Climatic classification systems serve as fundamental tools in climatology, providing a structured framework for understanding and categorizing the Earth’s diverse climates. These systems are crucial for various disciplines, including agriculture, hydrology, ecology, urban planning, and resource management, as they allow for the systematic study of climatic patterns and their interactions with the environment. Historically, climatic classification evolved from empirical approaches, which primarily relied on observable features like temperature and precipitation thresholds and their correlation with vegetation zones, to more rational or genetic approaches that sought to understand the underlying physical processes driving climate.

Among the pioneering rational classification systems, Charles Warren Thornthwaite’s contributions stand out. Dissatisfied with purely empirical methods that often lacked a strong theoretical basis, Thornthwaite developed a system that focused on the dynamic interplay between moisture supply (precipitation) and moisture demand (evapotranspiration). His work, particularly the revised system of 1948, introduced the groundbreaking concept of potential evapotranspiration and the water balance, fundamentally transforming the way climatologists conceived of and quantified climate. This system aimed to classify climates based on the actual physical processes governing the exchange of water and energy between the Earth’s surface and the atmosphere, thereby offering a more nuanced and application-oriented understanding of climatic conditions.

Bases of Thornthwaite’s Climatic Classification System

Thornthwaite’s approach to climatic classification underwent significant evolution, resulting in two primary systems: the earlier 1931 system and the more refined and widely recognized 1948 system. Both systems shared a common philosophical underpinning: to move beyond mere statistical averages of temperature and precipitation towards an understanding of their effectiveness in supporting plant growth and other environmental processes.

The 1931 System: Indices of Effectiveness

Thornthwaite’s initial system, proposed in 1931, laid the groundwork by introducing two key indices: the Precipitation Effectiveness (P-E) Index and the Temperature Effectiveness (T-E) Index. This system sought to quantify the climatic parameters in terms of their direct biological relevance, rather than just their absolute values.

  • Precipitation Effectiveness (P-E) Index: This index was designed to represent the efficiency of precipitation in supplying moisture for vegetation. Thornthwaite recognized that not all precipitation is equally effective; high temperatures lead to greater evaporation and less effective precipitation. The P-E Index was calculated by summing monthly P/E ratios, where E represented an idealized evaporation value derived from a formula that incorporated temperature. Specifically, the monthly P-E ratio was given by 115 * (P/T - 10)^(10/9), where P is monthly precipitation in inches and T is mean monthly temperature in degrees Fahrenheit. The annual P-E Index was the sum of the 12 monthly P-E ratios. Higher P-E indices indicated more humid conditions suitable for lush vegetation. Climates were then grouped into categories like Arid, Semi-arid, Subhumid, Humid, and Perhumid based on these index values.

  • Temperature Effectiveness (T-E) Index: Complementary to the P-E Index, the T-E Index aimed to quantify the thermal energy available for plant growth. It recognized that plant growth is limited by temperature, with certain thresholds required for metabolic activity. The monthly T-E ratio was expressed as (T - 32) / 4, where T is mean monthly temperature in degrees Fahrenheit. The annual T-E Index was the sum of the 12 monthly T-E ratios. This index allowed for the classification of climates into thermal regimes such as Microthermal, Mesothermal, and Megathermal, corresponding to different vegetation types.

In addition to these indices, the 1931 system also incorporated the concept of the seasonal distribution of precipitation effectiveness. This was represented by a letter (A’, B’, C’, D’) indicating whether precipitation was effective throughout the year, concentrated in summer, or concentrated in winter. While an improvement over previous systems, the 1931 system was still limited by its empirical nature and the lack of a robust theoretical basis for the “effectiveness” values, particularly its inability to precisely account for actual water loss from the surface.

The 1948 System: A Rational Approach Based on Water Balance

Thornthwaite’s 1948 classification system marked a paradigm shift, moving from empirical indices to a more rational, process-based approach rooted in the concept of the water balance. This system became his most significant contribution to climatology and is the one predominantly referred to when discussing “Thornthwaite’s climatic classification.” The core innovation was the introduction of Potential Evapotranspiration (PET) as a central climatic element, along with a detailed methodology for calculating the water balance.

