Climate Classification of the Thornthwaite

Climate Classification of the Thornthwaite

Climate classification systems have helped simplify global climate dynamics. These systems help interpret weather patterns, understand environmental changes, and plan for agriculture and urban development. The mid-20th century climate classification system developed by Charles Warren Thornthwaite is unique in its approach and methodology.

C.W. Thornthwaite’s 1948 system emphasised potential evapotranspiration (PET) to determine climate types, a major departure from previous methods. This focus on moisture availability and temperature helped explain climatic variations, especially their effects on water balance and vegetation.

While less well-known than the Köppen system, Thornthwaite’s classification has influenced hydrology and agricultural planning. Its detailed approach to understanding precipitation-evaporation interaction has illuminated water-resource management, soil moisture regimes, and regional ecology.

This article will discuss the Thornthwaite system’s history, methodology, categories, and relevance to modern climatology. This examination will demonstrate Thornthwaite’s ongoing relevance to climate, environment, and human activity discussions.

Historical Background of the Thornthwaite Climate Classification System

The development of climate classification systems can be traced back to ancient civilizations, but it was not until the 19th and 20th centuries that these systems began to take on a more scientific and systematic form. The historical background of the Thornthwaite Climate Classification System is intertwined with these developments, reflecting a period of significant advancement in the field of climatology.

Early Developments in Climate Classification

  • Pre-20th Century: Early efforts at climate classification were largely qualitative, often based on observable vegetation types and general temperature patterns.
  • Late 19th Century: The scientific community began to seek more quantifiable and systematic approaches to classify climates. This era saw the emergence of various systems, most notably the Köppen Climate Classification System, which focused on temperature and precipitation patterns.
  • Air masses: Origin and Classification
  • Fronts: Types and Significance

C.W. Thornthwaite’s Academic Background and Influences

  • Early 20th Century: Charles Warren Thornthwaite, an American climatologist, emerged as a significant figure in the field. His academic journey was marked by a keen interest in understanding the relationships between climate and physical geography.
  • Influences: Thornthwaite was influenced by contemporary advancements in meteorology and hydrology. He was particularly interested in the work of his predecessors and contemporaries who emphasized the ecological and hydrological aspects of climate.

The Genesis of Thornthwaite’s Climate Classification

  • 1948 Publication: Thornthwaite introduced his climate classification system in a groundbreaking paper published in the “Geographical Review.” This system was a result of his extensive research and observations on moisture and energy balance in the environment.
  • Core Concept: Thornthwaite’s system was revolutionary in that it introduced the concept of potential evapotranspiration (PET) as a central factor in classifying climates. This approach differed significantly from the temperature and precipitation focus of earlier systems.

Contextualizing Thornthwaite’s Work

  • Academic Environment: The period during which Thornthwaite developed his classification system was one of substantial scientific exploration and innovation. There was a growing recognition of the complexity of climatic systems and a need for more detailed and varied approaches to classification.
  • Global Influence: While Thornthwaite’s system initially gained less worldwide recognition compared to the Köppen system, it significantly influenced regional climatology studies, particularly in North America, and contributed to a more profound understanding of water balance in various climates.

Thornthwaite’s Climate Classification System

The Thornthwaite Climate Classification System, introduced by Charles Warren Thornthwaite in 1948, represents a significant development in the field of climatology. This system was revolutionary for its time, emphasizing the role of potential evapotranspiration (PET) in understanding climate types.

Key Concepts: Potential Evapotranspiration and Precipitation Effectiveness

  • Potential Evapotranspiration (PET): Thornthwaite’s system is centered around the concept of PET, which is the amount of water that would be evaporated and transpired by vegetation if sufficient water were available. It reflects the energy available to convert water from liquid or solid to vapor, indicating the climatic demand for water.
  • Precipitation Effectiveness: This refers to the relationship between precipitation and PET. In Thornthwaite’s system, the efficiency of precipitation in meeting the evaporative demand of the atmosphere plays a crucial role in defining climate types.


  • Thornthwaite’s system calculates PET based on temperature, day length, and latitude. These factors collectively determine the amount of solar radiation and thus the energy available for evaporation and transpiration.
  • The classification involves calculating a water balance and an aridity index, which helps in determining the moisture regime of an area.

Climate Categories

  • Thornthwaite’s system classifies climates into several types based on the aridity index and seasonal distribution of moisture. These categories include humid, subhumid, semiarid, arid, and perarid, each characterized by distinct precipitation and evapotranspiration patterns.
  • The system also acknowledges the seasonality of moisture and classifies regions into different moisture regimes like perhumid, humid, mesic, dry subhumid, semiarid, and arid, based on the duration of moisture surplus and deficit.

