XIV. 



A GRAPHIC METHOD OF DETERMINING THE ADIABATIC CHANGES IN THE 



CONDITION OF MOIST AIR.* 



By Dr. H. Hertz. 



The theoretical meteorologist daily has to discuss considerations as 

 to the changes of coudition that take place in moist air that is com- 

 pressed or expanded without the addition of any heat. Hence he 

 desires to attaiu answers to these questions with the least possible ex- 

 penditure of time, and he does not care to use any of the complicated 

 formulae of thermodynamics. Actually he generally uses the small 

 practical table that Professor Hann oommunicated in the year 1874 

 (Zeit. der Oest. Ges.f. Met., 1874, ix, p. 328). Still it appears that with 

 at least an equal convenience one may attain a greater completeness if 

 one makes use of the graphic method, and the table accompanying this 

 paper presents an attempt in this direction. This contains nothing 

 theoretically new except in so far as that it also completely considers 

 the peculiar behavior of moist air at 0° C, which, so far as I know, has 

 hitherto not been treated of.t In the following I will now in Section i, 

 collect together the exact formulae of the problem, since a complete col- 

 lection of such appears to be wanting. Under Section n, the presenta- 

 tion of the formulae by the graphic table is described. Finally under 

 Section in, I explain completely, although purely mechanically, the ap- 

 plication of the latter to a numerical example. If one follows this ex- 

 ample with the diagram in the hand, one attains a judgment as to the 

 use of the table and a knowledge of the method of using it without the 

 necessity of going through the computations of Sections I and n. 



In a kilogram of a mixture of air and aqueous vapor let X represent 

 the proportional weight of dry air and yu the proportional weight of un- 

 saturated aqueous vapor contained therein. Let the pressure of the 

 mixture be p and its absolute temperature be T. Our problem is: 

 What conditions will the mixture pass through when its pressure is di- 



* Translated from the Meteorologische Zeitschrift, 1884, vol. I, pp. 421-431. 



t See, however, Guldberg and Mohn, " Studies on the movement of the atmosphere," 

 part 1, pp. 9-16, and, also, by the same authors, Oest. Zeit.f. Meteorologie, 1878, xiii, 

 p. 117. 



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