Thornthwaite’s Climatic Classification
Thornthwaite’s climatic classification is more complex and empirical compared to Köppen’s, incorporating evaporation, precipitation efficiency, and potential evapotranspiration (PE) to define climate types.
Thornthwaite’s Climatic Classification: A Hydrological Perspective
Thornthwaite’s approach introduced a new way of classifying climate, focusing not only on temperature and precipitation but also on evaporation and water balance, making it particularly useful in hydrology, agriculture, and ecological studies.
📌 1931 Classification: Focus on Vegetation & Moisture Balance
1️⃣ Factors Considered
✅ Like Köppen, natural vegetation was considered a key indicator of climate.
✅ Instead of just temperature and precipitation, evaporation was added as a critical factor.
✅ Two key indices were introduced:
- Precipitation Effectiveness (P/E ratio & P/E index)
- Temperature Effectiveness (T-E index)
2️⃣ Precipitation Effectiveness (P/E Index)
💧 Precipitation effectiveness = Total precipitation available to vegetation (after accounting for evaporation losses).
- P/E Ratio = (Monthly Precipitation) ÷ (Monthly Evaporation)
- P/E Index = Sum of P/E ratios for all 12 months
🌍 5 Humidity Zones Based on P/E Index
| Humidity Zone | Vegetation | P/E Index |
| A (Wet) | Rainforest 🌴 | ≥127 |
| B (Humid) | Forest 🌳 | 64-126 |
| C (Sub-humid) | Grassland 🌾 | 32-63 |
| D (Semiarid) | Steppe 🌿 | 16-31 |
| E (Arid) | Desert 🌵 | <16 |
📌 Further Sub-Humidity Zones (Based on Seasonal Rainfall Distribution)
- r → Rainfall throughout the year
- w → Rainfall deficiency in winter
- s → Rainfall deficiency in summer
- d → Deficient rainfall for the entire year
📝 Total Possible Combinations → 5 Humidity Zones × 4 Seasonal Variations = 20 Climate Types
3️⃣ Thermal Effectiveness (T-E Index)
🌡️ Temperature effectiveness = Contribution of temperature to vegetation growth.
- Measured using positive deviations from freezing point (0°C)
- T-E Index = Sum of monthly mean temperatures above freezing
🌍 6 Temperature Provinces Based on T-E Index
| Temperature Province | T-E Index | Climate Type |
| A’ – Tropical ☀️ | ≥127 | Hot & humid |
| B’ – Mesothermal 🌲 | 64-127 | Moderate |
| C’ – Microthermal 🍂 | 32-63 | Cold winters |
| D’ – Taiga 🌲 | 16-31 | Boreal forests |
| E’ – Tundra ❄️ | 1-15 | Cold, short summers |
| F’ – Frost 🏔️ | 0 | Permafrost |
📌 1948 Classification: Introduction of Potential Evapotranspiration (PE)
Unlike the 1931 classification, which was based on vegetation, the 1948 classification focused on water balance and energy availability using the concept of Potential Evapotranspiration (PE).
1️⃣ Potential Evapotranspiration (PE)
PE measures the amount of moisture lost to evaporation and transpiration from vegetation and soil.
📌 Formula for PE:
Where:
- PE = Potential Evapotranspiration (cm)
- t = Mean monthly temperature (°C)
- I = Sum of (t/5)¹.⁵¹⁴ for 12 months
- a = A complex function of I
🔍 PE represents the energy available for water loss in a given climate.
2️⃣ Key Climatic Indices
Thornthwaite used four key indices to classify climate:
✅ 1. Moisture Index (Im)
- Measures water balance (surplus or deficit) using:
- S = Monthly moisture surplus
- D = Monthly moisture deficit
- PE = Potential Evapotranspiration
✅ 2. Thermal Efficiency Index
- Simply expressed as Potential Evapotranspiration (PE) in cm.
✅ 3. Aridity & Humidity Indices
- Measures seasonal variations in moisture adequacy.
✅ 4. Concentration of Thermal Efficiency (CTE)
- CTE = Percentage of total annual PE occurring in the 3 warmest months.
- Helps determine whether the climate is seasonal or evenly distributed throughout the year.
