In 1948 Thornthwaite proposed a new classification of climate which is his most valuable contribution.

His second classification is based on the concept of potential evapotranspiration, which represents the amount of moisture that would be transferred to the atmosphere by evaporation of liquid or solid water plus transpiration from living tissues, principally plants if it (the moisture) were available.”

The potential evapotranspiration (PE) is calculated from the mean monthly temperature (in °C), with corrections for the day length. For a 30-day month (12- hour days):

PE (in cm) = 1.6 (10 t/I)a


Where I = the sum of 12 months of (t/5) 1.514

a = a further complex function of I.

The monthly water surplus (S) or deficit (D) is calculated from a moisture budget assessment including stored soil moisture. A moisture index (Im) is given by the following formula:

Im = (100 S – 60 D) PE


The most characteristic feature of this classification scheme is that the temperature efficiency is calculated from the PE value, this being a function of temperature. Using computed indices of moisture and heat, Thornthwaite defined the moisture and thermal provinces.

Thus, on the basis of moisture index alone, 9 humidity provinces have been defined: A, B4, B3, B2, B1, C2, C1,D, and E.

Similarly, on the basis of average annual thermal efficiency and its summer concentration, 9 thermal provinces have been categorized: A’, B’4, B’3, B’2, B1, C’2, C’1, D’ and E’. Both the types of provinces referred to above have been further subdivided in accordance with the seasonal variation of effective moisture.

Thus, on the basis of potential evapotranspiration, average annual thermal efficiency, seasonal variation of effective moisture, and summer concentration of thermal efficiency the type of climate of any place can be determined.