36 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



portional to the pressure below normal, figure 18 indicates that 

 local observations of pressure are of some value in forecasting 

 changes in the weather conditions where the daily weather map is 

 not available. As stated, however, the relation of pressure to vege- 

 tative activities can best be expressed by the summation of certain 

 other factors correlated with* pressure and known to exert a direct 

 influence on the development of the plant. 



SUMMARY OF CLIMATIC COMPARISONS. 



The climajtic characteristics of the plant-type zone may be summed 

 up as follows: The mean annual temperature is highest in the least 

 elevated type zone and decreases gradually with the increase in 

 altitude until, in the spruce-fir association, the season of growth 

 covers a period of only TO days. In the oak-brush type zone the 

 growing season is approximately 120 days. Precipitation, on the 

 other hand, is normally only about half as heavy in the oak-brush 

 type as in the type zones above. In general, however, the precipi- 

 tation is somewhat heavier in the aspen-fir than in the spruce-fir 

 type zone. The precipitation is rather uniformly distributed 

 throughout the year. The evaporation is highest in the oak-brush 

 type, where the greatest heat units and least rainfall are recorded. 

 The evaporation factor is nearly as intensive in the spruce-fir type, 

 however, while in the aspen-fir association it is only about half as 

 great. The strong evaporation in the spruce-fir type is accounted 

 for by the high wind velocity, which often exceeds 40 miles per hour 

 for several hours in succession. The seasonal wind movement in the 

 spruce-fir type is approximately 100 per cent greater than in the 

 associations below. The possible and actual sunshine are found to 

 be practically identical in the respective types. The barometric pres- 

 sure, of course, varies with the elevation, but the seasonal fluctua- 

 tions in a given locality are slight and insignificant so far as con- 

 cerns any direct effect on the vegetation. 



TEMPERATURE SUMMATIONS. 



Owing to the mass of climatic data compiled, it was necessary to 

 simplify them by summarizing 1 on different bases. 



The temperature factor in the respective stations for the periods 

 during which the plants were under observation was summarized in 

 three ways: (1) By physiological temperature coefficients as de- 

 veloped by Lehenbauer 2 and later applied by Livingston; 3 (2) the 



1 The literature relative to methods of comparative summations of climate has been 

 reviewed by Abbe, Cleveland, First report on the relation between climate and crops. 

 U. S. Weather Bureau Bull. 36, 1905. 



2 Lehenbauer, P. A. Growth of maize seedlings in relation to temperature. Physiol. 

 Res. 1:247-288. 1914. 



8 Livingston, Burton " E. Physiological temperature indices for the study of plant 

 growth in relation to climatic conditions, Physiol. Res. 1 : 399-420. 1916. 



