
Real Estate UK

Catania

Berlino Quarter

Mina el Cerrejon
The Aridity Index (AI) is the ratio between mean annual precipitation (P) and potential evapotranspiration (PET): AI = P / PET. A low value indicates an arid area (evapotranspiration exceeds precipitation), while a high value indicates a humid area (precipitation exceeds evapotranspiration).
Aridity is a determining factor for vegetation distribution, water availability and ecosystem composition. Areas with a very low aridity index (<0.03) are classified as hyper-arid and host desert ecosystems with extremely limited biodiversity. Humid areas (AI > 0.65) support richer and more productive ecosystems.
Data are sourced from the Global Aridity Index v2 (CGIAR-CSI) at approximately 1 km resolution. The AI is calculated as the ratio between mean annual precipitation (WorldClim/CHELSA) and potential evapotranspiration calculated using the Penman-Monteith method.
The classification adopted follows the UNEP (1992) definition, the international standard for aridity classification:
| Class | AI Range | Description |
|---|---|---|
| Hyper-arid | < 0.03 | Desert; extremely rare precipitation |
| Arid | 0.03 – 0.20 | Semi-desert; sparse xerophilous vegetation |
| Semi-arid | 0.20 – 0.50 | Savanna, steppe; drought-adapted vegetation |
| Dry sub-humid | 0.50 – 0.65 | Grasslands, Mediterranean scrub |
| Humid | > 0.65 | Forests, water-rich ecosystems |
Unit: dimensionless (ratio of mm precipitation to mm evapotranspiration)
| Code | Name | Provider | Resolution | Availability |
|---|---|---|---|---|
WRD_ARIDX_22 | Global Aridity Index v2 | CGIAR-CSI | 1000 m | 2022 |
| Indicator | Unit | Range | Inverted |
|---|---|---|---|
aridity | — | [0, 0.05, 0.15, 0.35, 0.75, 1.5] | No |
Not inverted: higher values indicate more humid areas with greater water availability.
| Level | AI Range | UNEP Class | Interpretation |
|---|---|---|---|
| E (Critical) | < 0.05 | Hyper-arid | Desert; minimal biodiversity and water availability |
| D (Poor) | 0.05 – 0.15 | Arid | Semi-desert; sparse xerophilous vegetation |
| C (Moderate) | 0.15 – 0.35 | Semi-arid | Savanna/steppe; drought-adapted vegetation |
| B (Good) | 0.35 – 0.75 | Dry sub-humid to humid | Grasslands, scrub, open forests |
| A (Excellent) | > 0.75 | Humid | Rich forests; high water and ecological availability |
aridity
Ratio between mean annual precipitation and potential evapotranspiration (Penman-Monteith method). Source: Global Aridity Index v2 (CGIAR-CSI, ~1 km, reference period 1970–2000). UNEP classification: < 0.03 hyper-arid, 0.03–0.20 arid, 0.20–0.50 semi-arid, 0.50–0.65 dry sub-humid, > 0.65 humid.