aridity

Aridity is the long-term dryness of a region, resulting from a persistent deficit of moisture relative to the atmospheric demand for evaporation. It describes climate dryness, not a short-term event, and is distinct from drought, which is a temporary condition. Aridity arises when precipitation is consistently low or when high temperatures, winds, or low humidity increase evaporation and water loss from soils and vegetation. It is shaped by large-scale atmospheric circulation, latitude, elevation, seasonality, as well as local factors such as soil texture and land cover.

A variety of indices are used to quantify aridity. The most common is the aridity index AI = P / PET, where P is annual precipitation and PET is potential evapotranspiration. Lower AI indicates drier conditions. In many classifications, AI values below about 0.20 characterize arid or hyperarid regions, with AI below 0.05 often described as hyperarid, and values roughly 0.20–0.50 typical of semi-arid zones. Other indices include the De Martonne aridity index, which uses mean annual temperature and precipitation. Aridity is also mapped globally to identify deserts and drylands and to assess trends over time.

Implications of aridity include limitations on agriculture, water resource stress, and impacts on biodiversity and ecosystem services. Human responses focus on water conservation, irrigation efficiency, drought-tolerant crops, and land-management practices to reduce vulnerability. Climate change is expected to modify aridity in many regions through changes in precipitation patterns and evapotranspiration, potentially expanding drylands or shifting their boundaries.