS in 1890. NWS uses data from these Cooperative Observer Program stations to support weather forecasts and warnings and other public service programs. The data are also used by others, including state climatologists, farmers, and resource planners such as energy providers that use weather information to anticipate and plan for varying levels of energy consumption. NOAA’s National Climatic Data Center (NCDC) established the USHCN in 1987 by selecting a subset of weather-monitoring stations from the existing Cooperative Observer Program network of stations. The USHCN currently consists of 1,218 stations. NCDC has twice revised the makeup of stations that compose the USHCN—in 1996 and 2009—primarily to extend the weather records of stations that have closed over time as volunteer observers have discontinued their service. To address this issue, NCDC added data from nearby stations with similar temperature trends that are continuing to gather and report data. In all, NCDC has added over 100 stations as of the latest revision in 2009. NCDC does not have a direct role in managing USHCN stations but relies on NWS’s weather forecast offices throughout the contiguous United States to continue to manage the stations as part of the larger group of weather- monitoring stations in the Cooperative Observer Program. For example, NCDC relies on weather forecast offices to maintain records on the location of the stations and other conditions that can affect weather observations, including the types of equipment used to measure temperature and precipitation and the time of observation. NCDC uses USHCN data to assess and monitor climate variation and change, including to quantify national- and regional-scale temperature trends within the contiguous 48 states. On the basis of its analysis of USHCN data, NCDC estimates that the average surface temperature across the contiguous states has warmed by about 1.4 degrees Fahrenheit since 1895. NCDC’s analysis has also identified areas of the country where temperatures have cooled or remained relatively stable. NCDC combines temperature records from the USHCN with temperature records from weather-monitoring stations around the world to analyze global temperature trends. This analysis has in turn been summarized in the assessment reports of the Intergovernmental Panel on Climate Change, an international body that reviews and assesses the most recent scientific and technical information produced worldwide relevant to the understanding of climate change. NWS headquarters establishes the policies, standards, and requirements for managing the Cooperative Observer Program, and weather forecast offices in six NWS regions (central, eastern, southern, western, Alaska, and Pacific) have responsibility for recruiting and training observers and installing and maintaining temperature-measuring equipment and rain gauges on observers’ properties. NWS applies the same standards and requirements to all stations in its Cooperative Observer Program, including those in the USHCN. In particular, NWS has established siting standards for measuring air temperature to ensure uniformity in meeting national and international requirements for climate observation. The standards, which cover conditions in the immediate vicinity of the stations, specify that temperature-measuring instruments should  not be sited on rooftops;  be installed over level terrain;  be installed at least 100 feet from any extensive concrete or paved surface;  be mounted 4 to 6 feet above the surface; and  be no closer than four times the height of any nearby building, tree, fence, or similar obstruction. NWS guidelines state that implementation of these standards should be flexible and balanced with other factors, such as the availability of space. According to NWS, these siting standards are based in part on recommendations of the World Meteorological Organization, an agency of the United Nations that, among other things, coordinates the activities of member states to generate data and information on weather and climate in accordance with international standards. For example, according to World Meteorological Organization guidelines, the best sites for measuring air temperature are over level ground; freely exposed to sunshine and wind; and not shielded by or close to trees, buildings, and other obstructions. NWS has also established management requirements for weather- monitoring stations that call for inspections of stations and updates of station records to reflect any changes. The requirements for inspections call for a minimum of one inspection by weather forecast office officials per year and specify that during these inspections, the officials are to review observers’ practices for taking weather measurements, check equipment and perform any needed repairs, and assess the conditions surrounding the station, among other things. The management requirements state that, even if there are no changes at a station, officials from weather forecast offices should update each station record at least once every 5 years. To provide a complete and permanent record of a station, NWS has designed an information system that weather forecast offices are to use to record the dates of inspections and update station records. Such records are used by NCDC and other researchers to help interpret weather records from a station and determine how factors such as station location and measurement instruments affect the weather records. Figure 1 depicts the roles of NWS and NCDC in managing the USHCN. According to NCDC officials, achieving a relatively uniform geographic distribution across the contiguous 48 states was a high priority when selecting USHCN stations and was balanced with other factors, including how long stations had collected temperature records, limited periods of missing temperature data, and the stability of measurement conditions. According to NCDC officials, consideration of siting conditions in the immediate vicinity of stations played a limited role in both the initial selections in 1987 and when stations were added in 1996 and 2009 because they considered other factors, such as geographic distribution, to be more important to the analysis of long-term temperature trends. NCDC officials told us that in selecting stations for the USHCN, the agency placed a high priority on achieving geographic distribution across the contiguous 48 states, so that the network could help identify both national and regional warming and cooling trends. To achieve the geographic distribution needed to identify regional trends, according to agency officials, NCDC aimed to select a minimum of two stations from each of the 344 climate divisions across the country. NCDC officials acknowledged that they encountered difficulties achieving the desired geographic distribution in certain areas of western states—such as Nevada—that have a relatively low population density and thus fewer stations to choose from because of a lack of volunteers to serve as observers. As a result, according to NCDC officials, station density is slightly higher across the eastern states than in the western states. Our analysis of all 1,218 USHCN stations (including active stations and those that were inactive or closed) found that while NCDC generally met its aim of two stations per climate division, 14 percent of climate divisions had fewer than two stations. As of April 2011, 20 percent of climate divisions had fewer than two active stations (see fig. 2). According to NCDC officials, the existing climate divisions are only one way to partition the nation’s climate, and if divisions were being developed today, the climate divisions would differ in number and in the areas they cover. According to NCDC officials and documents describing the process used to select USHCN stations in 1987 and to amend the list of stations in 1996 and 2009, the agency also sought stations that had temperature records dating back to the early 20th century, had limited periods of missing data, and had a limited number of station changes, but sometimes made exceptions to these factors.  Number of years of temperature records. In order to detect long-term temperature trends, NCDC aimed to select stations that had a long history of temperature records, ideally dating back to the early 20th century. In some cases, however, NCDC selected stations with a shorter history of temperature records than was ideal to ensure geographic distribution of stations across the contiguous 48 states, according to officials. NCDC officials also told us that they created composite stations to achieve a minimum record length when no stations in a particular geographic area had been collecting temperature records as long as they sought. According to NCDC officials, they create a composite station by combining data from one or two stations that have closed with data from an active station in the same area whose temperature records overlap in time with records from the closed station or stations and continue to the present. NCDC officials told us that they compare the stations’ overlapping temperature trends before creating a composite to help ensure that the climates at the stations are similar. According to NCDC documents, the initial selection of USHCN stations in 1987 included 84 composite stations, and, as of the latest revision to the network in 2009, the number of composite stations had increased to 208, largely in response to station closures. Our analysis of the 1,218 stations that make up the USHCN as of the latest revision in 2009, including composite stations, found that NCDC has largely achieved its desired record length. Specifically, as of 2010, over 85 percent of the stations had a record length dating back more than 100 years, and another 14 percent had temperature record lengths of 76 to 100 years. Less than 1 percent of stations had record lengths of 75 or fewer years.  Extent of missing data. NCDC officials told us that they also attempted to select USHCN stations with limited periods of missing data but that they often had to select stations with incomplete temperature records, including stations that were missing data for multiple years, because few stations have complete records. For example, about half the data from the Little Falls Mill Street station, located in upstate New York, are missing.