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. 
Relative 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.* Tlte 
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 
17.41 6 598, 
2 
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 14.18 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 
‘ : : : 
For some very true remarks in this connection, see: Stevens, Neil 
E., “ A method for studying the humidity relations of fungi in culture,” 
Phytopathology 6: 428-432. 1916. Other references are there given, 
