373] B. E. Livingston 175 



day to day, for the same place, which makes it necessary in 

 climatic discussions to resort to averages and means. While 

 the index of relative humidity for any instant may be readily 

 interpreted by use of the corresponding air temperature., there 

 is no possible way by which an average of several such indices 

 may be so interpreted ; the average temperature for the period 

 is of no use for this purpose, since the march of temperature 

 for the period is not necessarily at all related to that of the 

 moisture condition. The only way to give definite meaning 

 to a relative humidity mean is to obtain the original humidity 

 values from which the mean was derived (together with the 

 corresponding air temperatures), to substitute for each indi- 

 vidual value the corresponding vapor pressure deficit, and to 

 derive the mean of the deficits, thus discarding relative hu- 

 midity altogether. 



For biological experimentation, for hygienic studies of the 

 air moisture condition in dwellings, and for general climato- 

 logical purposes, it is very obvious that the whole concept of 

 relative humidity is hopelessly misleading; the sooner this 

 concept can be forgotten the more rapidly will knowledge ad- 

 vance. When it is not desirable or expedient to employ the 

 index of atmospheric evaporating power itself (as determined 

 directly by some form of atmometer), the moisture condition 

 of the air should be stated in terms of the vapor pressure de- 

 ficit, which demands no correction for air temperature and 

 may represent evaporating power in all comparisons where the 

 index of effective air circulation may be considered as constant. 



