Which climate classification system is based on potential evapotranspiration?

The climate classification system based on potential evapotranspiration is Thornthwaite’s climate classification. Developed by American climatologist C. W. Thornthwaite, it estimates how much water the atmosphere could remove from a surface through evaporation and plant transpiration if sufficient moisture were available.

Thornthwaite introduced potential evapotranspiration, or PET, as a central concept in his 1948 revised system. The classification compares precipitation with PET to describe moisture conditions, using measures such as humidity, aridity, and a broader moisture index. It also includes thermal-efficiency categories based on annual PET.

This differs from Köppen, which primarily uses temperature and precipitation thresholds, and from Trewartha, which modifies temperature and seasonal criteria. Bergeron is a classification approach associated with air masses and weather types rather than a PET-based global climate system.

A useful distinction is between actual evapotranspiration and PET. Actual evapotranspiration is limited by available water; PET represents atmospheric demand under conditions where water is not limiting. Thornthwaite’s method uses that potential demand to make climate moisture comparisons.