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Source: D. Stooksbury, UGA Climatologist Graphics by C. Esco, UGA CAES

What is the Palmer Drought Severity Index?

The Palmer Drought Severity Index was developed in 1965 to "measure the departure of the moisture supply."

Climatologists figure the lack of rain, or other precipitation, and the length of time of below-average moisture levels into the Index. The time factor means an unusually 'wet' month in the middle of a drought won't significantly affect the Index numbers. The Index allows climatologists to standardize their moisture condition reports for accurate comparisons between areas and over time.

The PDSI measures only weather conditions, though. It doesn't measure or take into account streamflow or groundwater levels.

For more in-depth information, go to the National Drought Mitigation Center's information on the PDSI.

Source: NDMC