Temperature Data Continuity In The ASOS (Automated Surface Observing System) Era–Determining Station Averages

Implementation of the Automated Surface Observing System (ASOS) by the National Weather Service during the 1990s resulted in small but seemingly systematic changes in observed temperatures at airport weather stations when compared to the previous temperature measuring system. The Climate Data Continuity Project (CDCP) was initiated to quantify these changes for NOAA and the climate data user community. With few exceptions, ASOS was found to report cooler temperatures than the stations they replaced. However, with respect to laboratory-calibrated thermometers, the ASOS temperature readings have consistently tested within ± 0.3°F.

The ASOS Climate Data Continuity Study is now nearing completion. The final stages of this project are the determination of climatic averages for selected ASOS sites across the country and the determination of a methodology that can be applied at any ASOS site. Climatic averages for periods of a few years up to the standard three-decade averages used for determining "climatic normals" are frequently used for research, management, planning decisions, education and public information as well as for providing a reference from which temperature forecasts are made. A method for utilizing recent ASOS data in combination with long-term temperature data from surrounding NWS Cooperative Stations was developed for the St. Louis, MO ASOS. Results of this methodology will be presented, and the impact that the introduction of ASOS has had on 2000 climate averages will be discussed. It is quite easy to produce averages that approximate the current and recent climate to within about 2°F. Refining estimates of climate averages are surprisingly difficult due to pre-ASOS station inhomogeneities in combination with natural climate variations, local climate differences and data inhomogeneities from nearby NWS Cooperative stations.