Aridity Index (AI)
Index name: Aridity Index (AI).
Ease of use: Yellow.
Origins: Developed from work done by De Martonne in 1925; aridity is defined as the ratio of precipitation to mean temperature.
Characteristics: Can be used to classify the climates of various regions, because the ratio of precipitation to temperature provides a method for determining an area’s climate regime. Monthly calculation of AI can be used to determine the onset of drought, as the index takes into account temperature impacts as well as precipitation.
Input parameters: Monthly mean temperature and precipitation. For climate classification, annual values are used.
Applications: Mainly used to determine the development of drought over shorter timescales, which is helpful for identifying and monitoring agricultural and meteorological impacts.
Strengths: Easy to compute with just two inputs. Flexible in that various time steps can be analysed.
Weaknesses: Does not take into account carry-over of dryness from year to year. May be slow to react in certain climates.
Resources: Some maps and tools are available online.
Example: Iowa State University, Department of Agronomy, Lesson 4a: Derived Temperature Indices, Aridity Index.
Example: CGIAR Consortium for Spatial Information (CSI), Global Aridity and PET Database.
Example: ArcGIS, Aridity Index for Africa.
Baltas, E., 2007: Spatial Distribution of Climatic Indices in Northern Greece, Meteorological Applications, 14: 69–78. DOI: 10.1002/met.7.
De Martonne, E., 1925: Traité de Géographie Physique. Quatrième édition. A. Colin, Paris.
Currently used by: Bosnia and Herzegovina, Bulgaria, Iran, Jordan, Macedonia, Ukraine, Uzbekistan.