A major division of C. W. Thornthwaite's 1931 schemes of climatic classification, determined as a function of the temperature-efficiency index or the potential evapotranspiration. In the 1931 system, six main temperature provinces (climates) are distinguished: 1) tropical; 2) mesothermal; 3) microthermal; 4) taiga; 5) tundra; and 6) frost. In the 1948 system they are 1) megathermal; 2) mesothermal; 3) microthermal; 4) tundra; and 5) frost. Compare climatic province; see humidity province.