(ə′rid·əd·ē)

(climatology) The degree to which a climate lacks effective, life-promoting moisture.

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The degree to which a climate lacks moisture; in meteorology, the reverse of humidity. C. W. Thornthwaite's aridity index is based on the relationship between precipitation and potential evapotranspiration, and indicates the degree of water deficiency:

aridity index = 100dn

where d is the sum of monthly differences between precipitation and potential evapotranspiration when the former is less than the latter, and n is the sum of monthly values of evapotranspiration for the deficient months.