- The Standardized Precipitation Index (SPI) indicates how unusual the amount of accumulated precipitation is, compared to the historical record over a given time scale.
- The Standardized Precipitation-Evapotranspiration Index (SPEI) indicates how unusual a combination of precipitation and evapotranspiration (the atmospheric demand for water) is, compared to the historical record over a given time scale.
- The Palmer Drought Severity Index represents drought severity, framed from the perspective of soil moisture or dryness. The Palmer metrics are computed based upon a location’s recent precipitation, recent atmospheric evaporative demands, and its soil characteristics.
For each of the three indices described above, negative scores indicate drier conditions, and positive scores indicate wetter conditions.