How did C. Warren Thornthwaite classify climates using moisture and thermal efficiency?

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World map with continents shaded in colors representing climate zones, from warm reds and yellows near the equator to cool blues and whites at the poles. — Climate Even in our warming world, there will be some chilly evenings.

C. Warren Thornthwaite developed two distinct approaches to climate classification, both of which utilized the concepts of moisture and thermal efficiency to analyze atmospheric effects on the environment.1

In his 1931 classification, Thornthwaite employed a vegetation-based approach. He used the derived concepts of "precipitation effectiveness" and "temperature efficiency" to specify how the atmosphere influences vegetation.1

In his 1948 classification, Thornthwaite shifted his methodology significantly. While he retained the core concepts of moisture and thermal efficiency, he transitioned to a "rational" classification system based on numerical values rather than vegetation indicators. He replaced his earlier metrics with a "moisture index" and a "thermal efficiency index." Although this 1948 scheme is frequently cited in climatology literature, it is generally considered more complex than the Köppen climate classification and defines a much larger number of climatic regions.1