The Global Humidity Index is based on a ratio of annual precipitation and potential evapotrans- piration (these data layers are described elsewhere) as P/PET, and largely follows the classification used in a 1984 UNESCO study. The Global Humidity Index surface shows mean annual potential moisture availability for the period 1951-1980, classified into four aridity zones and one humid zone, defined in this data set as follows: Hyper-Arid Zone P/PET less than 0.05 Arid Zone 0.05 less equal P/PET less than 0.20 Semi-Arid Zone 0.20 less equal P/PET less than 0.50 Dry-Subhumid Zone 0.50 less equal P/PET less than 0.65 Humid Zone 0.65 less equal P/PET