Significance of Thornthwaite’s System

  • Hydrological Applications: The emphasis on PET and water balance made this system particularly useful in hydrology, irrigation planning, and agricultural zoning.
  • Ecological Insights: By focusing on moisture availability, Thornthwaite’s classification provides valuable insights into vegetation patterns, soil moisture regimes, and the general ecological characteristics of different regions.

Innovations and Adaptations

  • Thornthwaite’s system was ahead of its time in incorporating energy balance concepts into climatic studies. His methodology paved the way for more advanced and integrated climate models.
  • Over the years, the system has been adapted and refined by various climatologists to suit different regional contexts and to incorporate more comprehensive climatic data.

Methodology of Thornthwaite’s Climate Classification System

The methodology underpinning the Thornthwaite Climate Classification System is a blend of empirical observations and mathematical calculations, primarily focusing on the concept of potential evapotranspiration (PET) and its interaction with precipitation. This approach allows for a more nuanced understanding of climatic variations in relation to moisture availability.

Calculation of Potential Evapotranspiration (PET)

  • Temperature-Based Approach: Thornthwaite’s method for calculating PET is primarily temperature-driven. It assumes that temperature is a proxy for the energy available for evapotranspiration.
  • Thornthwaite’s Equation: PET is calculated using a specific formula that incorporates mean monthly temperature and a correction factor for day length and solar radiation, which varies with latitude and time of year.
  • Monthly and Annual PET: The calculation is done on a monthly basis, and then these values are summed to obtain annual PET, giving a comprehensive view of the yearly water demand of the atmosphere.

The Role of Climatic Factors

  • Temperature: As the primary driver in Thornthwaite’s method, temperature directly influences the rate of PET. Higher temperatures generally increase PET, indicating a greater demand for moisture.
  • Sunlight and Latitude: The amount of solar radiation, affected by day length and latitude, plays a crucial role in determining PET. Regions closer to the equator or with longer daylight hours have higher PET values.
  • Seasonal Variations: The system accounts for seasonal changes in temperature and solar radiation, reflecting the seasonal variability of moisture demand.

Classification Thresholds and Indices

  • Aridity and Humidity Indices: Thornthwaite’s system uses these indices to classify climates. The aridity index is a ratio of actual to potential evapotranspiration and helps in identifying moisture surplus or deficit.
  • Moisture Index: It is calculated by comparing the total annual precipitation with PET. This index is key in classifying climates into various types based on the degree of aridity or humidity.

Water Balance Concept

  • The methodology also involves calculating a water balance for different regions, considering both precipitation and PET. This balance is crucial in understanding the moisture regime of a region, particularly its ability to support vegetation and agriculture.

Climate Categories in Thornthwaite’s System

Thornthwaite’s Climate Classification System is distinct for its emphasis on moisture availability and potential evapotranspiration (PET), leading to a unique categorization of climates. The system classifies climates based on the Moisture Index (MI), which is calculated from the difference between total precipitation and PET over a year. This results in several climate categories, each characterized by specific moisture and temperature regimes.

Moisture Index (MI) and Climate Types

  • Moisture Index (MI): A key component in Thornthwaite’s classification, it is calculated by subtracting the annual potential evapotranspiration from the total annual precipitation and then normalizing this value. The MI determines the moisture regime of a climate.
  • Climatic Types Based on MI: These types range from extremely dry to extremely wet conditions.

Humidity Provinces

  • Perhumid (A): Regions with a constant surplus of water throughout the year (MI greater than 100).
  • Humid (B): Areas with sufficient moisture for the entire year, usually having a surplus for half the year (MI between 20 and 100).
  • Moist Subhumid (C): Regions with a moderate surplus of water for 6-9 months of the year (MI between 0 and 20).
  • Dry Subhumid (D): Areas experiencing a short period of water surplus, but with significant periods of moisture deficit (MI between -20 and 0).

Aridity Provinces

  • Semiarid (E): Regions characterized by a longer duration of moisture deficit, with brief periods of surplus, typically supporting grasslands (MI between -40 and -20).
  • Arid (F): Areas with prolonged periods of moisture deficit, supporting very sparse vegetation or desert conditions (MI between -60 and -40).
  • Perarid (G): Extremely dry regions with almost no moisture surplus at any time of the year (MI less than -60).