Potential Evapotranspiration (PET)

Thornthwaite defined Potential Evapotranspiration (PET) as the amount of water that would evaporate from the soil surface and transpire from plants if there were always sufficient water available. It represents the maximum possible water loss under prevailing atmospheric conditions and is primarily a function of thermal energy. Unlike actual evapotranspiration (AET), which is limited by water availability, PET represents the atmospheric demand for water. Thornthwaite argued that PET, being an indicator of thermal efficiency, is as important as temperature itself in determining the energy exchange at the Earth’s surface.

Calculation of PET: Thornthwaite developed an empirical formula to estimate monthly PET based primarily on mean monthly temperature and day length:

$PET = 1.6 \left(\frac{10T}{I}\right)^a$

Where:

  • PET is the monthly potential evapotranspiration in centimeters.
  • T is the mean monthly air temperature in degrees Celsius.
  • I is the annual heat index, which is the sum of 12 monthly heat indices (i): $I = \sum_ i_j$.
  • i is the monthly heat index, calculated as $i = \left(\frac{T}{5}\right)^{1.514}$. This component integrates the non-linear relationship between temperature and evaporation rates.
  • a is an exponent that is a complex cubic function of the annual heat index (I): $a = (0.000000675 I^3) - (0.0000771 I^2) + (0.01792 I) + 0.49239$.

Additionally, this initial PET value is unadjusted for day length. Thornthwaite provided a correction factor for each month based on latitude, adjusting the PET value to account for varying hours of daylight, which directly influences the amount of solar radiation available for evapotranspiration. This explicit incorporation of day length was a significant improvement, reflecting the seasonal variation in solar energy receipts.

The Water Balance Concept

The heart of the 1948 system lies in the water balance equation, which quantitatively describes the movement and storage of water in a given area over time. The fundamental equation is:

$P = AET + S + \Delta SM + R$

Where:

  • P = Precipitation (water supply)
  • AET = Actual Evapotranspiration (actual water loss from the surface)
  • S = Water Surplus (water runoff or deep percolation)
  • ΔSM = Change in Soil Moisture storage
  • R = Runoff (sometimes included in S, or separated for hydrological analysis)

Thornthwaite’s water balance model calculates monthly values of AET, water surplus, and water deficit by comparing monthly precipitation (P) with monthly PET. The model assumes a certain soil moisture storage capacity (typically 100mm or 150mm), which acts as a buffer.

  1. When P > PET: If precipitation exceeds the demand for evapotranspiration, the excess water first recharges the soil moisture storage up to its capacity. Any further excess becomes water surplus (S), leading to runoff or deep percolation.
  2. When P < PET: If precipitation is less than the potential demand, plants draw water from the soil moisture storage. This continues until the soil moisture is depleted. The difference between PET and the water actually supplied from precipitation and soil moisture is the water deficit (D). This deficit represents the amount of water required to achieve PET.

By performing these calculations monthly throughout the year, Thornthwaite’s system quantifies the seasonal variations in moisture availability, leading to a more dynamic understanding of climate.

Thornthwaite’s Classification Components

The 1948 system classifies climates based on four main indices derived from the water balance computations:

  1. Moisture Index (Im): This is the primary index for moisture classification and is derived from the annual water surplus (S) and water deficit (D) in relation to annual PET. It quantifies the degree of humidity or aridity.

    $I_m = 100 \times \left(\frac{S - D}{PET}\right)$

    Where S is annual water surplus and D is annual water deficit. The values of Im define the following moisture regimes:

    • A - Perhumid: Im > 100 (very wet, consistent surplus)
    • B - Humid: Im 20 to 100 (wet, moderate surplus)
      • B4: 80 to 100
      • B3: 60 to 80
      • B2: 40 to 60
      • B1: 20 to 40
    • C2 - Moist Subhumid: Im 0 to 20 (slightly moist, small surplus)
    • C1 - Dry Subhumid: Im -33.3 to 0 (slightly dry, small deficit)
    • D - Semi-arid: Im -66.7 to -33.3 (dry, moderate deficit)
    • E - Arid: Im < -66.7 (very dry, large deficit)

  2. Thermal Efficiency Index (PET): This index directly uses the calculated annual Potential Evapotranspiration as a measure of thermal energy available for plant growth. It essentially replaces the earlier T-E Index.

