Thornthwaite Climatic Classification – UPSC

• Thornthwaite’s classification grades climates by how effective precipitation and heat are for plants, not by their raw amounts.
• It was devised by the American climatologist Charles Warren Thornthwaite (1899–1963): a vegetation-based scheme for North America in 1931, extended to the world in 1933, and a rational scheme built on potential evapotranspiration in 1948.
• Its lasting gift is the water balance, basic to hydrology and irrigation planning.

The 1931 Scheme

• Basis: like Köppen, vegetation indicates climate, but two indices replace raw values.
• Precipitation effectiveness (P-E) index: the sum of 12 monthly P/E ratios.
  • Monthly P/E = 11.5 [P/(T − 10)]^(10/9), with P in inches and T in °F; the index is 10 times the sum.
• Thermal efficiency (T-E) index: the sum of monthly (T − 32)/4.

• Seasonal letters: r rain adequate in all seasons, s deficient in summer, w deficient in winter, d deficient in all seasons.
• 5 × 6 × 4 = 120 combinations were possible; 32 types appeared on the 1933 world map.

The 1948 Scheme

Potential Evapotranspiration

• Potential evapotranspiration (PE): the water that would pass to the air by evaporation plus transpiration from a full plant cover if soil water never ran short.
  • It is water need; actual evapotranspiration (AE) is what supply allows.
• Estimated from monthly temperature, corrected for day length: PE = 1.6 (10t/I)^a cm, where I is the annual heat index, the sum of (t/5)^1.514, and a is a function of I.

Water Balance

• Monthly rain is set against PE with a soil-moisture store:
  • Surplus (S): rain beyond PE once the soil is full feeds runoff and groundwater.
  • Utilisation: when PE exceeds rain, plants draw on stored water.
  • Deficit (D): the shortfall of AE below PE once the store is spent.
  • Recharge: the first rains refill the store before any surplus.
• Uses: irrigation scheduling, crop-water need and drought indices.

Indices and Provinces

• Humidity index Ih = 100S/PE; aridity index Ia = 100D/PE.
• Moisture index Im = Ih − 0.6 Ia, so a surplus counts more than an equal deficit.

• Seasonality: moist climates take r, s, w, s2, w2 from Ia (16.7 and 33.3 limits); dry climates take d, s, w, s2, w2 from Ih (10 and 20 limits).
• Summer concentration of PE (a’ to d’) completes a four-part code.
• Thornthwaite and John Mather (1955) later weighted surplus and deficit equally, Im = 100(S − D)/PE, moving the dry limits to −33.3, −66.7 and −100.

India under Thornthwaite

• V. P. Subrahmanyam (1956) classified India from about 250 stations: most of it is more arid than earlier schemes showed, and seasonal rainfall, not heat, limits growth.
• On 1991–2020 normals (100 mm soil store):

• All five are megathermal (A’).

Köppen and Thornthwaite Compared

Evaluation

• Merits: rational, judging climate by plant water need; the water balance is applied worldwide.
• Limitations:
  • Temperature-only PE ignores wind, humidity and radiation, and errs in tropical and dry lands.
  • Long calculations and too many types limited world mapping.
  • Ignores air masses, pressure and relief, the causes of climate.
• Current view: the Penman–Monteith equation (FAO, 1998) has replaced Thornthwaite’s PE in agronomy; Johannes Feddema (2005) mapped a revised Thornthwaite-type world.