aridity index Indication of moisture deficit. All climatic classifications include arid categories, defined either by quantitative or, more usually, by mainly subjective criteria. C. W. Thornthwaite first used the term ‘aridity index’ and calculated it as 100 times the water deficit, divided by the potential evaporation. See also RADIATION INDEX OF DRYNESS; and THORNTHWAITE CLIMATE CLASSIFICATION.

aridity index An indication of moisture deficit. All climatic classifications include arid categories, defined either by quantitative or, more usually, by mainly subjective criteria. C. W.Thornthwaite first used the term ‘aridity index’ and calculated it as 100 × the water deficit/the potential evaporation.

aridity index An indication of moisture deficit. All climatic classifications include arid categories, defined either by quantitative or, more usually, by mainly subjective criteria. C. W. Thornthwaite first used the term ‘aridity index’ and calculated it as 100X the water deficit/the potential evaporation.