Thornthwaite Climate Classification Calculator
Classifies the climate type by Thornthwaite using the annual water balance in millimeters and potential evapotranspiration.
—
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.
Related Tools
BR Köppen Climate Classification Calculator
Classifies the climate type by Köppen using monthly average temperatures and accumulated annual precipitation in millimeters.
Pulp Dilution Water
Computes how much water must be added to take a pulp from one mass percent solids to a lower one, Water = M × (C₁/C₂ − 1), where M is the incoming pulp mass (or mass flow). It follows from the mass balance: the solids mass does not change on dilution, so the final pulp mass is M·C₁/C₂ and the difference is water. This is the most routine operation in a mineral processing plant — grinding, desliming, flotation and thickening each demand their own percent-solids range, and getting the dilution water wrong throws off residence time, viscosity and reagent consumption. Both percentages are by mass (weight of solids per weight of pulp), and the result comes out in the same unit entered for M. Enter the pulp mass or flow, the current percent solids and the target percent solids.
Mass Recovery
Calculate the mass recovery (mass yield) of a mineral processing operation, R = (concentrate mass ÷ feed mass) × 100%, dividing the concentrate mass produced by the ore feed mass. The result, in %, is the fraction of mass reporting to the concentrate — different from metallurgical recovery (which measures the fraction of metal recovered). Low mass recovery is typical of lean ores (little concentrate from much feed); high indicates rich ore or poorly selective concentration. Combined with the grades, it closes the plant's mass balance. Enter the concentrate and feed masses.
Soil Group Index (HRB/AASHTO)
Computes the group index of the HRB/AASHTO M 145 classification, the number in parentheses that follows the soil symbol in a soil report: GI = 0.2a + 0.005ac + 0.01bd, where a and b come from the percentage passing the No. 200 sieve and c and d come from the liquid limit and the plasticity index. Each term is truncated — a and b from 0 to 40, c and d from 0 to 20 — and the result is rounded to an integer and never negative, which makes the index range from 0 to 20. The higher the group index, the worse the soil as a subgrade: 0 points to clean, well behaved granular material, values above 12 point to plastic clay that only works once replaced or stabilised, and this is the number that feeds the pavement thickness charts. The complete formula with both terms was adopted; for subgroups A-2-6 and A-2-7 the standard calls for the 0.01bd term alone, and the result is the same: every A-2 soil has at most 35% passing the No. 200 sieve, which is exactly where the a term goes to zero, so the first two terms drop out on their own. Enter the percentage passing the No. 200 sieve, the liquid limit and the plasticity index.
Mann-Kendall Trend Test
Computes the Mann-Kendall test, used to detect a monotonic trend (upward or downward) in a time series without assuming the data are normal or the trend is linear. It sums the signs of all pairwise comparisons between values: if the series tends to rise over time, positive signs prevail. It's widely used in hydrology, climatology and environmental science to assess trends in long series. The S statistic becomes a z via the normal approximation, with a tie correction. Enter the series in chronological order.
Water Absorption (Ceramic)
Compute the water absorption of a ceramic piece, WA = (wet mass − dry mass)/dry mass·100%, the amount of water the open pores absorb by immersion. It is the property that classifies ceramic tiles: porcelain (WA ≤ 0.5%, very dense and strong), stoneware, semi-stoneware, semi-porous and porous (wall tile, WA > 10%). The lower the absorption, the more sintered and resistant the piece. Enter the wet mass and the dry mass.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.