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Monthly maps of drought conditions in the contiguous US as measured by several Palmer Drought Indices (Palmer, 1965) are provided for the years through present. Animations of any set of monthly maps for any period desired can be viewed by selecting the beginning and ending year and the months of interest. The following indices are available:
Palmer Z Index: measures short-term drought on a monthly scale.
Palmer Drought Severity Index (PDSI): attempts to measure the duration and intensity of the long-term drought-inducing circulation patterns. Long-term drought is cumulative, so the intensity of drought during the current month is dependent on the current weather patterns plus the cumulative patterns of previous months. Since weather patterns can change almost literally overnight from a long-term drought pattern to a long-term wet pattern, the PDSI can respond fairly rapidly.
Palmer Modified Drought Index (PMDI): Operational version of the PDSI. Description available in Heddinghaus and Sabol (1991).
Palmer Hydrological Drought Index (PHDI): measures hydrological impacts of drought (e.g., reservoir levels, groundwater levels, etc.) which take longer to develop and longer to recover from. This long-term drought index was developed to quantify these hydrological effects, and it responds more slowly to changing conditions than the PDSI.
Heddinghaus, T.R. and P. Sabol, 1991: A review of the Palmer Drought Severity Index and where do we go from here? Proceedings, 7th Conf. on Appl. Climatol., 10-13 September 1991, Boston: American Meteorological Society, 242-246.
Heim, Jr., Richard R., 2002: A review of Twentieth-Century drought indices used in the United States. Bulletin of the American Meteorological Society, vol. 83, pp. 1149-1165.
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