The cluster analysis consisted of grouping stations together based upon each station's latitude, longitude, elevation and precipitation characteristics. The precipitation characteristics were developed for each season (winter, spring, summer, and autumn), so we had the ability to cluster the stations based upon data from any of the seasons. We decided that the summer season, because of the widespread convective nature of precipitation, would be our best choice to determine our clusters. We acknowledged that this is not true for all regions of the United States, but were satisfied with the results.
In the final cluster analysis based upon the station attributes and the summer precipitation characteristics for each station, 139 unique clusters were developed. The average cluster size contained 22 stations and the range of cluster populations was from 5 to 49 stations. As the clusters were being developed and stations were being placed into their clusters, statistical tests were conducted for homogeneity and discordance. After the 139 clusters were decided upon, only 37 points in clusters were considered discordant for the summer precipitation (1.21%). Discordance was tested using the clusters developed for the summer season, but with the precipitation characteristics of the other seasons. For autumn, there were 80 discordant points (2.62%); for spring, 84 discordant points (2.75%); and for winter, 75 discordant points (2.45%). These results were considered acceptable as it was noted that several points were discordant for any cluster they were put into, they were put into the cluster that made most sense geographically. We could have dismissed these stations from the Drought Atlas, but we felt that if the stations were of high quality data, we did not want to discharge any stations if we did not have to.