In GAEZ, the procedures proposed by Nelson and Outcalt (1987) are applied to calculate an air frost index (FI) which is used to characterize climate-derived permafrost conditions into four classes:

(i) Continuous permafrost

(ii) Discontinuous permafrost

(iii) Sporadic permafrost

(iv) No permafrost

Reference permafrost zones are determined based on prevailing daily mean air temperature (Ta). The air frost index (FI) is calculated and used to characterize permafrost areas. For this calculation, accumulated degree-days, above and below 0oC, are used to calculate the thawing index (DDT) and the freezing index (DDF). CRUTS32 as climate data source is used based on historical data for the time period 1981-2010.

The Permafrost zones classification dataset is part of the GAEZ v4 Theme 2 Agro-Climatic Resources, Climate Classification sub-theme.

For additional information on agro-climatic resources and indicators, please to Chapter 3 of the GAEZ v4 Model Documentation.