194 



FACTORS AFFECTING TRANSPIRATION 



in temperature means that a higher vapor pressure will be required before the 

 atmosphere is saturated. 



Expression of humidity values in terms of relative humidity, although 

 a common practice, is unsatisfactory for physiological purposes because the 

 same relative humidity, 50 per cent for example, may refer to widely dif- 

 ferent vapor pressures and vapor pressure deficits (Table 23). A relative 

 humidity of 50 per cent at 10° C. is equivalent to a vapor pressure deficit 

 of 4.60 mm. Hg, while a relative humidity of 50 per cent at 50° C. is 

 equivalent to a vapor pressure deficit of 46.26 mm. Hg. In other words, 

 with the same relative humiditj^, evaporation from moist surfaces exposed to 

 the air will be many times as rapid at 50° C. as at 10° C. Only when all of 

 the relative humidity values are recorded at the same temperature are they 

 an expression of the relative differences in the vapor pressure of an atmosphere. 

 The general relations between relative humidity, vapor pressure, and vapor 

 pressure deficit are summarized in Table 23. 



TABLE 23 THE RELATION BETWEEN RELATIVE HUMIDITY, VAPOR PRESSURE DEFICIT AND 



VAPOR PRESSURE AT DIFFERENT TEMPERATURES 



Temper- 

 ature 



°C. °F. 



o 32 



5 41 

 10 50 



15 59 

 20 63 



25 77 



30 86 



35 95 

 40 104 



45 113 

 50 122 



o 

 o 

 o 

 o 

 o 

 o 

 o 

 o 

 o 

 o 

 o 



Actual vapor pressure (mm. Hg) at indicated relative humidity 



(Read down) 



10% 



20% 



30% 40% 50% 60% 70% 80% 



90% 



0.46 

 0.65 

 0.92 

 1.28 



1-75 

 2.38 



3.18 



4.22 



5-53 

 7.19 

 9.25 



o 

 I 

 I 



'^ 



3 

 4 

 6 

 8 

 II 



14 

 18 



92 



31 

 84 

 56 

 51 

 75 

 36 



44 

 06 



38 

 50 



1-37 



96 



76 

 84 

 26 



13 



55 



12.65 

 16.60 

 21. 56 



27.75 



1.83 



2.62 



3.68 



5.12 



7.02 



9.50 



12.73 



16.87 



22. 13 



28.75 



37.00 



100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 



Vapor pressure deficit (mm. Hg) at indicated relative humidity 



(Read up) 



100% 



58 



54 

 21 



79 

 54 

 76 

 82 

 18 



32 

 88 



si 





In general the greater the vapor pressure of an atmosphere, other factors 

 remaining unchanged, the slower the rate of transpiration. The rate of dif- 

 fusion of water-vapor out of a leaf depends upon the difference between the 

 vapor pressure in the intercellular spaces and the vapor pressure of the out- 

 side atmosphere, since the vapor pressure is a measure of the diffusion pressure 



