Palmer Drought Severity Index (PDSI)
Developed in 1965 by W.C. Palmer, this index quantitatively compares the actual amount of precipitation received in an area during a specified time period with the normal or average amount expected during that same period.
For the most current Index, see what the National Climatic Data Center has available.
The Index is based on the supply-and-demand concept of the water balance equation, taking into account more than only the precipitation deficit at specific locations.
The range is usually between -4.00 or less, extreme drought, and +4.00 or greater, extremely moist.
The PDSI is a meteorological drought index and responds to weather conditions that have been abnormally dry or abnormally wet. This index does not take into account lake and reservoir level and river and stream flow.
It is "calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil. From the inputs, all the basic terms of the water balance equation can be determined, including evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer. Human impacts on the water balance, such as irrigation, are not considered."
Check out what makes a heat wave or a drought.
[Source: the National Drought Mitigation Center. Additional information courtesy of the Encyclopedia of Climate and Weather, Stephen H. Schneider, Editor In Chief.]