The Standardized Precipitation Index (SPI), developed by T.B. McKee, N.J. Doesken, and J. Kleist and in 1993, is based only on precipitation. One unique feature is that the SPI can be used to monitor conditions on a variety of time scales. This temporal flexibility allows the SPI to be useful in both short-term agricultural and long-term hydrological applications.
View climate division based 1-, 2-, 3-, 6-, 9-, 12-, and 24-month, SPI maps going back to 1896. The data used to produce these maps are available from the National Centers for Environmental Information.
Calculating the SPI
A discussion of time steps and how each is used.
Download a program which will allow you to calculate SPI values from a custom dataset.