344 



ENVIRONMENTAL CONDITIONS. 



humid, semihumid, and semiarid conditions included in the arid, 

 semiarid, etc., areas as above described. A detailed moisture-ratio 

 chart of a single county in Aiizona or Nevada would doubtless be 

 vastly more complicated than is any of our general charts for the entire 

 country. 



D. AQUEOUS-VAPOR PRESSURE. 

 (1) Preliminary Considerations. 



The pressure of aqueous vapor in the air should be an index of the 

 relative influence of the air (aside from its rate of movement) toward 

 the retardation of evaporation from wet surfaces. Roughly, this 

 index should be somewhat nearly inversely proportional to the evapora- 

 ting power of the air, disregarding the wind factor. This is, therefore, 

 a climatic dimension the measurement of which should be valuable in 

 studies of the relations between plant activities and environmental 

 conditions. 



The vapor-pressure data for the United States, for the years 1873 to 

 1905, have been reduced to a system of homogeneous monthly and 

 annual means by Bigelow (Bull. S., 1909), and these means have fur- 

 nished the basis for our work in this connection. We have employed 



(1) the normal mean vapor-pressure for the period of the average frost- 

 less season, and (2) the normal mean annual vapor pressure. 



(2) Normal Mean Aqueous- Vapor Pressures for Period of Average Frostless Season 



(Table 19, Plate 63.) 



The indices here employed were derived from the monthly means 

 of Bigelow, in the way already described for such cases. Our results 

 are given in the last column of table 19, and plate 63 represents them 

 graphically. 



Table 19. — Normal mean relative humidities, for the year and for the period of the average 

 frostless season, ynean relative humidities for the three summer months, 1908, and normal 

 mean vapor-pressures for the year and. for the period of the average frostless season. 



