240 JOHN ERNST WEAVER 



root-systems themselves to consider briefly the above-ground 

 environment. 



Enough will have been said about summer temperatures, if 

 we add that the long days, mostly cloudless, are followed by 

 cool nights, during which the temperature usually reaches 45°- 

 58°F. During the winter the ground seldom freezes below 

 4 inches in depth, and, of course, on the expqsed slopes the proc- 

 ess of alternate freezing and thawing is most pronounced. 



A continuous record of humidity has been kept for more than 

 two complete growing seasons and it has been" found that the 

 air is often 5 per cent to 10 per cent drier on the exposed than on 

 the sheltered slopes. It is not uncommon on the dry slopes and 

 during late afternoons for the humidity to fall to 15-30 per cent, 

 while during the night it may rise again to about 75 per cent or 

 even 95 per cent. 



The wind, prevailingly from the southwest, is of great impor- 

 tance to vegetation, because it increases the evaporating power 

 of the air, and the greater saturation deficit increases transpi- 

 ration. During the season of 1913 (April 16 to September 3) 

 a total of 13,605 miles of wind passed over the southwest slope 

 at a height of 0.5 meter, while only 56 per cent as many miles 

 were recorded by the anemometer similarly placed on a northeast 

 slope. In general, these conditions were duplicated in the season 

 of 1914 with the ratio of 100 : 49 (April 18 to August 15). 



However, these factors of temperature, humidity, and wind 

 movement, as Livingston has shown, 5 may be quite satisfactorily 

 summed up by measuring the evaporating power of the air. A 

 rather detailed report of evaporation rates as determined by 

 Livingston's standardized porous cup atmometers in prairie 

 and other stations has already been made, 6 in which it was shown 

 that from May 5 to September 23, 1913, the average daily 

 evaporation on the northeast prairie slopes was only 64 per cent 

 of that on southwest exposures. Similar records for 1914 bear 



5 Livingston, B. E., Evaporation and Plant Habitats. Plant World 11 : 1-10, 

 190S. 



6 Weaver, John Ernst, Evaporation and Plant Succession in Southeastern 

 Washington and Adjacent Idaho. Plant World 17: 273-295, 1914. 



