Thornthwaite Climate Classification Calculator

Thornthwaite climate classification

In 1948 Charles W. Thornthwaite suggested classifying climates by their monthly water balance instead of by raw temperature and rainfall cutoffs. His method works through the year, comparing precipitation (P) against potential evapotranspiration (PET) one month at a time, and from that it pulls out three indices. There is a humidity index, Ih = 100·SUR/PET; an aridity index, Ia = 100·DEF/PET; and a moisture index, Im = Ih - 0.6·Ia, with SUR standing for the annual water surplus and DEF for the annual deficit. Take an area where P = 1500 mm and PET = 1200 mm and there is no real dry season: Im usually lands near 25, which puts it in the humid class.

That Im value sorts the climate into one of nine moisture classes. At the top sits A (perhumid, Im above 100), then B1-B4 (humid), C2 (moist subhumid), C1 (dry subhumid), D (semiarid), and at the bottom E (arid, Im below -40). A second letter, drawn from the PET total, marks the thermal regime: megathermal, mesothermal, microthermal, taiga, tundra or frost. Add the letters for seasonal deficit and surplus and a Thornthwaite code such as B3rB'4a' tells you not only how big the climatic water swings are but when in the year they happen.

Applications

The Brazilian agroclimatic zoning that EMBRAPA and INMET use to map risk areas for soybean, maize, coffee and citrus rests on this method, and it shows up in Agronomy, Meteorology and Geography courses. Most water-balance software, BHnorm and BHC among them, runs on the Thornthwaite-Mather (1955) book-keeping procedure, which FAO Irrigation and Drainage Paper 56 cites alongside Penman-Monteith.

FAQ

Why use Thornthwaite instead of Koppen? Koppen leans on raw temperature and rainfall. Thornthwaite actually models the water balance, so it picks up atmospheric demand (PET) and reacts better to evapotranspirative droughts.

How is PET estimated? Thornthwaite's original formula relies on monthly mean temperature and a heat index I. Newer variants reach for Hargreaves or Penman-Monteith, and FAO 56 recommends the latter whenever you have humidity, wind and radiation data to feed it.

What soil water-holding capacity should I assume? Thornthwaite-Mather started with 100 mm. In Brazil, agronomic studies tend to drop that to 75 mm for shallow soils and raise it to 125 mm for deep latossolos.