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This classification is based on two main climatic indices
a) The moisture index (Im)
b) The annual potential evapotranspiration (P.E)
Im is given as follows Im = (100s-100D)/PE
Where s is the annual water surplus and D is the annual water deficit. Using this index Thornthwite arrived at the following climatic types
Climatic Type Moisture Index (Im)
A Perhumid 100 and more
B4. Humid 80 – 100
B3. Humid 60 – 80
B2 Humid 40 – 60
B1. Humid 20 – 40
C2 Moist Sub-humid 0–20
C1. Dry Sub-humid -33.3 – 0.0
D. Semi-arid -66.7 - -33.3
E. Arid -100 – -66.7
Thornthwaites’s approach to climatic classification has been found useful in diverse fields such as ecology, agriculture and water resource development. Thornthwaite’s classification scheme has however not escaped criticism. Values of potential evapotranspiration on which the moisture index is based are not readily available for many areas. Thornwaites’ empirical formula for estimating values of P.E. has failed to produce satisfactory estimates over many parts of the world particularly the arid and semiarid areas. The ratio of actual evapotranspiration to potential evapotranspiration (AE/PE) has been shown to be a better index of delimiting moisture regions than the moisture index.
francis1897 answered the question on October 5, 2022 at 11:00
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