264 THE MECHANICS OF THE EARTH's ATMOSPHERE. 



These considerations show that mixtures of saturated warmer with 

 unsaturated cooler air gives rise to condensations much more easily 

 than do mixtures of saturated cooler with dritr and warmer air. 



The tiow of a jet of saturated warmer air into a cool space must 

 therefore be accompanied by much more powerful condensation than 

 is the inflow of saturated cohler air into a space filled with unsaturated 

 warmer air. 



The fact that clou<ls of vapor so easily arise over every open vessel 

 filled with warm water, while the formation of fog near very cold 

 bodies in warmer regions is much more rarely to be observed, gives an 

 assurance of the correctness of this principle. 



Whenever during moderately cool weather the door of a wash-house 

 is opened great clouds of vapor pour out, but the opening of an ice 

 cellar on a hot day has not a similar result. 



!Now that the limits have been determined within which, in general, 

 mixture can occur, it is proper to give the quantity that can be precip- 

 itated by the condensation. Such precipitation occurs whenever the 

 i:)oint Fr^ lies above the saturation curve. For then the limit of satura- 

 tion is exceeded, and by a quantity that is represented by the length 



This quantity, which will be designated by «3, is that of which, before 

 the writings of Wettstein and Hann, it was assumed that it was i)re- 

 cipitated as water as the result of the mixing. 



To what extent one was led into error by this assumption is most 

 easily seen from the figure by the following considerations: 



Let it be assumed that at first actual saturation occurs in the mix- 

 ture, and let the whole quantity j/^ be actually present in the form of 

 vapor or aqueous gas, then will the gradual precipitation of the vapor 

 be accompanied by a simultaneous warming. 



The increase of temperature herebj* brought about is found from the 

 equation 



1000 aJt = - rdy, 



where c is the capacity for heat of the moist air under constant pressure,, 

 and r is the latent heat of evaporation, and where e is to be multiplied 

 by 1,000, since we have taken a kilogram of the mixture, whereas y is 

 expressed in grams. 



Since now, as will subsequently become evident, the temperature t 

 rises only a few degrees even for a very considerable supersaturation, 



therefore we can consider as constant in each individual case, and 



corresponding to this we obtain 



lO^c 



y2-y = -^{t-h) (3) 



in which y and t represent those values that are obtained after the 

 precipitation of the water that is in excess of the limit of saturation. 



