2.1.1.3. The UNESCO Climate Classification System

2.1.1.3. The UNESCO Climate Classification of Arid Zones

The UNESCO system for differentiating agroclimatic zones is straightforward. Similar to the Papadakis system (Papadakis, 1970), it relies on three main criteria:

  • Moisture regimes
  • Winter types
  • Summer types

In this classification, the moisture regime is determined by the ratio of annual rainfall to annual potential evapotranspiration (PET), calculated using the Penman method[1]. This ratio, also known as the aridity index, reflects the interaction between moisture supply (precipitation) and moisture demand (PET), resulting in distinct moisture regime classes.
The winter type is defined by the mean temperature of the coldest month, while the summer type is defined by the mean temperature of the warmest month.
Tables 4, 5, and 6 present the classifications for moisture regime classes, winter types, and summer types, respectively.

[1] For details on the calculation of potential evapotranspiration is referred to here and here. Note that the UNESCO system used the Standard Penman method, which is quite similar to the newer Penman-Monteith method.

Although originally designed for the differentiation of arid zones, the system can easily be extended to include also more humid climates. For example, the moisture regime Per-humid (aridity index >1), which was not included in the original system, can be added to improve differentiation within humid zones.

The UNESCO system is a closed classification, in which combinations of moisture regimes, winter and summer types form unique climatic patterns, or agroclimatic zones. For instance, the pattern ‘PH-C-M’ represents a climate characterized by a per-humid moisture regime, cool winter type and mild summer type.
By excluding theoretically impossible combinations, the system identifies a maximum of 78 distinct climates, all of which are listed with brief descriptions here.

Despite its simplicity and wide class limits, the UNESCO system enables effective differentiation and mapping of climatic patterns, especially in dryland areas. Examples of its application can be found in Pub.39 (Arabian Peninsula), Pub.48 (Iran), Pub.60 (Central Asia), Pub.67 (West Asia & North Africa), and Pub.112 (Africa).