The American climatologist, C. W. Thronthwaite, first presented a classification of the climate of North America in 1931. In 1933, he classified world climates based on the concept of climate classification in North America. He further revised the climate classification in 1948 in which he added the concept of transpiration.
Thornthwaite concept of climate classification comes in three phases:
Thornthwaite Climate Classification of 1931:
In 1931, Thornthwaite uses two concepts in his climatic classification of North America.
Where T is the total monthly temperature.
Based on P/E Index, there are 5 humidity groups:
There are also subcategories of the humid zone:
It is the summation of monthly temperature which ranges from 0 to 127; 0 for frost and 127 for the tropical region. There is six-zone based on thermal efficiency:
In 1933, he used both features( that is precipitation and temperature efficiency) for world climatic classification, he divided world climate into more than 120 climatic zones which makes it very complex to study the world climate.
Concept of Potential Evapotranspiration :
Evapotranspiration is made up of two things Evaporation and transpiration. Potential Evapotranspiration means the total amount of evaporation and transpiration that occurs if sufficient water sources were available.
The Following are the calculation technique of Potential Evapotranspiration by Thornthwaite.
Moisture Index:
Water demand for Plants is measured by Potential evapotranspiration(PET).
Evaluation of Thornthwaite Climate Classification:
- Thornthwaite Climate classification of the USA in 1931.
- Thornthwaite World climate Classification in 1933.
- Revised Thornthwaite Climate classification in 1948.
Thornthwaite Climate Classification of 1931:
In 1931, Thornthwaite uses two concepts in his climatic classification of North America.
- Precipitation Effectiveness
- Temperature Efficiency
Precipitation Effectiveness is measured by the following formula:
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| Precipitation Effectiveness |
Based on P/E Index, there are 5 humidity groups:
- A: (P/E Index is greater than 127): Wet Climate (Rainforest )
- B:(P/E Index 64-127): Humid Climate (Deciduous )
- C:(P/E Index 32-63): Sub Humid (Grassland)
- D:(P/E Index 16-32): Semi-Arid (Steppe)
- E:(P/E Index less than 16): Arid (Desert)
There are also subcategories of the humid zone:
- r: Precipitation throughout the year
- s: deficient rainfall in summer
- w: deficient rainfall in Winter
- d: rainfall deficient all over the year.
Temperature Efficiency:
The following are a technique to calculate the Temperature efficiency: |
| Temperature Efficiency |
It is the summation of monthly temperature which ranges from 0 to 127; 0 for frost and 127 for the tropical region. There is six-zone based on thermal efficiency:
- A': Tropical (128 and above)
- B': Mesotherms( 64-127)
- C' Microtherms (32-63)
- D': Taiga (16-31)
- E': Tundra(1-15)
- F': Frost (0)
Thornthwait World climate Classification of 1933.
In 1933, he used both features( that is precipitation and temperature efficiency) for world climatic classification, he divided world climate into more than 120 climatic zones which makes it very complex to study the world climate.
Revised Thornthwait Climate classification of 1948.
In 1948, he the revised system, and he added a new concept of the Potential Evapotranspiration and Moisture Index.Concept of Potential Evapotranspiration :
Evapotranspiration is made up of two things Evaporation and transpiration. Potential Evapotranspiration means the total amount of evaporation and transpiration that occurs if sufficient water sources were available.
The Following are the calculation technique of Potential Evapotranspiration by Thornthwaite.
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| Potential Evapotranspiration Calculation technique |
Moisture Index:
Water demand for Plants is measured by Potential evapotranspiration(PET).
Im( moisture index)=100(P-PET)/PET.
Evaluation of Thornthwaite Climate Classification:
- Temperature is the only modifying factor for calculating PET, not a deciding factor as in Koppen climate classifications
- Moisture index and potential Evapotranspiration, further classified 9 major humid zones, if we combine 4 indexes that are Precipitation effectiveness, Temperature efficiency, Potential Evapotranspiration, and Moisture Index then there will be more than 600 microclimatic zones in the world.
- It is much more complex than Koppen's climate classification and makes it unsuitable for the study.
Try to solve the following questions:
- Explain the techniques to calculate potential evapotranspiration suggested by Thornthwaite. (UPSC geography optional paper 1 2019, 10 Marks)
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- विश्व जलवायु का थोर्न्थवेट वर्गीकरण
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