CLIMATIC CONDITIONS OF THE UNITED STATES. 287 



(within the soUd or liquid phase) to vaporize water, then the apparent 

 rate of evaporation becomes zero. Going further, the external condi- 

 tions frequently become such that the resistance offered by the air to 

 vaporization of water is greater than the tendency of the internal 

 conditions to cause vaporization, this resistance being due to a ten- 

 dency of the air to deposit water on the surface, and actual condensa- 

 tion ensues; i. e., the evaporating power of the air becomes negative 

 and the evaporating surface gains water instead of losing it. 



There seems never to have been any attempt to define air according 

 to its chemical content; it would still be air if it were largely carbon 

 dioxide or hydrogen, etc., and it seems unadvisable to attempt a 

 restriction of the terms ''air" and ''atmosphere" at this late day. 

 For the rest, the expression "evaporating power of the air" has been 

 in use among students of this power at least since 1906 (when Living- 

 ston used it). It will probably appeal to most students of this poweras 

 quite unobjectionable and it need not be dropped. Livingston (1917, 1) 

 has suggested atmometric index as still another term, to avoid the 

 difficulty just mentioned and to avoid the necessity of employing 

 evaporation to mean both the process and one of the conditions 

 controlling its rate. We shall not employ this new expression, how- 

 ever, preferring to allow others to decide the question thus raised. 



The evaporating power of the air is of the utmost physiological 

 importance to vegetation, and it can be rather readily and directly 

 measured, in relative terms. Nevertheless it has not been seriously 

 studied in the United States, and most of the information so far 

 obtained in regard to it is only indirect. To appreciate what ones of 

 the climatic conditions usually measured may be valuable here, it is 

 necessary to consider the secondary conditions, upon which depends 

 the atmospheric evaporating power. 



The vapor-tension deficit. — Without air-movement, and supposing 

 the air and water temperatures to be the same, the evaporation-rate 

 should be nearly proportional to the vapor-tension deficit; that is, to 

 the difference between the maximum vapor-tension for the given air- 

 temperature and the tension of water-vapor actually present in the 

 air. The actual vapor-tension in the air is a closely approximate 

 measure of the tendency toward condensation and the maximum vapor- 

 tension for the given temperature and pressure is a measure of the 

 whole tendency toward evaporation; the former tendency overcomes 

 a portion of the latter and what remains is very nearly the actual 

 tendency toward evaporation. The maximum vapor-tension of water 

 is, of course, a constant for any temperature and barometric pressure, 

 and its value may be obtained from physical tabled. The actual 

 vapor-tension is seldom as great as the maximum; it is so only in the 

 case of water-saturated air. If we allow E and E' to represent evapora- 

 tion-rates from the same surface at different times, P and P' the 



