284 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



At the upper limit of the cloud, especially in stratus clouds, the processes 

 of growth and dissolution of the cloud by direct loss or gain of heat by 

 radiation are carried on like the formation and dissolution of fog in the 

 lowest strata of air. 



The formation of clouds by adiabatic expansion as well as the disso- 

 lution by compression occurs wherever we have to do with ascending 

 or descending currents of air. This process has in recent times been so 

 frequently treated that the subject may here be treated very briefly. 

 The cumulus clouds of summer with horizontal bases, the thunder 

 cloud and the rain cloud, properly so called, owe their origin to this 

 process. To what extent "nocturnal radiation " influences the upper 

 layers of such clouds can only be made clear by further investigation. 



Still more complicated than the two methods hitherto considered 

 in the formation and dissolution of clouds and fog are the processes that 

 accompany mixture. In both the above mentioned pairs of processes a 

 steady increase of cooling or warming is accompanied by a steadily 

 progressive condensation or dissolution. It is quite otherwise in mix- 

 tures. A process of mixture can progress in the same direction and 

 yet cause at first condensation and in its subsequent stages dissolution. 

 The breath which we exhale into the cool air leaves the mouth satu- 

 rated but not yet in the condition of fog ; only after the beginning of 

 the mixing with the colder air does the formation of the cloud of vapor 

 begin, which then through further mixture with colder, drier air, again 

 dissolves. We see this process depicted in a strictly mathematical way 

 in Fig. 38. Jf for instance we assume that a small quantity of air at the 

 temperature t { is mixed with a larger quantity at the higher temperature 



% 



AM 



t 2 , then all possible mixing-ratios will occur trom — =0 up to the final 



result, which we will assume to be greater than that which corresponds 

 to the higher value y 2 *. In this case the quantity of contained water y 

 passes through all values belonging to the ordinates of the line F { F 2 

 until reaching the final value y>y 2 *. In this process condensation must 

 occur as soon as the mixing-ratio exceeds the value which corresponds 

 to the ordinate y x *; if it increases still further then beyond a definite 

 point as it approaches towards the ordinate y 2 * dissolution again begins, 

 which becomes complete for a mixing-ratio corresponding to the ordi- 

 nate y 2 * and thus again results an unsaturated mixture. 



If a smaller quantity of nearly saturated warmer air mixes with a 

 larger quantity of colder air then will the mixture pass through its 

 conditions in an inverse order, and again the initial condensation and 

 the subsequent dissolution will occur under the conditions assumed in 

 Fig. 38. 



Although now in both cases condensation occurs first and then dis- 

 solution, still there is an important difference between them. For if we 

 imagine the mixing-ratio to undergo steady change between the points 



