What is the input data and details of calculations for RAI (Rainfall Anomaly Index)

The Rainfall Anomaly Index (RAI) was developed by van Rooy (1965), and incorporates a ranking procedure to assign magnitudes to positive and negative precipitation anomalies. This index is a rain-base drought index. It is a meteorological drought index. The Rainfall Anomaly Index (RAI) Uses normalized precipitation values based upon the station history of a particular location. Comparison to the current period puts the output into a historical perspective.

When you want to use it, you can use daily or monthly precipitation data. The RAI considers two anomalies, i.e., positive anomaly and negative anomaly. First, the precipitation data are arranged in descending order. The ten highest values are averaged to form a threshold for positive anomaly and the ten lowest values are averaged to form a threshold for negative anomaly. For the details of equations refer to AgMerra Drought paper.

The arbitrary threshold values of +3 and -3 have, respectively, been assigned to the mean of the ten most extreme positive and negative anomalies. Nine abnormality classes, ranging from extremely wet to extremely dry conditions, are then given against a scale of numerical values of the relative rainfall anomaly index.

Oladipo (1985) found that differences between the RAI and the more complicated indices of Palmer and Bhalme-Mooley were negligible.

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