What is RAI (Rainfall Anomaly Index)?

The RAI (Rainfall Anomaly Index was developed by van Rooy (1965), and incorporates a ranking procedure to assign magnitudes to positive and negative precipitation anomalies. The RAI (Rainfall Anomaly Index 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. The thresholds are calculated by Equations 6 and 7, respectively: 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.

RAI easy to calculate, with a single input (precipitation) that can be analysed on monthly, seasonal and annual timescales. RAI has weaknesse including: Requires a serially complete dataset with estimates of missing values. Variations within the year need to be small compared to temporal variations. Oladipo (1985) found that differences between the RAI and the more complicated indices of Palmer and Bhalme-Mooley were negligible.

If you want to calculate RAI with draw severity graphs you can use Drought Indices tool.

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