Dryness; aridity is the most common criterion for the definition and classification of drylands, occurring when evapotranspiration exceeds precipitation (Adamo and Crews-Meyer (2006) Appl. Geog. 26, 1). Thomas, in J. Holden (ed.) (2012) considers the annual overall net negative moisture balance to be the defining characteristic of deserts and drylands.

An aridity index is a numerical indicator of the degree of dryness of the climate at a given location, expressed as the ratio of annual potential evaporation to precipitation, and the UNEP index of aridity (1992 World Atlas of Desertification) is defined as:

where PET is the potential evapotranspiration and P is the average annual precipitation. PET and P must be expressed in millimetres.

Moriyama et al. (2010, Int. Archs Photogrammetry, Remote Sensing & Spatial Information Science, 38, part 8) propose a satellite-derived aridity index, derived from the day and night surface temperatures and the maximum solar flux (See Arora (2002) J. Hydrol. 265).