Observer Bias in Daily Precipitation Measurements at United States Cooperative Network Stations

899 JUNE 2007 AMERICAN METEOROLOGICAL SOCIETY | T he Cooperative Observer Program (COOP) was established in the 1890s to make daily meteorological observations across the United States, primarily for agricultural purposes. The COOP network has since become the backbone of temperature and precipitation data that characterize means, trends, and extremes in U.S. climate. COOP data are routinely used in a wide variety of applications, such as agricultural planning, environmental impact statements, road and dam safety regulations, building codes, forensic meteorology, water supply forecasting, weather forecast model initialization, climate mapping, flood hazard assessment, and many others. A subset of COOP stations with relatively complete, long periods of record, and few station moves forms the U.S. Historical Climate Network (USHCN). The USHCN provides much of the country’s official data on climate trends and variability over the past century (Karl et al. 1990; Easterling et al. 1999; Williams et al. 2004). Precipitation data (rain and melted snow) are recorded manually every day by over 12,000 COOP observers across the United States. The measuring equipment is very simple, and has not changed appreciably since the network was established. Precipitation data from most COOP sites are read from a calibrated stick placed into a narrow tube within an 8-in.-diameter rain gauge, much like the oil level is measured in an automobile (Fig. 1). The National Weather Service COOP Observing Handbook (NOAA–NWS 1989) describes the procedure for measuring precipitation from 8-in. nonrecording gauges as follows: