ADIABATIC CHANGES OF MOIST AIR NEUHOFF 461 



theadiabats, it is perfectly practicable to introduce into the rain stage 

 the value m u = 3.60 as a constant moisture factor corresponding 

 to an average quantity of moisture of about 8 or 10 grams; in this 

 case the slight departures for values above or below this have but 

 little importance. 



If we desire to compute the final pressure at o° C. of the adiabat 

 of 20 and 76o mm assuming m = 3.60 then we have the expression 



log p' -1^1? = 2.5753 



P' 



whence we find p' = 46o mm , or the same value as that which we 

 found for the pseudo-adiabat. 



If now we seek to find how large the difference will be in the 

 value of mwhen we entirely neglect the influence of the moisture, 

 that is to say, when we assume for the rain stage the value m = 

 3.44, or the same as for dry air, then in this same example we 

 obtain the same equation 



log *' -1^5 = 2.5802 

 g P' 



whence p' = 464 mm and the departure as compared with the previous 

 value is only 4 mm in excess. 



Therefore the extreme result of the entire neglect of a quantity 

 of moisture amounting to 10 grams in comparison with the weight 

 of air, in the factor m, amounts to only a difference of pressure of 

 4 mm in the expansion and cooling of the air from the temperature 

 of 20 C. down to o° C. Therefore for small variations in moisture 

 on either side of 10 grams when we use m n = 3.60 as a constant for 

 the rain stage we introduce departures that scarcely come into 

 consideration. 



In many cases when the quantity of moisture in the air is small 

 it suffices to assume the factor m = 3.44 as a constant even for the 

 rain stage, especially when we do not keep the higher value in 

 memory or wish to avoid using the tables. 



In the snow stage the original quantity of moisture present will 

 scarcely ever need to be used. It is always so slight that here also 

 we may assume an average quantity of moisture of about 2 or 3 

 grams and a corresponding value of m = 3.46, and since it scarcely 

 matters in the computation whether we use 3.46, or 3.44 therefore 

 we adopt for the snow stage m = 3.44 or the value which holds 

 good for dry air. 



