20 MISC. PUBLICATION 305, U. S. DEPT. OF AGRICULTURE 



The limiting effect of temperature on plants was expressed in the 

 same manner. The coefficient for temperature efficiency Thorn - 

 thwaite called the T-E ratio. The sum of the 12. monthly ratios for 

 any station he called the T-E index. Determining by empirical 

 means an equation that gave the poleward limit of tundra a T-E 

 index of zero and the poleward limit of the tropical rain forest and 

 savannah a T-E index of 128 (which was the same as the P-E index 

 of the arid margin of the rain forest), six temperature provinces 

 were defined as follows : 



A' (tropical), T-E index 128 and above. 

 B' (mesothermal), T-E index 64-127. 

 C' (microthermal), T-E index 32-63. 

 D' (taiga), T-E index 16-31. 

 E' (tundra), T-E index 1-15. 

 F' (frost), T-E index 0. 



The sixth province rarely occurs in the United States. 



Where temperature efficiency is adequate, variations in precipita- 

 tion effectiveness establish the primary climatic boundaries; other- 

 wise temperature efficiency is the limiting factor and determines the 

 boundaries. The combination of the two produces a climatic prov- 

 ince, in which various combinations of temperature effectiveness, 

 precipitation effectiveness, and seasonal distribution of precipitation 

 are possible, as shown in figure 2. 



LENGTH OF THE GROWING SEASON 



The figures for the length of growing season are taken from the 

 Atlas of American Agriculture (45) . Eef erence to the map showing 

 average annual frost-free season will show any portion of the country 

 to be included between lines that indicate the average length of 

 growing season. Between any two such lines is an area wiierein the 

 average annual frost-free season changes with the distance from any 

 designated line. It is obvious that an average or mean cannot vary, 

 but it is equally plain that it may be different in one place from what 

 it is in another. Therefore, in the characterizations under each 

 region when the statement is made, for instance, that the average 

 is from 10 to 20 in the northern portion and from 40 to 50 in the 

 southern, it is meant that in the northern portion of the region in 

 question the average of the extreme north may be 10, but at a short 

 distance south it may be 20. In the southern portion likewise, the 

 average may be 50 in the extreme south and a certain number of 

 miles north only 40. But it should be clear from this example that 

 the growing season in the northern portion, changing in length from 

 one place to another, varies from 10 to 20 and that the growing sea- 

 son of the southern portion varies likewise from 40 to 50. 



SNOW COVER 



The figures for number of days with snow cover are taken from 

 the Atlas of American Agriculture (45} . Here, as in the discussion 

 of the growing season, the average annual figure is said to vary. 

 Again it is obvious that averages do not vary, but may change with 

 distance. When it is said, for instance, that the average annual 

 number of days with snow cover is from 1 to 10, it is to be under- 

 stood that the average figure changes with distance; that somewhere 



