PROPERTIES OF WATER VAPOR 



(Further explanation on p. 331.) 



The compressibility factor C v of water vapor is defined by 



Pv = e/C*R V T, (19) 



where p v is its density according to the Goff-Gratch formulation, e is its pressure, and 

 R v is the gas constant for water vapor, namely, 



R v = 0.46150 X 10 T erg g." 1 "K." 1 



= 0.110226 ITcal. g." 1 "K; 1 (20) 



Values of compressibility factor Cv are listed in Table 91. 



The enthalpy, entropy, and isobaric specific heat residuals of water vapor are defined 

 by the equations 



hv = 4Rv(t + 1354.74) + Ah, 



Sv = 4R v [\og. (W/ 4 ) + 1.15901] + As, (21) 



c pv = 4R V + Ac P v = 0.4409 + Ac pv 



Each of these expressions is obtained from the corresponding one for moist air on dividing 

 the latter through by mixing ratio r and then putting r = 00. Clearly, therefore, h v 

 denotes the enthalpy of water vapor per unit mass of water vapor, that is, specific 

 enthalpy — and similarly for Sv and c pv . 



Values of the above residuals are listed in Table 91 along with values of the compres- 

 sibility factor Cv. Linear interpolation is valid throughout. In the whole range covered 

 by the table, namely, — 100 to 60 °C. by to 199.26 millibars, the compressibility factor 

 lies between 1.0000 and 0.9948, the enthalpy residual lies between — 0.6 and +0.4 

 ITcal. gm." 1 the entropy residual lies between —0.0012 and +0.0013 ITcal. g." 1 "K." 1 and 

 the isobaric specific heat residual lies between + 0.0009 and + 0.0241 ITcal. g." 1 °K. _1 



As mentioned previously, values of the saturation pressure of water vapor with respect 

 to liquid e*(T) and with respect to ice ei(T) as calculated from the Goff-Gratch formu- 

 lation appear in Tables 94-97. From the evidence currently available, extrapolation of the 

 formula for ev,(T) to obtain values of the saturation pressure with respect to undercooled 

 liquid down to — 50 C C. appears to be justified pending further research. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



