Online Drought Indicators and Indices

Standardized Anomaly Index (SAI)


Index name: Standardized Anomaly Index (SAI).

Ease of use: Yellow.

Origins: Introduced by E.B. Kraus in the mid-1970s and was examined closely by Katz and Glantz at the National Center for Atmospheric Research, United States, in the early 1980s. SAI was developed based on RAI, and RAI is a component of SAI. They are similar, but both are unique.

Characteristics: Based upon the results of RAI, and was developed to help identify droughts in susceptible regions, such as the West African Sahel and north-east Brazil. RAI accounts for station-based precipitation in a region and standardizes annual amounts. Deviations are then averaged over all stations in the region to obtain a single SAI value.

Input parameters: Precipitation at monthly, seasonal or annual time steps.

Applications: Identifying drought events, especially in areas frequented by drought.

Strengths: Single input, which can be calculated for any defined period.

Weaknesses: Only uses precipitation, and calculations are dependent on quality data.

Resources: Equations for the calculations are provided in the literature.


Katz, R.W. and M.H. Glantz, 1986: Anatomy of a rainfall index. Monthly Weather Review, 114: 764–771.

Kraus, E.B., 1977: Subtropical droughts and cross-equatorial energy transports. Monthly Weather Review, 105(8): 1009–1018.

Leave a comment