282 



JOHN ERNST WEAVER 



a station with two atmometers was established on the southwest 

 slope about 20 m. from the crest of a hill somewhat less than the 

 average height, and another station with two instruments was 

 placed at an equal distance from the hilltop on an average north- 

 east slope. These atmometers were read at intervals of a week 

 or less from May 5 to September 23, as shown in the graph (fig. 3), 

 on which chart the rainfall at Pullman is also recorded in cm. 

 An inspection of these graphs shows a remarkable similarity in 



Fig. 4. Station No. 4, in the prairie formation near Pullman, Washington. 

 The atmometers are to the left and not shown in the picture 



their general course throughout the season, but the difference 

 in the amount of evaporation is just as striking. The average 

 daily evaporation on the north slope is 16 cc, which is only 64% 

 of that (25 cc.) on the south slope. The greatest factor in caus- 

 ing this difference is the same one that has so profoundly influenced 

 the topography, namely, the wind. During the 141 days of 

 observation, the wind was from the southwest 43% of the time; 