    The thermal classification is as follows:

    • A’ - Megathermal: PET > 114 cm (hot, tropical climates)
    • B’ - Mesothermal: PET 57 to 114 cm (warm temperate climates)
      • B’4: 99.7 to 114 cm
      • B’3: 85.5 to 99.7 cm
      • B’2: 71.2 to 85.5 cm
      • B’1: 57.0 to 71.2 cm
    • C’ - Microthermal: PET 28.5 to 57 cm (cool temperate climates)
      • C’2: 42.7 to 57.0 cm
      • C’1: 28.5 to 42.7 cm
    • D’ - Taiga: PET 14.25 to 28.5 cm (cold, boreal climates)
    • E’ - Tundra: PET 7.1 to 14.25 cm (very cold, polar margin)
    • F’ - Frost: PET < 7.1 cm (permanently frozen, polar regions)

  3. Summer Concentration of Thermal Efficiency: This index (s) indicates the percentage of annual PET that occurs during the three warmest consecutive months. It captures the seasonality of thermal energy availability, which is crucial for distinguishing climates with similar annual PET but different seasonal distributions of heat.

    • a’: > 48.0% (Strong summer concentration)
    • b’4: 42.0 - 48.0%
    • b’3: 36.0 - 42.0%
    • b’2: 30.0 - 36.0%
    • b’1: 24.0 - 30.0%
    • c’2: 18.0 - 24.0%
    • c’1: 12.0 - 18.0%
    • d’: < 12.0% (Very little summer concentration, often indicative of equatorial climates or cold climates with short, weak summers)

  4. Seasonal Variation of Effective Moisture: This index describes the timing and severity of water surplus or deficit. It uses the aridity index (Ia = 100 * D/PET) and humidity index (Ih = 100 * S/PET) to specify whether a climate experiences moisture deficit (d) or surplus (s) and during which season.

    • r (little or no water deficit): Indicates that the climate is humid enough that there is little or no moisture deficit, or a small deficit that is balanced by a small surplus. Applicable to Perhumid (A), Humid (B), and some Moist Subhumid (C2) climates.
    • s (moderate summer water deficit): Applicable to humid climates (A, B, C2) where a moderate water deficit occurs in summer.
    • w (moderate winter water deficit): Applicable to humid climates (A, B, C2) where a moderate water deficit occurs in winter.
    • s2 (severe summer water deficit): Applicable to humid climates (A, B, C2) with a severe summer deficit.
    • w2 (severe winter water deficit): Applicable to humid climates (A, B, C2) with a severe winter deficit.
    • S (summer water surplus): Applicable to arid and semi-arid climates (D, E, C1) where a water surplus occurs in summer.
    • W (winter water surplus): Applicable to arid and semi-arid climates (D, E, C1) where a water surplus occurs in winter.
    • d (little or no water surplus): For arid or semi-arid climates with no significant surplus.

The Complete Climate Formula

Thornthwaite’s complete climate classification symbol combines these four indices, forming a comprehensive descriptor for each location. A typical classification might look like B4 s b’3.

  • B4: Represents the moisture regime (Humid, specifically the wetter end of Humid).
  • s: Indicates the seasonal variation of effective moisture (moderate summer water deficit).
  • b’3: Indicates the thermal efficiency and its summer concentration (Mesothermal, with a specific range of summer concentration).

This four-part code provides a detailed quantitative description of a climate’s moisture balance, overall heat budget, and the seasonal distribution of both.

Evaluation of Thornthwaite’s Classification System

Thornthwaite’s 1948 classification system represents a landmark achievement in climatology, introducing concepts and methodologies that profoundly influenced subsequent research. However, like any scientific model, it has both significant strengths and notable limitations.

Strengths (Advantages)

  1. Rational and Quantitative Basis: Thornthwaite’s system is lauded for its rational and genetic approach. Instead of simply categorizing climates based on observed averages, it attempts to model the underlying physical processes of water and energy exchange. This quantitative basis provides precise, measurable indices that allow for rigorous comparison between different regions.
  2. Focus on Water Balance: The central role of the water balance concept was revolutionary. By integrating precipitation, potential evapotranspiration, soil moisture storage, surplus, and deficit, the system provides a holistic view of the hydrologic cycle at a location. This makes it highly relevant for applications in hydrology, water resource management, and drought assessment.
  3. Applicability in Applied Climatology: The system’s emphasis on moisture availability and thermal efficiency makes it exceptionally valuable for practical applications.
    • Agriculture: Helps in irrigation planning by identifying periods and magnitudes of water deficit, and in crop selection based on thermal requirements.
    • Forestry and Ecology: Aids in understanding the distribution of natural vegetation and forest types, as plant growth is directly linked to moisture and heat availability.
    • Urban Planning: Informs decisions regarding water supply, drainage systems, and green infrastructure.
    • Climatic Change Studies: The indices can be used to track changes in moisture and thermal regimes over time, providing insights into the impacts of climate change.
  4. Global Comparability: The use of standardized indices based on physical principles allows for consistent comparison of climates across different parts of the world, fostering a deeper understanding of global climatic patterns.
  5. Reflects Vegetation: The chosen indices, particularly the moisture index and thermal efficiency, often show a strong correlation with the distribution of major vegetation zones, indicating its ecological relevance.
  6. Incorporation of Soil Moisture: Acknowledging the role of soil as a temporary reservoir for water (soil moisture storage) was a critical advancement, providing a more realistic portrayal of water availability for plants.

