Thornthwaite's Classification of Climates
Thornthwaite's classification system, developed by C. Warren Thornthwaite, is a method for classifying climates based on two main factors: potential evapotranspiration (PE) and moisture index (Im). This system is particularly useful for understanding the relationship between climate and vegetation, making it relevant for fields like agriculture and ecology, and is a key topic in competitive exams like the UPSC Geography.
Key Concepts: Potential Evapotranspiration (PE)
Potential evapotranspiration (PE) is the amount of water that would evaporate from the soil and transpire from plants if water were sufficiently available.
PE is a measure of the atmospheric demand for water. It is influenced by temperature, solar radiation, and wind speed. Higher temperatures and more sunlight generally lead to higher PE.
Thornthwaite defined potential evapotranspiration (PE) as the amount of water that would be lost from the soil and plants through evaporation and transpiration, assuming an adequate supply of moisture. This concept quantifies the atmospheric demand for water. It is primarily a function of temperature, with higher temperatures leading to increased PE. Other factors like solar radiation, humidity, and wind speed also play a role, but Thornthwaite's original formulation focused heavily on temperature as the primary driver.
Key Concepts: Moisture Index (Im)
The moisture index (Im) quantifies the degree of humidity or aridity of a climate.
It is calculated by comparing actual evapotranspiration (AE) to potential evapotranspiration (PE). A positive Im indicates a humid climate, while a negative Im indicates an arid climate.
The moisture index (Im) is the cornerstone of Thornthwaite's classification. It is derived from the difference between actual evapotranspiration (AE) and potential evapotranspiration (PE), normalized by PE. The formula is typically expressed as: Im = (Precipitation - PE) / PE. A positive Im signifies a surplus of moisture (humid climate), a negative Im indicates a deficit (arid climate), and an Im close to zero suggests a moisture-neutral condition. This index helps differentiate between climates based on their water balance.
Classification Categories
Thornthwaite's system uses a combination of letters to denote different climatic characteristics. The primary classification is based on the moisture index (Im), which is further divided into humidity and aridity classes. A secondary classification is based on the thermal efficiency (TE) or heat index, which is derived from monthly PE values and indicates the thermal regime of a region.
| Moisture Index (Im) | Climate Type | Description |
|---|---|---|
100 | A - Humid | Excess moisture |
| 20 to 100 | B - Moist Subhumid | Moderate moisture surplus |
| 0 to 20 | C1 - Dry Subhumid | Slight moisture deficit |
| -33.3 to 0 | C2 - Semiarid | Moderate moisture deficit |
| -66.7 to -33.3 | D - Arid | Significant moisture deficit |
| < -66.7 | E - Extremely Arid | Extreme moisture deficit |
Thermal Efficiency (TE) and Seasons
Beyond moisture, Thornthwaite also classified climates based on their thermal efficiency, represented by the Thermal Efficiency (TE), which is the sum of monthly PE values. This classification helps in understanding the thermal regime and the length and intensity of the growing season. The thermal classes are denoted by subscripts (e.g., r for megathermal, s for mesothermal, w for microthermal, and v for tundra).
Thornthwaite's classification uses a grid system where the horizontal axis represents the moisture index (Im) ranging from extremely humid (A) to extremely arid (E), and the vertical axis represents thermal efficiency (TE) or heat index, indicating the thermal regime. Each climate type is a combination of these two dimensions, often with further subdivisions based on seasonal precipitation patterns. For example, a climate might be classified as 'A' (Humid) with a 'r' subscript indicating a megathermal regime (high heat and moisture year-round). The system visually maps out climatic zones based on these two key variables, allowing for a nuanced understanding of regional climates and their suitability for different types of vegetation.
Text-based content
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Potential Evapotranspiration (PE) and Moisture Index (Im).
Applications and Limitations
Thornthwaite's classification is widely used in hydrology, agriculture, and ecology for its focus on water balance and its implications for vegetation. However, it has limitations, particularly in its reliance on temperature as the sole determinant of PE and its less precise handling of precipitation seasonality compared to other systems like Köppen's. Despite these, it remains a valuable tool for understanding climate-vegetation relationships.
Remember that Thornthwaite's system is particularly strong in relating climate to agricultural productivity due to its emphasis on water availability and thermal efficiency.
Learning Resources
Provides a clear overview of Thornthwaite's classification, its parameters, and its application in geography.
An excerpt from Encyclopedia Britannica detailing the Thornthwaite climate classification system and its historical context.
A National Park Service resource that briefly touches upon various climate classification systems, including Thornthwaite's, in the context of climate change.
Details the Thornthwaite system, its components, and its use in ecological studies.
The UK Met Office's explanation of different climate classification systems, offering context for Thornthwaite's place among them.
A scientific overview from ScienceDirect explaining the Thornthwaite classification and its relevance in environmental science research.
An educational tutorial that introduces various climate classification systems, including a mention of Thornthwaite's approach.
A research paper discussing the Thornthwaite classification system, often used in academic studies of climate and its impacts.
ThoughtCo provides a concise explanation of the Thornthwaite climate classification, its basis, and its categories.
A review of the Thornthwaite system, likely found in academic databases, offering in-depth analysis and historical perspective.