In C. W. Thornthwaite's 1931 climatic classification, a region in which the precipitation effectiveness of its climate produces a definite type of biological consequence, in particular the climatic climax formations of vegetation (rain forest, tundra, etc. ). Five main classes of humidity province are distinguished, bounded by values of precipitation effectiveness index (P–E index): 1) wet or rain forest; 2) humid or forest; 3) subhumid or grassland; 4) semiarid or steppe; and 5) arid or desert. Thornthwaite (1948) used values of moisture index to limit similar but purely climatic (nonbiological) zones as follows: 1) perhumid; 2) humid; 3) subhumid; 4) semiarid; and 5) arid. The moist climates are those with a positive moisture index; dry climates have negative values. Compare hyetal region, climatic province, temperature province.