48 CONSTRUCTION OF ISOBARIC CHARTS 



Example: During a kite ascension made at Omaha on Sept. 23, 1898, at 11.25 

 A. M., 75th meridian standard time, there was observed jj = 24.20 inches, t = 68° F., 

 r = 51 per cent. 



Table 7, for p = 24.20 inches and t = 68° F., gives t^ - t = 5°. 5 ; and 

 Table 8, for t,^ — t = 5°. 5 and r = 51 per cent., gives tr— t = 3°.0. The virtual 

 temperature is thus found to be 68° -|- 3° = 71° F. 



Formulae (18) and (19) can be made valid for moist air if i^ be substituted for t in 

 them, and they then read 



. Ep„ =1837.-3 (C + 459.4)r/(logp), (21) 



n;;=^^^(l-10 l««-3Jr„<,+459.4)_ (22) 



The condition for atmospheric equilibrium may be so formulated that the num- 

 ber n y\ of isobaric surfaces contained between two level surfaces, V — I'o and V = \\ 

 is everywhere the same. From equation (22) it appears that this is the case when 

 tr can be expressed as a function of F alone, i. e., when the surfaces of equal values of 

 tr coincide with the level surfaces of gravity. Whence it appears that in an atmos- 

 phere in the condition of static equilibrium the surfaces of equal values of t^ as well 

 as the isobaric surfaces, coincide with the level surfaces of gravity. 



The values of E^j and of 11 {^' may be easily tabulated if we restrict ourselves once 

 for all to a small number of limiting values of ^^o and p^ as well as of Vq and T'',. For 

 example, we choose respectively every half-inch of barometric pressure and every 

 10 000th level surface of gravity, that is to say we compute the following values : 



E29..5 T729.0 C-SS.S J?28.0 UiT.S .f. 

 30.0 '-'29.5 '-'29.0 -'-'28.5 ■'-'28.0 '='''.'•> 



For such small intervals the average values of Ir may be readily found by graphic inter- 

 polation. When these values are substituted in (21) and (22) and the lattei- are then 

 integrated we obtain : 



Ej;j = 1 837.3(^, -f 459.4)'^^^-" (23) 



and 



n Jj = p r^( 1 - 1 1837.3(^+459.4))_ (24) 



