El Nino Watch
Yearly rain compared with how much water the air could evaporate (P/ET0, the UNEP aridity index), plus a penalty for long dry seasons.
Hazards
ET0 is the reference evapotranspiration: how much water a well-watered grass would lose to the air. Rain divided by ET0 is the aridity index used by UNEP and the JRC World Atlas of Desertification.
A place can be humid over the year and still run dry for months: the score gets +1 if 3 or more consecutive months receive less than half of their evaporation. Local context (for example Samui's small island reservoirs and the 2026 rationing) is added to the note, with its source, but does not change the score.
The current "Water" factor (place level) uses a climate profile. It compares recent ERA5 rain with the place's own 1991-2020 normal for the same days, but how long a window counts depends on where the water comes from:
Limit today: only the last 120 days of ERA5 are stored, so the 180- and 365-day windows cannot be computed yet. The temperate level then rests on 90 days only and the explanation calls it provisional (longer windows are never estimated). A dry quarter in Normandy (for example 38% of normal over 90 days in 2026) is therefore Watch, not Act.
Water security is where El Nino hits Samui hardest. The current rain deficit is the "Water" factor of the place level.
| Aridity index P/ET0 | Class | Exposure |
|---|---|---|
| >= 0.65 | humid | 0 |
| 0.5-0.65 | dry sub-humid | 1 |
| 0.2-0.5 | semi-arid | 2 |
| 0.05-0.2 | arid | 3 |
| < 0.05 | hyper-arid | 4 |
+1 (max 4) for 3+ consecutive months with rain below half of ET0. Exposure scale: 0 very low, 1 low, 2 moderate, 3 high, 4 very high. Higher = more exposed. The colour of the matrix cell follows the same scale.