82 



or even more. Occasionally, in high winds, the absolute humidity is 

 greater over the trees. Over the field station the daily progress of ab- 

 solute humidity was about the same as in the forest, but the maximum 

 difference was only about half as great. The absolute humidity in and 

 above the forest is greater than that over the open fields, and there is 

 some trace of an increase of this difference to the time of maximum." 

 A greater relative humidity has been found over evergreen trees 

 than over deciduous trees, which is slight (I.e.: 104), but the psy- 

 chrometer was close to the evergreens and farther above the decidu- 

 ous ones. 



Intensity of evaporation 



Sta. A. 20 rods (330 ft.) from wind- 

 break, 25 to 40 feet Ugh. 

 Standard 



Sta. B. 12 rods (198 ft.) from wind- 

 break 



Sta. C. 3 rods (49.5 ft.) from wind- 

 break 



Fig. 12. Diagram showing relative retardation of evaporation by a windbreak, 

 Lincoln, Nebraska. [Adapted from Card ('97).] 



The border of the Illinois forest and prairie was characterized by 

 tongues and isolated groves of forest and by glades. The forest had 

 the same kind of influence as windbreaks upon the leeward areas and 

 glades, and therefore the influence of windbreaks upon the evaporating 

 power of the air is of interest. Card ('97) made a valuable study of 

 this series of problems at Lincoln, Nebraska. The influence of wind- 

 breaks upon evaporation is summarized in Figure 12. This diagram 

 shows that leeward of a close windbreak ranging from 25 to 40 feet 

 in height, the rate of evaporation in terms of the standard (A), which 

 was 330 feet leeward, was 91 per cent, at a distance of 198 feet (B), 

 and 71 per cent, at 49.5 feet (C), thus showing a marked reduction 

 with proximity to the windbreak. These observations covered 62 days. 



Nearer to Illinois, similar though very limited observations were 

 made in central Wisconsin by King ('95) which agree with Card's 

 on the retardation of evaporation by windbreaks. His results are 

 shown graphically in Figure 13. 



Recently Bates ('11) has made an elaborate study of the effects of 

 windbreaks upon light, soil, moisture, velocity of wind, evaporation, 

 humidity, and temperature. His results confirm those just given and 

 give additional facts which, however, with one exception, will not be 

 mentioned. The paper itself should be consulted. This investigation 

 by Bates shows that in proportion to the perfection of the windbreak 



