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What do the letters a, b, c, and d show in Type C and D climate codes?
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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, the third symbols—a, b, c, and d—are used to indicate the degree of summer warmth or winter coldness in Type C and Type D climates.12
The specific criteria for these designations are as follows:
- a: The temperature of the warmest month is 22 °C (71.6 °F) or above.32
- b: The temperature of each of the four warmest months is 10 °C (50 °F) or above, but the warmest month is less than 22 °C.32
- c: One to three months have temperatures of 10 °C or above, but the warmest month is less than 22 °C.32
- d: This designation applies only to Type D climates and indicates that the temperature of the coldest month is less than –38 °C (–36.4 °F). When this condition is met, the "d" symbol is used instead of a, b, or c.32