Climate division data is provided on a monthly basis by the National Centers for Environmental Information. Parameters include precipitation, temperature, and various drought indices.
Navigate to NCEI's nClimDiv page to retrieve climate division data as far back as 1895, or read on to learn more about these drought indices.
The Palmer Drought Severity Index 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 (PDI) can respond fairly rapidly. Note that man-made changes are not considered in this calculation. PDSI index values generally range from -6 to +6, where negative values denote dry spells, and positive values denote wet spells.
The Palmer Hydrological Drought Index is another long-term drought index that takes into account hydrological impacts (e.g., reservoir levels, groundwater levels, etc.), as such impacts take longer to develop, and it takes longer to recover from them. The PHDI responds more slowly to changing conditions than the PDSI (PDI). Note that man-made changes, such as increased irrigation, new reservoirs, and added industrial water use were not included in the calculations for this index. As with PDSI, PHDI values generally range from -6 to +6, where negative values denote dry spells, and positive values denote wet spells.
The Palmer "Z" Index shows how the moisture conditions depart from normal, and can reflect above-normal precipitation events, even during periods of drought. Z index values typically range from -2.75 to 3.5, where negative values indicate dry spells, and positive values indicate wet spells.
The Modified Palmer Drought Severity Index was instituted by the National Weather Service Analysis Center, who modified the PDSI for operational meteorological purposes. The selection for the modified PDSI value for a given month is made internally by the program based on probabilities, in that the modification incorporates a weighted average of the wet and dry index terms using probability as the weighting factor. In general, PMDI and PDSI values will be the same during an established wet or dry spell, but will differ during periods of transition.
The Standardized Precipitation Index is a probability index that considers only precipitation. The SPI is based on the probability of recording a given amount of precipitation, and the probabilities are standardized so that an index of zero indicates the median precipitation amount (half of the historical precipitation amounts are below the median, and half are above the median). The index is negative for drought, and positive for wet conditions. As the dry or wet conditions become more severe, the index becomes more negative or positive. The SPI is computed by NCDC for several time scales, ranging from one month to 24 months, to capture the various scales of both short-term and long-term drought.