Thornthwaite, C Warren. 1948. An Approach toward a Rational Classification of Climate. The Geographical Review 38 (1): 55-94.


The main argument in Warren C Thornthwaite’s article “An Approach toward a Rational Classification of Climate” is that “climatology is nothing more than statistical meteorology … of elements like temperature, precipitation, atmospheric humidity, and pressure, and wind velocity” (p. 55). Further, Thornthwaite argues that climate is not a product of precipitation alone, but a combination of precipitation with evaporation and transpiration. According to Thornthwaite, “where precipitation is in excess of water need, the climate is moist. Where water deficiency is large in comparison with the need, the climate is dry. Where precipitation and water need are equal or nearly equal, the climate is neither humid nor arid” (p. 56).

Additionally, he talks about potential evapotranspiration to support his argument. “[Potential evapotranspiration] does not represent an actual transfer of water to the atmosphere but rather the transfer…under ideal conditions of soil moisture and vegetation…determined experimentally” (p. 56). It is an index of thermal efficiency. The rate of evapotranspiration depends on four things: climate, soil-moisture supply, plant cover, and land management. The actual evapotranspiration is better explained by the “vapor transfer” method (p. 57). He then puts forth the idea of computed potential evapotranspirationto discussmeasurements of runoff “compared with computed values of water surplus” (p. 71). He under-looked the climatic classification made by Koppen claiming that the moisture factor and moisture index are essential factors for the classification of climate. Thornthwaite utilized evaporation data from the continental United States and derived an empirical equation by which the precipitation evaporation ratio could be determined from monthly values of precipitation and temperature. He used this moisture index in making a climatic map of North America and a later one of the earth (p. 74). Finally, he proposes four symbols to give a complete description of a climate: water need, the percentage that summer potential evapotranspiration is of the annual total, surplus as a percentage of need and deficiency as a percentage of need (p. 88). This classification was successful to determine that climatic boundaries are rational by comparing precipitation and evapotranspiration.

Thornthwaite made a number of meteorological situations and came up with the concept of climatic regions developed independently of other geographical factors such as vegetation, soils, and land use may provide the key to their geographical distribution (p. 87). Unlike Koppen’s climatic classification, the present classification is a physical mechanism by means of which water is transported from the soil to the atmosphere; it is the machinery of evaporation as the cloud is the machinery of precipitation (p. 88). For Thornthwaite, the most important thing was the climate is not determined by precipitation alone, but with a combination of precipitation and evapotranspiration.

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