It is a classic index of climatology, proposed by Emmanuel de Martonne in 1926, which combines rainfall with temperature to classify the climate of a place on a single scale.
I = P / (T + 10)
- P: average annual precipitation, in mm
- T: average annual temperature, in °C
The logic is simple: the same rain yields less where it is hotter, because it evaporates more. Dividing by temperature corrects that, and the result allows you to compare places that rain alone would confuse.
| Index | Class |
|---|---|
| less than 5 | Arid |
| 5 to 10 | Semi-arid |
| 10 to 20 | Dry subhumid |
| 20 to 30 | Subhumid |
| 30 to 60 | Wet |
| more than 60 | Very humid |
It is useful as a quick label of the environment. For productive decisions, it is advisable to look at the water balance, which is more direct: it says how many millimeters are missing, not what category the place falls into.