What scale measures drought severity using precipitation, temperature, and soil-water data?

The Palmer Drought Severity Index measures drought severity using precipitation, temperature, and estimates of soil-water supply and demand.

Climatologist Wayne Palmer developed the index in the United States during the 1960s for the U.S. Weather Bureau. It compares current moisture conditions with what would be expected for a location, taking account of temperature, precipitation, and the estimated water balance.

Positive values indicate unusually wet conditions, while negative values indicate drought. Categories commonly range from extreme wetness through near normal to extreme drought. Because the index responds over longer periods, it is useful for monitoring agricultural and hydrological conditions rather than describing a single dry week.

The index has limitations. Its calculations were designed around conditions in the United States, and its fixed assumptions about soil and climate can make comparisons difficult in other regions. Other tools, including the Standardized Precipitation Index, focus more narrowly on rainfall.