Exploratory Analysis of the Difference between Temperature Observations Recorded by ASOS and Conventional Methods

Abstract

The Automated Surface Observing System is currently replacing conventional observations at the National Weather Service, the Federal Aviation Administration, and other stations that report hourly observations. From a climatological viewpoint, it is necessary to compare the data from the old and new measuring systems in order to gain an understanding of their differences. These differences may become important when using time series for applications such as the computation of climatic normals, the development of homogeneous datasets for long periods of record for the investigation of climatic change, the placing of events into historical perspective, or the analysis of extreme weather events. This exploratory study of temperature data was undertaken to determine first whether there is a data continuity problem between the two observing systems and second, if there is a problem, to identify the magnitude of the problem. The most important conclusion from this study is that differences in site characteristics, even at the same airport, play as much, if not more, of a role in assessing the comparability of measurements from the two observing systems as does the instrument system bias. The instrument bias at most stations is on the order of a few tenths of a degree Fahrenheit, but the siting differences can lead to biases on the order of a couple of degrees. Not only is there a difference in the magnitude of the biases, but there is also a difference in the direction; the instrument bias is usually negative, but the siting biases can be either positive or negative.