Table 93 (concluded) 349 



DEFINITIONS AND SPECIFICATIONS OF WATER VAPOR IN THE 



ATMOSPHERE 



The relationship between T„ as defined and the wet-bulb temperature as indicated 

 by a particular psychrometer is a matter to be determined by carefully controlled 

 experiment, taking account of the various parameters concerned; e.g., ventilation, 

 size of thermometer bulb, radiation, etc. 3 



(16)* The thermodynamic equivalent temperature T, of moist air at pressure p, 

 temperature T and mixing ratio r, is the temperature which the air assumes by 

 means of adiabatic condensation at constant pressure of all the water vapor which 

 it contains, the condensed water falling out of the system immediately. 

 T e is determined by the equation 



los T-m — ^r- Iog ( 1 + 77 r j 



where : 



L V (T) = heat of vaporization of water at temperature T, 

 Lv(T e ) = heat of vaporization of water at temperature T», 

 Cv, = specific heat of liquid water, 

 c p = specific heat of dry air at constant pressure, 

 c f v = specific heat of water vapor at constant pressure. 



Here c v and c pv are assumed to be independent of temperature in the interval T w 

 to 7%. 



*The Working Subcommittee of the International Joint Committee on Psychrometric Data (Goff, 

 J. A., Amer. Soc. Mech. Eng. Trans., vol. 71, 1949) recommends as preferable that thermodynamic wet- 

 bulb temperature be defined as the solution Tw (.p,T,r) of the equation: 



HP, T,r) —r • h'w(p, Tv,) = h,{p, Tv) — *•«(/>, Tv) • h'v(p, Tv) 

 where h {p, T, r) = specific enthalpy of moist air, h'w(p, Tv>) = specific enthalpy of pure compressed 

 liquid (or solid) water, and hn(p, Tv>) — specific enthalpy of saturated air at pressure p and tempera- 

 ture Tv>. This definition combines the correct energy and weight accountings for the steady-flow 

 process of injecting pure compressed liquid (or solid) water at pressure p and temperature Tu into 

 a stream of moist air at pressure p, temperature T, and mixing ratio r to bring the air adiabatically 

 to saturation at pressure p and temperature Tv. This is to be regarded as the appropriate idealiza- 

 tion of the actual process by which the thin film of water on a wet-bulb thermometer immersed in a 

 stream of moist air maintains, at any rate for a time, a steady value below that of the air itself. 



* Definitions (13), (15), and (16) were rescinded and the definitions in note (a) under (13) and in 

 footnote 3 were adopted for relative humidity and for thermodynamic wet-bulb temperature by the 

 WMO Commission for Aerology, First Session, Toronto, 1953, and approved by the Executive Commit- 

 tee, WMO, 1953. 



SMITHSONIAN METEOROLOGICAL TABLES 