Seasonal Distribution of Moisture

  • The system further categorizes climates based on the seasonality of moisture, such as summer or winter dry periods. This aspect is crucial in regions where the distribution of rainfall across the year significantly impacts the climate and vegetation patterns.

Regional Examples

  • Each of these climate types can be exemplified by specific regions across the world, showcasing the diverse climatic conditions encompassed by Thornthwaite’s system. For instance, perhumid climates are often found in tropical rainforests, while perarid climates are typical of extreme deserts.

Comparison with Other Classification Systems

Thornthwaite’s Climate Classification System, developed by C.W. Thornthwaite, represents a unique approach in the realm of climatology. It’s particularly interesting to compare it with other prominent climate classification systems, notably the Köppen Climate Classification, to understand its distinct features and applications.

Comparison with Köppen’s System

  • Köppen Climate Classification: Developed by Wladimir Köppen, this is perhaps the most widely used climate classification system. It primarily uses temperature and precipitation patterns to define different climate zones.
  • Focus on Temperature and Precipitation: Köppen’s system emphasizes the distribution of temperature and precipitation throughout the year to classify climates. It is relatively simpler and more straightforward than Thornthwaite’s system.
  • Vegetation-Based Categories: Köppen’s classification correlates closely with vegetation types, making it particularly useful in biogeographical studies.

Differences and Similarities

  • Methodological Differences: The most significant difference lies in the methodologies. Thornthwaite’s system is based on the concept of potential evapotranspiration (PET) and focuses on the energy balance and moisture availability. In contrast, Köppen’s system is more directly based on observable temperature and precipitation data.
  • Categorization: While both systems categorize climates into broad types, the criteria for these categorizations differ. Thornthwaite’s approach leads to a more nuanced understanding of moisture regimes, whereas Köppen’s method is more straightforward and easier to apply.
  • Similarity in Broad Categorization: Despite methodological differences, both systems recognize similar broad categories such as humid, arid, and semi-arid climates.

Advantages and Limitations

  • Thornthwaite’s System:
    • Advantages: Offers a detailed understanding of the water balance, which is crucial for hydrology and agricultural planning.
    • Limitations: More complex and requires more data (particularly temperature) for accurate classification.
  • Köppen’s System:
    • Advantages: Simplicity and ease of use with readily observable climatic data.
    • Limitations: Less detailed in terms of understanding the energy balance and moisture regime.

Contemporary Relevance

  • Thornthwaite’s System: Despite its complexity, it remains relevant in specific fields like hydrology and agriculture, particularly for water resource management and understanding drought conditions.
  • Köppen’s System: Continues to be widely used due to its simplicity and effectiveness in general climatic studies and biogeographical applications.

Applications and Significance of Thornthwaite’s Climate Classification System

The Thornthwaite Climate Classification System, despite being less well-known than other systems like Köppen’s, has significant applications and has made substantial contributions to various fields. Its unique approach to classifying climates based on potential evapotranspiration (PET) and moisture regimes offers valuable insights in several areas.

Applications in Hydrology and Agricultural Planning

  • Water Resource Management: Thornthwaite’s emphasis on the balance between precipitation and evapotranspiration is crucial for understanding water availability and managing water resources, especially in regions prone to drought or water scarcity.
  • Agricultural Zoning: The system aids in determining suitable crops and irrigation practices by providing detailed information on the moisture regime of different regions. This is particularly important for sustainable agricultural planning and crop selection.

Significance in Contemporary Climatology Studies

  • Understanding Climate Change Impacts: Thornthwaite’s system is relevant in studying the impacts of climate change, as shifts in temperature and precipitation patterns directly affect PET and, consequently, water balances in ecosystems.
  • Ecosystem and Vegetation Analysis: The classification helps in understanding and predicting changes in vegetation patterns and ecosystem dynamics in response to varying climate conditions.

Contribution to Environmental Planning and Management

  • Soil Moisture Regimes: The system offers insights into soil moisture conditions, essential for soil conservation, forestry, and land-use planning.
  • Drought Assessment and Management: It is instrumental in drought assessment, providing a framework for identifying areas at risk and developing drought mitigation strategies.

Influence on Climatological Research and Theory

  • Advancing Climatological Methods: Thornthwaite’s approach introduced the concept of energy balance in climatology, paving the way for more sophisticated climate models.
  • Interdisciplinary Impact: The system has influenced interdisciplinary research, linking climatology with hydrology, agriculture, and ecology, thus fostering a more integrated understanding of environmental systems.

