STATION STATION STATION STATION 



''a" ''b" ''c" "d" 



Eelative Humidity .63— .75 .64— .78 .68— .86 .59— .74 

 Piche 3.28 2.80 3.30 3.60 



Pan 102.00 cc. 97.70 cc. 80.20 cc. 129.00 cc. 



Station ''a" is located on the sand pit, station "b" is four 

 feet from the lake shore, station "c" is at the base of the more 

 abrupt slope 160 feet north of the lake, and station "d" is at 

 the summit of the first elevation 940 feet north of the lake and 

 51 feet above its surface. 



It is again notable that the greatest relative humidity is at the 

 very base of the steeper slope (station "c"), even where the 

 slopes are not abrupt. The differences in relative humidity are 

 not gi'eat however, all the stations being quite dry during the 

 warmest part of the day. Station "a" is quite remarkable in 

 that it is on a narrow sand spit which extends some distance into 

 the bay, and yet shows a high rate of evaporation, evidently 

 because of its exposed position, and the inability of the vapors 

 to rise sufficiently rapidly above the wind-swept lake to keep the 

 air humid. The reason for the greater evaporation at station 

 "d, " and for the treelessness of this entire slope is to be 

 found in the position of the tract with reference to surround- 

 ing topography. The higher prairie rises westward from the 

 lake to the Lookout, but between the high point and the Lab- 

 oratory there is a depression which forms a trough leading from 

 the general prairie to the southwest directly to the slope north 

 of the bay, as shown in figure 1. The southwesterly winds which 

 sweep across the prairie are therefore directed to the slope in 

 question, and render it quite xerophytic, notwithstanding the 

 proximity of the lake. The vapors from the latter are usually 

 swept to the opposite side and make possible the existence of the 

 forest border along the east side of the lake. 



The great variation in rate of evaporation at different points 

 in the same vicinity is well illustrated by the series of observa- 

 tions made by the writer in August, 1909, at the Laboratory. 

 Six stations were selected. Station 1 was located near the sum- 

 mit of the Lookout, facing southwest, — the most exposed station 

 (see Plate I, fig. 1) ; station 2 was located near the west side of 

 the Laboratory grounds, and near the edge of the "trough" 



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