Aridity index

#As used by C. W. Thornthwaite in his 1948 climatic classification: an index of the degree of water deficit below water need at any given station; a measure of aridity.
It is calculated, independently of the opposite humidity index, as follows:

where d (the water deficit) is the sum of the monthly differences between precipitation and potential evapotranspiration for those months when the normal precipitation is less than the normal potential evapotranspiration; and where n is the sum of monthly values of potential evapotranspiration for the deficient months.
Thornthwaite puts the aridity index to two uses: 1) as a component of the moisture index; 2) as a basis for the more detailed classification of moist climates (perhumid, humid, and moist subhumid climates).

1. 
   See index of aridity.

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1. As used by C. W. Thornthwaite in his 1948 climatic classification: an index of the degree of water deficit below water need at any given station; a measure of aridity.