Educational and Research Applications

  • Academic Curriculum: Thornthwaite’s classification is taught in climatology and environmental science courses, highlighting its theoretical importance.
  • Basis for Further Research: The system has served as a foundation for further climatological research, particularly studies focused on the interactions between climate, water resources, and land use.

Adaptations and Updates to Thornthwaite’s Climate Classification System

Since its inception in 1948, Thornthwaite’s Climate Classification System has undergone various adaptations and updates to enhance its accuracy and applicability in light of evolving climatological knowledge and technological advancements.

Modifications and Adaptations

  • Incorporating New Data: With the availability of more comprehensive and accurate climatic data, the system has been adapted to incorporate these datasets for more precise climate classification.
  • Refinement of PET Calculation: Advances in understanding the factors affecting evapotranspiration have led to refinements in the formula used to calculate PET, making it more accurate in different environmental settings.
  • Integration with Geographic Information Systems (GIS): The use of GIS technology has enabled more detailed and spatially precise applications of the Thornthwaite system, facilitating better analysis and visualization of climate data.

Current Relevance and Use in Climate Research

  • Climate Change Studies: Thornthwaite’s system, especially its focus on moisture regimes, remains relevant in assessing the impacts of climate change on regional water balances and ecosystems.
  • Water Resource Management: The classification is still used in hydrological studies, particularly in regions where water scarcity or the management of water resources is a critical concern.
  • Agricultural and Ecological Applications: Updated versions of the system continue to inform agricultural zoning and ecological research, especially studies related to drought risk and soil moisture.

Challenges and Improvements

  • Addressing Limitations: While the system has been praised for its innovative approach, it also faces criticism, particularly regarding its heavy reliance on temperature for PET calculation. Efforts have been made to incorporate other climatic factors like wind speed and solar radiation more explicitly.
  • Global Applicability: Initially, the system was more focused on North American climates. Updates have been made to broaden its applicability to a wider range of global climatic conditions.

Collaboration with Other Climatic Models

  • Hybrid Models: In some instances, Thornthwaite’s methodology has been combined with other climatic models and classification systems to create more comprehensive and versatile tools for climate analysis.

Challenges and Criticisms of Thornthwaite’s Climate Classification System

While Thornthwaite’s Climate Classification System has been influential in climatology and related fields, it has also faced several challenges and criticisms over the years. These critiques mainly focus on its methodology, scope, and applicability in various contexts.

Reliance on Temperature for PET Calculation

  • Temperature-Centric Approach: The system’s heavy reliance on temperature as the primary variable for calculating potential evapotranspiration (PET) is a major point of criticism. Critics argue that this approach oversimplifies the complex process of evapotranspiration, which is also influenced by other factors like wind speed, humidity, and solar radiation.
  • Regional Limitations: This temperature-centric method may not be entirely accurate in regions where other factors play a more significant role in evapotranspiration than temperature.

Accuracy and Comprehensiveness

  • Data Requirements: Accurate application of Thornthwaite’s system requires detailed and reliable temperature data, which may not be readily available in all regions, especially for historical climate analysis.
  • Generalization Issues: The system tends to generalize climate types based on broad categories, which might overlook microclimatic variations and specific local climatic conditions.

Comparisons with Other Systems

  • Lesser Flexibility Than Köppen’s System: Compared to the Köppen Climate Classification, Thornthwaite’s system is often considered less flexible and adaptable to a wide range of climatic conditions.
  • Complexity in Application: The methodology, involving detailed calculations of PET and moisture indices, is more complex and data-intensive than some other classification systems, making it less accessible for general use.

Modern Relevance and Adaptability

  • Climate Change Dynamics: As climate patterns shift globally due to climate change, there is a need to continuously update and adapt the system to reflect these new realities, which can be a challenging process.
  • Integration with Advanced Climatic Models: With advancements in climate modeling, there is a growing need to integrate Thornthwaite’s system with more sophisticated models to enhance its utility and accuracy.

Application in Interdisciplinary Contexts

  • Hydrology and Agriculture: While the system is highly useful in specific fields like hydrology and agriculture, its application in broader environmental and ecological studies may be limited by its focus on PET and temperature.

In summary, while Thornthwaite’s Climate Classification System is a significant contribution to the field of climatology, it is not without its limitations and areas for improvement. Addressing these challenges through ongoing research and adaptation will be crucial for maintaining the system’s relevance and utility in the face of changing global climate dynamics.


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