Weaknesses (Limitations and Criticisms)

  1. Complexity of PET Calculation: While innovative, the empirical formula for PET is purely based on temperature and day length. It notably does not account for other critical meteorological factors that influence evapotranspiration, such as wind speed, humidity, and net radiation. This can lead to inaccuracies, particularly in regions with high wind, low humidity, or complex terrain where radiation patterns are highly variable. For instance, in windy, arid regions, actual evapotranspiration might be higher than predicted by Thornthwaite’s formula, which would underestimate the moisture deficit.
  2. Assumptions about Soil Moisture Capacity: The system typically assumes a fixed soil moisture storage capacity (e.g., 100mm, 150mm, or 300mm), which is a simplification. In reality, soil moisture capacity varies significantly based on soil type (sand vs. clay), soil depth, vegetation cover, and topography. This fixed assumption can lead to errors in calculating water surplus and deficit, especially in areas with diverse soil characteristics.
  3. Data Requirements: The system requires comprehensive monthly average temperature and precipitation data. For many remote or underdeveloped regions, such long-term, reliable meteorological data may not be readily available, limiting the applicability of the system globally.
  4. Arbitrary Boundaries: Although based on calculations, the specific threshold values used to define the boundaries between climate categories (e.g., humid vs. subhumid, mesothermal vs. microthermal) are ultimately arbitrary. They might not always perfectly align with ecological transitions or local perceptions of climate.
  5. Ignores Extreme Events: The reliance on monthly average values smooths out daily or extreme climatic events, such as intense downpours followed by prolonged dry spells, or sudden heatwaves. These short-term events, however, can have significant ecological and hydrological impacts that are not captured by the system.
  6. Lack of Synoptic Basis: Thornthwaite’s system is descriptive and quantitative but lacks a direct link to the synoptic climatology, i.e., the large-scale atmospheric circulation patterns, air masses, and frontal systems that fundamentally drive weather and climate. It describes the result of climatic processes but not the causes in terms of atmospheric dynamics.
  7. Difficulty in Interpretation for Laypersons: The multi-part alphanumeric classification codes (e.g., B4 s b’3) are highly informative for specialists but can be less intuitive and harder to grasp for general users compared to simpler, more descriptive systems like Köppen’s.
  8. Initial Overestimation in Cold Climates: Early applications showed that Thornthwaite’s PET formula tended to overestimate evapotranspiration in very cold climates. Subsequent researchers, like Mather, proposed refinements to address this, demonstrating the need for continuous adjustment and validation of empirical models.

In conclusion, Thornthwaite’s climatic classification system, particularly the 1948 version, represents a pivotal advancement in climatology. By pioneering the concept of potential evapotranspiration and grounding its classification in the principles of the water balance, Thornthwaite moved beyond simple empirical descriptions to a more rational and physically based understanding of climate. This focus on the dynamic interplay between moisture supply and demand provided a robust framework for quantifying climatic conditions relevant to plant growth and water resource management, establishing a legacy that continues to influence modern hydrological and ecological modeling.

Despite its methodological complexities and inherent limitations, such as the empirical nature of its PET calculation and simplifying assumptions about soil moisture capacity, Thornthwaite’s system remains a cornerstone in applied climatology. Its enduring utility in fields ranging from agriculture and forestry to regional planning underscores its conceptual power. While more sophisticated models and extensive data now enable highly detailed water balance analyses, the fundamental insights and methodology developed by Thornthwaite laid crucial groundwork, solidifying its place as a classic and influential approach to understanding the Earth’s diverse climatic patterns.