North American Drought Monitor Indices
Canada is not included on the Palmer maps because the applicability of the Palmer Index to high polar latitudes is still being evaluated. The maps are based on station data for Mexico and on climate division data for the contiguous U.S. plotted on the divisions' center points.
The operational Palmer Drought Index (PDI) 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 PDI can respond fairly rapidly.
The hydrological impacts of drought (e.g., reservoir levels, groundwater levels, etc.) take longer to develop and it takes longer to recover from them. The Palmer Hydrological Drought Index (PHDI), another long-term drought index, was developed to quantify these hydrological effects. The PHDI responds more slowly to changing conditions than the PDI.
The Palmer Z Index measures short-term drought on a monthly scale.