SPI

The Standardized Precipitation Index (SPI) is the global go-to tool for tracking droughts and wet spells, mostly because it keeps things simple by only looking at rainfall data. Developed by McKee et al. in 1993 and expanded by Edwards and McKee in 1997, it works by comparing recent rainfall over specific timeframes like 1, 3, or 12 months against long-term historical data. While direct rainfall shortages can sometimes feel a bit more intense at a local level than what the index shows, the SPI's real strength is its standardized math, which makes it incredibly easy to compare climate conditions across different regions and time periods.