The Palmer Drought Index, sometimes called the Palmer Drought Severity Index, is an often-used measurement of dryness based on recent precipitation and temperature.
It was developed by meteorologist Wayne Palmer , who first published his method in the 1965 paper Meteorological Drought for the Office of Climatology of the U.S. Weather Bureau.
The Palmer Drought Index is based on a supply-and-demand model of soil moisture. Supply is comparatively straightforward to calculate, but demand is more complicated as it depends on many factors - not just temperature and the amount of moisture in the soil but hard-to-calibrate factors including evapotranspiration and recharge rates.
The index has proved most effective in determining long-term drought — a matter of several months — and not as good with forecasts over a matter of weeks. It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought.
