The aridity index is a climatic indicator that quantifies the degree of dryness of a region by comparing precipitation (P) with potential evapotranspiration (PET). The most widely used formulation internationally is the UNESCO/UNEP version: AI = P / PET. A low value indicates a dry climate; a high value, a humid climate.
UNESCO classification
- Hyper-arid: AI < 0.05 (extreme deserts: central Sahara, Atacama)
- Arid: 0.05 - 0.20 (deserts with scattered vegetation)
- Semi-arid: 0.20 - 0.50 (steppes, scrubland)
- Dry sub-humid: 0.50 - 0.65 (dry Mediterranean, savannas)
- Humid: > 0.65 (humid temperate, equatorial climates)
Spain exhibits enormous diversity: from humid zones in Galicia and the Cantabrian coast (AI > 1.0) to semi-arid zones in Almeria, Murcia, and the south-east Peninsula (AI = 0.20-0.30), which include mainland Europe's only desert (Tabernas). This gradient explains the extraordinary landscape and agricultural diversity of the Peninsula.
Climate change is shifting isohyets northward on the Iberian Peninsula, with a trend toward aridification of southern and central Spain. Models project that an additional 30-40 % of the peninsular surface will shift from sub-humid to semi-arid before 2100. See also: return period.