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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 51 



example, air having a temperature of 20 C. and a relative humidity 

 of 67 per cent rises from sea level, then under adiabatic expan- 

 sion the condensation will begin at the altitude 770 meters. 



Then at 1500 meters altitude a temperature of 8.6° C. will prevail. 

 The difference therefore would amount to 11.4 for 1500 meters; 

 the rate of decrease would, however, be only 0.76 per 100 meters. 

 Notwithstanding this we should make a grave error if in this case 

 we should assume a condition of stable equilibrium. Therefore this 

 < ircumstance must never be lost sight of in judging as to the stability 

 of atmospheric equilibrium, for instance, from observations on 

 mountains and in valleys, where we generally must rely on observa- 

 tions at two points only. 



This difficulty generally disappears in the deductions from balloon 

 observations, since in most cases these give us complete curves of con- 

 dition for long distances. But this point deserves consideration often 

 in case of rapid ascensions, where not infrequently the first obser- 

 vation in the balloon can be made only after passing the limit of 

 condensation. 



It may be incidentally mentioned that Reye even in his time, 

 although in different form, has shown what importance attaches, 

 for the whirlwind storms, to this "knick" in the adiabatic curve, or 

 the steeper ascent in the condensation stage. Of course, these 

 remarks of Reye hold good, with appropriate changes, for the 

 thunderstorm phenomena of our regions. 



The question as to the "critical" gradient of temperature (a term 

 that we may properly apply to the limiting value characteristic 

 of unstable equilibrium) becomes especially complicated in one 

 particular case that I will now explain more fully. 



Assume that the curve of condition has some such appearance as 

 shown by the line Z Z in fig. 38 and that the 

 limit of condensation lies at 5. Then it can 

 happen that the adiabat of the condensation 

 stage has the course 5 A 2 while that of the 

 1 1 ry stage is represented by A , 5. Under these 

 conditions stable equilibrium prevails below 

 the limit of condensation but unstable equi- 

 librium above that limit. 



This case cannot easily occur in masses of 

 air that ascend as broad currents, for an ascent 

 along the curve Z S can only happen when the ascending air after 

 leaving the ground continually receives so much heat, eitl er by 

 radiation or by mixture with other air, winch of course must bring 



fig. 38 



