(press release). This article from 2000 describes how scientists use data from satellites and rain gauges along with tree-rings and lakebed sediments to understand and predict drought in North America.">


back: Dry Times in North America


A sophisticated system for measuring drought was developed by National Weather Service meteorologist Wayne Palmer in 1965. Called the Palmer Drought Severity Index, it uses temperature and rainfall information in a formula to determine dryness. This index is now the semi-official indicator of drought. The Palmer Index is most effective in determining long-term drought (a matter of several months) and is not as effective with short-term forecasts (a matter of weeks). The index uses a 0 as normal, and drought is shown in terms of negative numbers; for example, -2 is moderate drought, -3 is severe drought, and -4 is extreme drought. In mid-August 2000, sections of many western states were in the -4 range. [Images courtesy National Oceanographic and Atmospheric Administration (NOAA)]