Soils Home | Advanced | World Map

Global Humidity Zones Map Legend

Our Global Humidity Zones map was produced from the UNEP/GRID Documentation Summary for Data Set: Global Humidity Index from GRID and UEA/CRU, derived from the World Atlas of Desertification that was published by UNEP in 1992. The atlas and data base were produced through a cooperative effort between UNEP's Desertification Control Programme Activity Centre (DC/PAC), the Global Environment Monitoring System (GEMS) and the Global Resource Information Database (GRID). The Global Humidity Index Map is based on a ratio of annual precipitation and potential evapotranspiration (P/PET), and largely follows the classification used in a 1984 UNESCO study. The Global Humidity Index Map shows mean annual potential moisture availability for the period 1951-1980, classified into four aridity zones, one humid zone, and one cold tundra mountains zone.

Hyper-Arid Zone                     P/PET  <  0.05
Arid Zone                 0.05  <=  P/PET  <  0.20
Semi-Arid Zone            0.20  <=  P/PET  <  0.50
Dry-Subhumid Zone         0.50  <=  P/PET  <  0.65
Humid Zone                0.65  <=  P/PET
Cold                      Areas that have more than six months of an average
                          temperature below 0 degrees and not more than three
                          months where the temperatures reach above 6 degrees
                          centigrade (roughly equivalent to Koeppen climate
                          zones Dc, Df E).

The nomenclature for the aridity zones is standard in the scientific literature, and each is characterized by a series of general criteria described by Arnold (1992, p. 5).

The source document for the Global Humidity Index data set is as follows:

Deichmann, Uwe and Lars Eklundh. July 1991. "Global digital data sets for land degradation studies: a GIS approach." GRID Case Study Series No. 4; UNEP/GEMS and GRID; Nairobi, Kenya; 103 pages

Koeppen, W. 1931. Das Geographische System der Klimate. Handbuch der Klimatologie. Volume 1. Berlin, Germany.

UEA/CRU Report. August 1990. Phase II Global Humidity Index for 1930-59 and 1960-89. Draft version.

UEA/CRU Report. September 1990. Additional Data for Phases I and II.

Arnold, E. 1992. World Atlas of Desertification. UNEP's Desertification Control Programme Activity Centre, Global Environment Monitoring System, and the Global Resource Information Database. London (UK).

UNESCO. 1984. Map of the world distribution of arid regions. Paris, France.

Global Resource Information Database compiled and/or derived most of the global and regional databases, produced the maps, and carried out the data analyses and tabulations for the Atlas, assisted by a Technical Advisory Group on Desertification Assessment and Mapping composed of various international experts. The climate data were obtained from the Climate Research Unit of the University of East Anglia, U.K.


Last revised: 16 November 2001
Site created and maintained by the Office of Arid Lands Studies