CLIMATIC CONDITIONS OF THE UNITED STATES. 359 



much more satisfactory charts might probably now be made, it appears 

 that these charts of relative humidity must be regarded as perhaps 

 the most valuable of all air-moisture charts with which we have been 

 able to deal in a practical way. 



F. WIND. (TABLE 20, PLATE 68.) 



Wind influence upon plant growth occurs in several ways, the most 

 generally important of which is probably effective through increased 

 transpiration. As has been emphasized, air-movement is very influen- 

 tial in determining the evaporating power of the air, which, in turn, 

 is the main external condition governing the transpiration-rate as well 

 as the rate of water-loss directly from the soil-surface. This influence 

 depends upon the velocity of air-movement. Strong winds tend to 

 break plants as well as to dry them, but this feature of wind influence 

 appears to be of importance to vegetation only in relatively restricted 

 areas. If wind were to be studied in this regard, it is very high wind- 

 pressure and its duration that would require attention. The direction 

 of the wind, so important in weather predictions, is of no importance 

 to plants in general. If the ecology of individual plants or small 

 groups is to be studied, then wind direction may sometimes become 

 important. 



Wind as a climatic condition influencing vegetation may be con- 

 sidered: (1) in terms of its average velocity— perhaps the most useful 

 wind-dimension as far as the evaporating power of the air is con- 

 cerned; (2) in terms of its maximum velocity and the duration of very 

 high rates of air-movement; and (3) in terms of the maximum pres- 

 sure developed and the duration of very high pressures. The United 

 States Weather Bureau has accumulated anemometric data for many 

 stations in the United States, and we have employed some of these 

 data to prepare a chart of average wind-velocities for the period of 

 the average frostless season. The data employed are as yet unpub- 

 Hshed, but have been very kindly furnished us by Professor P. C. Day, 

 of the United States Weather Bureau. They consist in a table of 

 average wind-velocities (in miles per hour) for each month of the year 

 and for 151 stations, based generally upon the 20-year period 1891- 

 1910. For each of these stations the average wind-velocity has been 

 calculated for the period of the average frostless season, as in the other 

 cases where monthly averages were used in deriving means for the 

 frostless season. The results of our computations, together with the 

 annual average velocity in each case (obtained from the records of the 

 United States Weather Bureau) and the height above the ground of the 

 anemometer are given in table 20. 



