174 Vapor Tension Deficit [372 



vapor pressure of water vapor in the air, while the vapor pres- 

 sure deficit is the difference between these two vapor pressures. 

 Eelative humidity percentages are without value unless the 

 air temperature is also given, whereas the deficit values need 

 no reference to air temperature for their interpretation. 1 The 

 fallacy in the employment of relative humidity clearly lies in 

 the fact that its values are ratios and that the denominator of 

 the ratio varies with air temperature; different percentage 

 values cannot .be comparable unless they are calculated to the 

 same base. 



In the illustration given above, the relative humidity index 

 for room 1 is 3.25 times as great as that for room 2, and the 

 popular conception of relative humidity might lead to the 

 erroneous supposition that the evaporating power of the air 

 for room 2 should be 3.25 times as great as that for room 1, 

 whereas this last number should be the value of the fraction 



!M! or 5.98. 

 2.91 



The real uselessness of the concept of relative humidity and 

 the manner in which this concept is frequently misleading 

 are brought out by the fact that the index of relative humidity 

 may be identical for two rooms or for two climatic stations, 

 and (owing to a difference in air temperature) the mois- 

 ture factor of the evaporating power of the air may be very 

 different in the two cases. Thus, a relative humidity of 60 

 per cent, corresponds to an air moisture factor of 10.44 mm. 

 at 20, and to one of 14.13 mm. at 25. The moisture con- 

 dition of the air in the second case is much higher, but the 

 relative humidity values fail to suggest any difference. 



One of the most serious reasons for discontinuing the use 

 of relative humidity lies in the fact that the moisture con- 

 dition of the air generally varies from hour to hour and from 



1 For some very true remarks in this connection, see : Stevens, Neil 

 E., " A method for studying the humidity relations of fungi in culture.' 1 

 Phytopathology 6: 428-432. 1916. Other references are there given. 



