274 THE MECHANICS OF THE EARTh'S ATMOSPHERE. 



view the proof must first be given that the super-saturation, which we 

 have hitherto only known in laboratory experiments, also occurs in the 

 free atmosphere. 



The mixture of super saturated air with other quantities of air scarcely 

 needs a special consideration, since we at once see the result of such 

 mixture when we imagine, in Fig. 41, one of the points, Fi or Fo, trans- 

 posed to the upper side of the curve F' F\ and then execute the further 

 constructions according to the rules previously given. 



(C.) MOIST AIR WITH INTERMIXED WATER OR ICE. 



Water occurs in the atmosphere not only as vapor, but also in the 

 form of drops of rain, crystals of ice, and particles of fog. The psy- 

 chrometer and hygrometer teach us that the air is not necessarily sat- 

 urated with vapor when water is mixed with it in this manner. Unfor- 

 tunately we possess only very imi)erfect data as to how great a quan- 

 tity of water can in this way be mechanically mixed with the atmos- 

 phere.* But there can l)e scarcely any doubt that the sum of the water 

 mechanically mixed and that which is present in the form of v;ipor 

 may together be smaller, or equal to, or even greater than the quan- 

 tity corresponding to saturation for the given temperature. Corre- 

 sponding to this statement, I will designate such mixtures as air which 

 is " partly saturated mechanically," '' wholly saturated mechanically," 

 or " super-saturated mechanically." And now, first of all, we will inves- 

 tigate how such masses of air behave when mixed with ordinary air more 

 or less moist. 



By this investigation we shall come to learn the conditions under 

 which the dissolution of fog or clouds or the evaporation of falling rain- 

 drops may occur. Such dissolution is, as we at once see, to be attained 

 by mixture only when the intermixed air, which at first may be as- 

 sumed to be the warmer component, is relatively dry. 



Therefore, we will at first investigate the phenomena of mixture 

 under the following conditions: 



Let Bi > 100 and composed of two parts, of which the one Ri is in the 

 form of vapor and the other Ri is liquid, and moreover let Ri < 100 while 



Ri-i-Rj =Bi. 



Furthermore let ^2 < 100 and <2 > <i. This being assumed, the follow- 

 ing formuliTB hold good, using a notation which by analogy is intelligi- 

 ble of itself: 



y\>yi 

 yi<yi'- 



*Hann, Met. Zeit. lf^89, vol. vi, pp. :303-306. 



