These main types are further divided into climatic provinces. The K ppen 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. K ppen s 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.