332 Tables 84—92 (continued) 



THERMODYNAMIC PROPERTIES OF MOIST AIR 



relatively flat surface of separation, with an associated liquid phase. When capable of 

 such coexistence, the moist air is said to be saturated with respect to liquid and its mixing 

 ratio r assumes a definite value r w (p, T) depending on the pressure p and temperature T. 

 According to the redefinition, moist air at pressure p, temperature T, and mixing ratio r 

 is said to have relative humidity U, expressed decimally, 



U = r/ rv , (1) 



Thermodynamic temperature. — The Second Law of Thermodynamics asserts the 

 existence of a universal temperature function T which, according to experimental and 

 other evidence, is strictly proportional to the zero-pressure value of the pressure-volume 

 product in the case of a gas or gas mixture. The relation 



(pv)"=R*T (2) 



where v denotes volume per mol and the superscript ° refers specifically to zero pressure, 

 has come to be regarded as exact. For practical purposes, the so-called Kelvin scale 

 may be defined by arbitrary assignment of the value 273.17 C K. to the triple point of pure 

 ordinary water, the corresponding ice-point temperature being 



To = 273.16 °K. (3) 



In terms of the Kelvin degree (°K.) thus defined, the present best value of the universal 

 gas constant R* is that recommended in 1941 by Birge, 6 namely, 



R* = 8.31436 X 10 T erg "K." 1 gmol.- 1 (4) 



From the Kelvin scale T(°K.) is derived the so-called thermodynamic Celsius (centi- 

 grade) scale f(°C.) through the relation, 



t = T-T (5) 



It departs from the scale of the platinum resistance thermometer, that is, from the inter- 

 national Celsius scale, by at most 0.008 °C. in the range to 100 °C. ; and the departure 

 itself is known in this range with an accuracy probably better than 0.002 °C. It is tem- 

 perature on this thermodynamic Celsius scale that is used as independent argument in the 

 tables to follow. 



Table 84 

 COMPRESSIBILITY FACTOR OF MOIST AIR 



(Further explanation on p. 331. See also p. 295.) 



The compressibility factor C of moist air is defined by T 



pv = C(l+ r/c)RT, (6) 



where v is the volume of moist air per unit mass of dry air according to the Goff-Gratch 

 formulation, e is the ratio of the molecular weight of water (M„ = 18.016 g. gmol" 1 ) to 

 the apparent molecular weight of dry air (M = 28.966 g. gmol" 1 ), namely, 



6 = 0.62197; (7) 



and R is the gas constant for dry air, namely, 



R = 0.28704 X 10 T erg g." 1 °K." 1 



= 6.8557 X 10"* ITcal. g." 1 "K: 1 (8) 



In Table 84 are listed values of compressibility factor C as a function of pressure p, 

 thermodynamic Celsius temperature t, and relative humidity U. Linear interpolation is 

 valid throughout the table. To aid interpolation, zero-pressure values for dry air (r = 0) 

 are listed in the column (7 = even though relative humidity is not defined at this pressure 

 (see footnote 5, p. 331). In the region covered by Table 84, the compressibility factor C 

 lies between 1.0000 and 0.9956, which means that its departure from unity can safely be 

 disregarded in rough calculations. 



•Birge, R. T., Rev. Mod. Phys., vol. 13, pp. 233-239, 1941. 



T Equaion (6) can also be written as p = p/CRTv where p is the density of the moist air and 

 Tv is its virtual temperature (see Table 72). 



(continued) 

 SMITHSONIAN METEOROLOGICAL TABLES 



