4 BULLETIN 1405, IT. S. DEPARTMENT OF AGRICULTURE 



spruce-fir at 10,000 feet. Such factors as air temperature, soil 

 temperature, precipitation, air humidity, evaporation, wind velocity, 

 and sunshine duration and intensity all directly or indirectly affect 

 plant growth, but those most closely related to the grazing periods 

 are air temperature and precipitation. Air-temperature records 

 were obtained by using thermographs and maximum and minimum 

 thermometers exposed in shelters 4% feet above the ground. Precip- 

 itation was measured in standard rain gauges. 



AIR TEMPERATURE 



The records show temperature characteristics typical of mountain 

 regions and clearly indicate the effect of elevation. The highest 



SPRUCE. -FIR TYPE, iqOOO ftsr 



ASPEN-FIR TYPE, 8.7QO FCET 



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Fig. 1. — Typical thermographic records in the first half of June, 1922, showing 

 the relation of temperatures and the differences in the daily range 



maximum temperature and the widest daily variation are found in 

 the lowest type; with increase in altitude there is a slow decrease 

 in the minimum temperature and a larger decrease in the maximum, 

 which is responsible for the flattening of the daily temperature 

 curve for the spruce-fir type, as shown in Figure 1. Mean tem- 

 peratures accordingly diminish regularly with increased elevation. 

 During the main growing season, from June to September, in- 

 clusive, the mean-temperature decrease gradient for every 1,000 

 feet difference in elevation is 4.05° F. from the oak-brush to the 

 spruce-fir type, 3.91° from the oak-brush to the aspen-fir type, 

 and 4.23° from the aspen-fir to the spruce-fir type. The mean- 

 temperature decrease gradient for the year is somewhat similar to 

 that for the growing season, although owing to temperature in- 

 versions in winter the exact rates of decrease are somewhat different. 



