Monthly aridity index I = [p/(t + 10)/]">12]

i: Monthly aridity index, p: Monthly precipitation amount (mm), t: Monthly average temperature (˚C).

Annual aridity index I = [(P/(T + 10)) + (12p/(t + 10))]/2I

Annual aridity index, P: Annual precipitation amount (mm), T: Annual average temperature (˚C). p and t: Precipitation and Temperature of the driest month.