Palmer Drought Index
Encyclopedia
The Palmer Drought Index, sometimes called the Palmer Drought Severity Index and often abbreviated PDSI, is a measurement of dryness based on recent precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

. 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
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...

 and recharge rates. Palmer tried to overcome these difficulties by developing an algorithm that approximated them based on the most readily available data — precipitation and temperature.

The index has proven most effective in determining long-term drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...

 — a matter of several months — and not as good with conditions over a matter of weeks. It uses a 0 as normal, and drought is shown in terms of negative numbers; for example, negative 2 is moderate drought, negative 3 is severe drought, and negative 4 is extreme drought. Palmer's algorithm also is used to describe wet spells, using corresponding positive numbers. Palmer also developed a formula for standardizing drought calculations for each individual location based on the variability of precipitation and temperature at that location. The Palmer index can therefore be applied to any site for which sufficient precipitation and temperature data is available.

Critics have complained that the utility of the Palmer index is weakened by the arbitrary nature of Palmer's algorithms, including the technique used for standardization. The Palmer index's inability to account for snow and frozen ground also is cited as a weakness.

The Palmer index is widely used operationally, with Palmer maps published weekly by the United States Government's National Oceanic and Atmospheric Administration
National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration , pronounced , like "noah", is a scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere...

. It also has been used by climatologists to standardize global long-term drought analysis. Global Palmer data sets have been developed based on instrumental records beginning in the 19th century. In addition, dendrochronology
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

 has been used to generate estimated Palmer index values for North America for the past 2000 years, allowing analysis of long term drought trends. The Index has also been used as a means of explaining the late Bronze Age collapse
Bronze Age collapse
The Bronze Age collapse is a transition in southwestern Asia and the Eastern Mediterranean from the Late Bronze Age to the Early Iron Age that some historians believe was violent, sudden and culturally disruptive...

.

In the US, regional Palmer maps are featured on the cable channel Weatherscan
Weatherscan
Weatherscan is a TV channel offered by The Weather Channel. Carried on select cable systems in the United States, Weatherscan features uninterrupted local weather information in graphical format on a continuous loop...

.

External links

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