The Comprehensive Aerological Reference Data Set (CARDS) is a database of radiosonde records from worldwide sources. However, many of the records are short or incomplete, and there are clear spatial distribution deficiencies. The objective of this investigation is to select a subset of core stations, with data that give the best possible temporal and spatial coverage for studies of atmospheric temperature or water vapor trends. This is accomplished by assessing the records for content, and calculating the areas for which stations are representative, from ECMWF gridpoint analysis correlation decay. A subset of 188 core stations is chosen. Their records still contain some periods of missing data; leave some of the earth’s surface unrepresented, mostly over the southern oceans; and may have quality or homogeneity deficiencies. However, coverage, as assessed from information currently stored in CARDS, has been optimized.