83 



a quiet, stagnant air strip is formed to the leeward, and that this fa- 

 vors excessive heating during clear days and low temperatures on clear 

 nights. Years ago Harrington ('93:119) suggested this idea and 

 called attention to the close relation existing between the leeward con- 

 ditions of windbreaks and forest glades. The glade climate is more 

 rigorous, or extreme, than that upon plains (I.e.: 19, 84-88, 119). 

 Such a climate is thus a bit more "continental" during the spring, sum- 



Intensity of evaporation 



Distance from windbreak 12 inches high : 



Sta. F. 500 feet leeward. Standard 



Sta. E. 400 feet leeward 



Sta. D. 300 feet leeward 



Sta. C. 200 feet leeward 



Sta. B. 100 feet leeward 



Sta. A. 20 feet leeward 



Fig. 13. Diagram showing the relative evaporation, May 31, at different dis- 

 tances leeward of a windbreak, Almond, Wis. [Adapted from King ('95).] 



mer, and autumn. These glades are very hot in the early afternoon 

 and cool on clear nights, and the air is relatively stagnant; as Harring- 

 ton says, it is "lee for winds from all directions." The center of a 

 dense forest may thus possess physical conditions quite different from 

 those of the glade forest margin or in the open. Beginning with the 

 relatively stable conditions within a forest toward its margin, the diur- 

 nal temperature variations are much more extreme (Harrington, 

 1. c. : 89) "to a distance of a score or so of rods where it reaches a max- 

 imum. The amplitude is greater in glades. Hence the extremes of 

 temperature are exaggerated just outside the forest." The annual soil 

 temperatures of a glade are intermediate between that of the forest and 

 the plain. The forest margin is thus seen to possess many of the char- 

 acteristics of the glade, for its climate is somewhat more extreme than 

 that in the open, far from the forest. 



5. Temperature Relations in the Open and in Forests 



The temperature relations in open and forested regions are often 

 very different. The density of the vegetable covering in the open and 

 in the forests varies much and may have considerable influence upon 

 animals. Yapp ('09) observed that the marsh vegetation in England 



