Thornthwait's Classification of Climate Types

Sunday, July 26, 2020

C.W. Thornthwait, an American Climatologist presented a new classification of climate in 1931 which he modified in 1948 again. (Also Read: Koeppen's Classification of Climate)

1931 Classification

In his classification Koeppen in his classification of climate he included one more factor i.e. 'evaporation in his classification. For this he devised two indexes- 

1. Precipitation Effectiveness

In this he defined climates on the basis of precipitation available for vegetation for this he devised a formulae for finding out Precipitation Efficiency Ratio (P/E Ratio):-
r - Mean Monthly Rainfall(in inches)
t - Mean monthly temperature (°F)

On the basis of above formulae he devised world climate in to five types of Moisture Zones:-
Humidity Zones Vegetation P/E Index
Rainforest 127
Forest 64-127
C (sub-Humid) Grasslands 32-63
D (Semi Arid) Steppe 16-31
E (Arid) Desert Less than 16

Further he divided boundary between above by using 4 different subtypes dependent precipitation distribution:- 

r - Adequate Rainfall in all seasons
s - Summer dry region
w -  Winter dry region
d - All year rainfall deficit

Based above two classification he devised total 20 types climatic zones:-
1. Ar, 2. As, 3. Aw, 4. Ad, 5. Br, 6. Bs, 7. Bw, 8. Bd, 9. Cr, 10. CS, 11. Cw, 12. Cd, 13. Dr, 14. Ds, 15. Dw, 16. Dd, 17. Er, 18. Es, 19. Re, 20. Ed[post_ads]

Temperature/Thermal Effectiveness

This is basically the heat provided by soil and vegetation to environment. This is also defined as positive departure from freezing point. 
Thermal Efficiency Ratio(T/E Ratio) is calculated by the following formula:-

t - mean monthly temperature (°F)

Base on above formulae he classified world into six Temperature provinces:- 
Temperature ProvincesClimateT/E Index
TropicalMore than 127
F' Frost 0
Based on previous 3 classifications P/E Index, T/E Index and Precipitation distribution across seasons there are 120 probable climatic zones are possible but he classified world climate into 32 types on the world map. 1. AA'r, 2. AB'r, 3. AC'r, 4.BA'r, 5. BA'w, 6. BB'r, 7. BB'w, 8. BB's, 9. BC'r, 10. BC's, 11. CA'r, 12. CA'w, 13. CA'd, 14. CB'r, 15. CB'w, 16. CB'w, 17. CB'd, 18. CC'r, 19. CC's, 20. CC'd, 21. DA'w, 22. DA'd, 23. DB'w, 24. DB's, 25. DB'd, 26, DC'd, 27. EA'd, 28. EB'd, 29. EC'd, 30. D'(Taiga Climate), 31. E'(Tundra Climate), 32. F'(Polar Climate).

1948 Classification 

In this classification he kept all factors which were devised earlier and added an additional method based on Potential Evapotranspiration which is discussed below:-

Potential Evapotranspiration

Basically this is a function of both the mean monthly  temperature and mean monthly rainfall summed up on annual basis.
This indicates Thermal Efficiency and water loss i.e. the amount of moisture and heat transfer from the soil and vegetation thus it is a function of energy received by sun. 
It is calculated by using mean monthly temperature. The formula is as follows:-

Thornthwait further devised 4 indices to define boundary between different climates as follows:- 

1. Moisture Index

It represents moisture deficit and surplus. 
Im = (100S - 60D)/PE

Im = Annual Moisture Index 
S = Monthly surplus moisture 
D = Monthly deficit Moisture.

based on this he devised nine type of climates: -
Humidity ProvincesClimateIm
Sub-Humid (moist)0-20
Sub Humid (dry)-33.3-0
semiarid-66.7 to -33.3
EArid-100 to -66.7

2. Thermal Efficiency Index 

It represents basically potential Evapotranspiration and calculated as above.
Based on this climate is again divided into nine Thermal provinces.
Humidity ProvincesClimateIm

3. Aridity and Humidity Index

Aridity Index - In moist climate, Annual water deficit, taken as PE (Annual) becomes Aridity Index. 
Humidity Index - In Dry climates, Annual water surplus taken annual PE becomes Humidity Index. 
Based on this climate is divided in two main and 10 subtypes [post_ads_2]

I. Moist Climate(A,B,C2)

 Moist ProvincesClimatic ConditionsAridity Index
 rLittle or no water deficit 0-10
sModerate Summer deficit10-20
wModerate Winter water deficit 10-20
s2Large Summer water deficit Above 20
w2Large winter water deficitAbove 20

 II. Dry Climate(C'1, D, E)

Dry ProvincesClimatic ConditionsHumidity Index
dLittle or no water surplus 0-16.7
sModerate summer water surplus16.7-33.3
wModerate winter water surplus16.7-33.3
s2Large Summer water surplusAbove 33.3
w2Large winter winter water surplusAbove 33.3

4. Concentration of Temperature Effectiveness

It refers to percentage annual mean potential Evapotranspirtion in three summer months. 
Based on this eight types of climates are devised further 
Summer Concentration Climate TypesConcentration of Thermal Efficiency
d'Above 88.0

Evaluation and Merits of the Scheme

Similar to Koeppen's Scheme in three ways:-

  1. Based on calculation and empirical formulae.
  2. Both related vegetation nd climates similarly. 
  3. Both used letter symbols to indicate climate types. 

But it differs with Koeppen :-

  • In terms of P/E, T/E and PE, which was not used in Koeppon's Classification of Climate.


  • It is comprehensive and formulae based approach.
  • Scientific in nature.
  • Links vegetation with climatic zones.


  1. Very complex formulae which is difficult to calculate and comprehend.
  2. Delimitation of boundaries between two climatic zones become difficult and vogue due to a lot of calculation.
  3. Too much use of letter symbols make it difficult ot remember.
  4. Data of evaporation and moisture are difficult to collect. 
  5. Thornthwait yielded 32 climate types, three times more than the Koeppon's Classification of Climate.