Tables 108 and 109 3gX 



DENSITY OF PURE WATER VAPOR AT SATURATION 



The density of pure water vapor (vapor unadmixed with air) at saturation over a 

 plane surface of liquid water, p w , in cgs units is 



Pw — e " CD 



H C V R„T 



where 



R v z= gas constant for water vapor, 4.6150 x10 s erg g." 1 °K."\ 

 T = temperature of the vapor, °K., 



e v = saturation vapor pressure over water at temperature T, 

 C, = "compressibility factor" for water vapor (Table 91). 



The factor C\> is introduced into equation (1) to correct for the deviations of water vapor 

 from ideal gas laws. 



For pressures measured in millibars, the density p w in g. m." 3 (1 g. m.^^lO -8 g. 

 cm. -3 ) is 



Pk = 216.68 ^gN- (2) 



In a similar manner, the density of pure water vapor at saturation over a plane surface 

 of ice, pi, is found by substituting ei, the saturation vapor pressure over ice, for e* in 

 equations (1) or (2). 



Concentrations of the constitutents of moist air. — It is necessary to distinguish be- 

 tween the density of a gas or vapor unadmixed and the concentrations of the constituents 

 of a mixture; this is especially true in dealing with real gases. The vapor concentration 

 d v and the dry-air concentration d a are defined as the ratios of the masses of vapor 

 w» and of dry air m a , respectively, to the volume V occupied by the mixture 



d*=^ (3) 



d*=^ (4) 



Since the mixing ratio r = m v /m a and the density of moist air p = d a + d v , equation 

 (3) becomes 



dv = -t— — p (5) 



1 + r 



On introducing the mixing ratio at saturation over water r w , where r = Ur v and U is 

 the relative humidity, equation (5) becomes 



Using values of p and r v from Tables 71 and 73, respectively, dv may be readily computed 

 from (6). 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



