482 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 51 



where h is the change of altitude in meters; y is the quantity of 

 frozen water in grams; therefore 1 gram of freezing water corre- 

 sponds to a change of altitude of 27 meters. 



The evaporation of water can be assumed proportional to the 

 quantity of water present; the amount is very slight and is only 

 0.1 gram for 5 grams of water; this would introduce an error of 

 three meters in the altitude when there is an isothermal ascent of 

 135 meters but the empirical value has considered this fact. 



If the condensation adiabat reaches o° C. then the gram-line 

 passing this point gives the quantity of aqueous vapor still present, 

 for instance, 6 grams at 4000 meters. If the original quantity of 

 moisture be 10 grams then at the altitude of 2000 meters condensa- 

 tion occurs and 4 grams of water are condensed. If these grams 

 remain suspended in the air and freeze while the temperature re- 

 mains constant at o° then the hail stage would represent a change 

 of altitude of 108 meters or a jump of this amount at o° C. line; for 

 further expansion one must then follow the new adiabat. 



It is easy to memorize the number 30 meters as the change of 

 altitude for each gram of water; this results approximately from 

 the formula 



424 X 80 



e _ 



A 



1000 



The initial level of the adiabat diagram is taken at 76o mm of 

 pressure. If we have some other initial pressure, as, for instance, 

 73o mm and a temperature -f- 20 then must the altitudes be moved 

 up a corresponding 250 meters, but we easily recognize the fact 

 that in the use of the diagram where we only care for altitudes and 

 temperatures it is indifferent whether at the initial level a pressure 

 of 76o mm prevails or any other pressure. 



According to table 7, page 490, for the condensation adiabat we 

 have the data given in table M. 



Table M 



With these data we compute the corresponding values for the 

 initial pressures 750 and 770 as shown in tables N and O. 