Evaluation of Thornthwaite’s Climatic Classification
Thornthwaite’s classification brought a scientific, hydrological approach to climate studies, but it also faced practical and methodological challenges. Here’s a critical evaluation:
🌟 Strengths of Thornthwaite’s Classification
✅ 1. Quantitative & Empirical Like Köppen
- Both Köppen and Thornthwaite used empirical methods, with climate types quantitatively determined by precipitation and temperature.
- Vegetation was used as a climate indicator, similar to Köppen.
- Letter combinations were used to designate different climate types (e.g., r, w, s, d).
✅ 2. Introduced Precipitation Efficiency & Thermal Efficiency
- Thornthwaite’s scheme improved on Köppen’s by incorporating precipitation efficiency and thermal efficiency, making it more hydrologically relevant.
- This better explains droughts, soil moisture, and plant water demand, which is crucial for agriculture and water resource management.
✅ 3. Recognized Seasonal Variations in Rainfall
- Unlike Köppen, Thornthwaite included rainfall distribution patterns (r, w, s, d), making it more realistic for places with seasonal rainfall.
✅ 4. Basis for Further Research
- Though not widely used for general climatic classification, Thornthwaite’s work influenced climate modeling, hydrology, and agricultural planning.
- His Potential Evapotranspiration (PE) concept is still used today in water resource studies and climate change research.
⚠️ Weaknesses & Criticisms of Thornthwaite’s Classification
❌ 1. Too Many Climate Types
- Thornthwaite’s system resulted in 32 times more climatic types than Köppen’s.
- This overcomplication made it difficult to apply on a global scale.
❌ 2. Difficult to Delimit Climate Boundaries
- While Köppen used temperature and precipitation thresholds to clearly mark climatic zones, Thornthwaite’s system relied on indices that made boundary delineation vague and difficult.
- The use of two indices (P/E and T-E) added complexity without necessarily improving climate classification.
❌ 3. Not Popular Among Climatologists & Meteorologists
- While botanists, zoologists, and geographers found it useful, climatologists criticized it because:
- It did not include key atmospheric factors (e.g., air masses, wind patterns, ocean currents).
- It focused too much on water balance rather than broader climatic controls.
❌ 4. Lack of Evaporation Data for All Places
- Evaporation data was (and still is) difficult to obtain, making the application of Thornthwaite’s system impractical for many regions.
- Unlike Köppen’s method, which can be applied globally, Thornthwaite’s method requires local climate data that is often unavailable.
❌ 5. 1948 Scheme Was Too Complex for Mapping
- Though the 1948 classification was more refined, it was too complex to be represented on a world climate map.
- The large number of climate types made cartographic representation impractical.
❌ 6. Complex Calculations Required
- Thornthwaite’s empirical formula for Potential Evapotranspiration (PE) requires:
- Regular, detailed climatic data
- Extensive calculations
- Data that is not always available
- This made it difficult to apply in many parts of the world.
🔍 Comparison: Köppen vs. Thornthwaite
| Feature | Köppen (Descriptive) | Thornthwaite (Hydrological) |
| Basis of Classification | Temperature & Vegetation | Water Balance & Energy Availability |
| Main Focus | Biomes (Vegetation) | Evapotranspiration (Water Loss) |
| Complexity | Simple | Complex |
| Key Variables | Temperature + Precipitation | Temperature + Precipitation + Evaporation |
| Utility | Biogeography, Geography | Agriculture, Hydrology, Climate Modeling |
| Seasonal Variations | Not considered | Included (rainfall distribution) |
| Moisture Balance | Only for deserts | Fully included (PE, moisture surplus/deficit) |
| Genetic Explanation | No (descriptive) | Partial (hydrological factors) |
🔍 Conclusion: Why Köppen Is More Widely Used Than Thornthwaite
✔ Köppen’s system remains the most widely used because it is simpler, easy to apply, and globally adaptable.
✔ Thornthwaite’s system is scientifically superior for water balance studies, but its complexity and data requirements limit its practicality.
✔ Today, modern climate classifications integrate elements from both Köppen and Thornthwaite to get a more complete picture of climate systems.
