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Why are Type B climates classified by dryness rather than temperature?

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Köppen climate classification map — Köppen climate classification The major climatic types are based on patterns of average precipitation, average temperature, and natural vegetation. This map depicts the world distribution of climate types based on the classification originally invented by Wladimir Köppen in 1900.

In the Köppen climate classification system, Type B climates are classified by dryness because, in these regions, aridity is the primary factor controlling vegetation rather than temperature.12

While the other four major climate types (A, C, D, and E) are defined by temperature criteria, Type B designates areas where the lack of moisture is the dominant environmental constraint. Aridity in this system is not determined by precipitation alone; rather, it is defined by the relationship between the moisture input to the soil and the evaporative losses. Because evaporation is difficult to measure directly at standard meteorological stations, Köppen utilized a temperature-precipitation index as a proxy, assuming that evaporation is controlled by temperature. Consequently, any climate that meets the specific criteria for a B type is classified as such, regardless of its other characteristics.12