CLIMATIC FACTORS 



145 



quotient map of the United States by Jenny (1930) (Fig. 79).^ Jenny 

 has also taken advantage of this to show graphically the relations 

 between the nitrogen content of the soil and the climate along two 

 isotherms in the United States. The one line (11°C.) belongs to the 

 temperate region and includes territory in Colorado, Kansas, Missouri, 

 Illinois, Indiana, Ohio, and New Jersey; the other (19°C.) passes 

 through the subtropical states of Texas, Louisiana, and Mississippi 

 (Fig. 80). 



5. WIND 



Wind becomes of importance as a habitat factor on shores, on 

 broad open plains, and on mountain ridges. 



OJOO 



Ohio !nd New 



Kansas Mo. III. Jersey 



50 100 150 200 250 300 350 400 450 500N.5.Q. 



Texas Louisiana Miss 



Fig. 80. — The relation of the soil nitrogen content (A'^) and the humidity factor 

 (N.S.Q.) for grassland soils of the United States; along the annual isotherms (.1) of 

 11°C.; and (5) of 19°C. {After Jenny.) 



Winds are stronger the farther one goes above ground level. 

 According to Hellmann {Meteor. Zeitschr. 32: 1915), the mean annual 

 velocity of wind in an open field in Nauen was 3.29 m. per second at a 

 height of 2 m. ; 4.86 m. at 16 m. ; and 5.54 m. at 32 m. ; that of the wind- 

 swept coast of southern Ireland has 7.4 m. per second (Valencia). 

 This is almost as much as the alpine summits of Santis (2,440 m.) 

 with 7.7 m. per second; and Mount Washington, which stands quite 

 alone (1,950 m.), with 15 m. per second (annual means). ^ 



* A similar climatic map of North America based on precipitation, effectiveness 

 and temperature efficiency has been constructed by Thornwaite (1931). 



2 Since Kihlman's (1890) epochal investigations on the effect of wind upon 

 vegetation, a mass of data has been collected on the wind factor. Among the 



