CLIMATIC TYPES 

Any classification of climate depends to a large extent on the purpose of the classification. For instance, a classification for the purpose of establishing air stations where favorable flying conditions are important, would differ con-siderably from one for establishing the limits of areas that are favorable for the growing of crops. There are three classifications that merit particular attention. They are the classifications of C. W. Thornthwaite, W. Koppen, and G. T. Trewartha.

Figure 6-4-1.Temperature zones.

Thomthwaits classification of climates places a great deal of emphasis on the effectiveness of precipitation. Effectiveness of precipitation refers to the relationship between precipitation and evaporation at a certain locality. Thornthwaite classified climates into eight main climatic groups; five groups give primary emphasis to precipita-tion and the other three groups are based on temperature.

Kppens classification includes five main climatic types. They are TROPICAL RAIN, DRY, WARM TEMPERATE RAINY, COOL SNOW FOREST (Boreal), and POLAR climates. These main types are further divided into climatic provinces. The Kppen classification is based mainly on temperature, precipitation amount, and season of maximum precipita-tion. Numerical values for these elements con-stitute the boundaries of the above types, which were selected primarily according to their effect on plant growth. Figure 6-4-2, a foldout at the end of the chapter, shows Kppens climatic types.

Kppens climatic types are still considered valid today. His climatic zones, like others, are by no means static. Climatic zones shift with long-range weather patterns. The most noticeable shifts in these climatic zones have been observed over the northern portions of North America and Asia and over Africa. Russia and Canada, for example, have been able to farm land at higher latitudes over the past 200 years due to milder temperatures. Recent studies, however, indicate a general return of cooler temperatures at high latitudes, and now the growing region is gradually moving southward again where temperatures are more moderate. In Africa, desert regions have made notable shifts southward due to decreasing precipitation.

Trewartha is the most recent classifier of climate. Initially, his climatic classifications were based on Koppens; however, over the years, he has made significant changes and is now recognized for developing his own six climatic groups. These six groups are TROPICAL, DRY, SUBTROPICAL, TEMPERATE, BOREAL, AND POLAR. Five of these groups are based on temperature and one is based on precipitation (see Table 6-4-1). Trewarthas climatic groups, like Kppens, are also further broken down into climatic types and subtypes.

Table 6-4-1.Trewarthas Climatic Groups and Their Poleward Boundaries

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