The "Standardized Precipitation Index" (SPI) is used to describe extremely dry or wet climate situations. The World Meteorological Organization (WMO) recommends, that all national meteorological and hydrological services should use the SPI for monitoring of dry spells (Press report December 2009, WMO No. 872).
The advantages of SPI usage are:
- Only precipitation data are needed for the calculation of the index.
- The index is a standardized measure for precipitation in different climatic regions and for seasonal differences.
- Calculated for different time scales: meteorological, agricultural-economic and hydrological.