This scheme of climatic classification has been successfully applied to many regions, but because of certain limitations inherent in it no world map has yet been prepared.

This classification system has proved most satisfactory in case of North America where vegetation boundaries nearly coincide with particular PE values. But it is not satisfactory for the tropical and semi arid areas.

However, Thornthwaite does not determine his climatic boundaries on the basis of vegetation boundaries. In this respect, he differs from Koppen in case of his second classification.

This classification is quantitative as well as empirical, but it does not take into account various causative factors of climate. It also employs symbols for designating the climatic types.


The classification involves a lot of calculations, hence it is more difficult to use in determining the climatic type of a particular place or locality. That is why Thornthwaite’s system could not enjoy the same degree of popularity as that of Koppen.

Lastly, Thornthwaite as a climatologist developed the concept of the soil-moisture balance, and he preferred to use it as the foundation of his climate system.

He was convinced that the soil- moisture balance represents availability of moisture for plants, and an assessment of the availability of surplus moisture to supply stream flow and ground water.

This concept implies that precipitation alone does not indicate the amount of water actually available to plants. The amount and availability of soil moisture is also affected, besides other factors, by the losses due to evapotranspiration.


Thus, moisture requirements of plants, according to Thornthwaite, become higher as temperature increases.

Even though this scheme of climatic classification is based on two variables: (1) precipitation, and (2) evapotranspiration, temperature is by no means ignored in it. Temperature is also accounted for while calculating evapotranspiration.

If a comparison is made between the amount of water available from precipitation and the water need, it is easy to assess the moisture conditions to determine the seasonal distribution of moisture deficits or surpluses and whether a climate is wet or dry.

The most significant contribution of Thornthwaite has been that the concept of evapo­transpiration has been applied in practical studies of the water balance as regards the problems of water use.


However, in spite of numerous plus points of Thornthwaite’s classification of the climate, his system is seldom used in an introductory course because it involves a lot of very complicated calculations.