THERMODYNAMICS OF ATMOSPHERE VON BEZOLD 257 



puffing up and pushing forth of new heads from the cumulus clouds 

 I perceive evidence that within the cloud itself there must be present 

 a source of power and that we have hot to do with the simple results 

 of a quickly ascending current of air. 



The observations made by Moedebeck and Gross in the interior 

 of a cumulus cloud during the above-mentioned balloon voyage, 

 according to which, the balloon was set into great oscillations and 

 the drops of water whirled in confusion past each other, show that 

 within the clouds powerful movements take place independent 

 of the general movement of the air. 



Precisely such sources of power must exist within the cloud in 

 connection with the dissipation of supersaturation. 



On the other hand, I cannot conceal the fact that the very low 

 temperatures observed within the clouds are not easily reconciled 

 with any such assumption, although the deprivation of the sun- 

 shine from the interior of the cloud must bring it about that parti- 

 cles of water which have condensed in the upper portion of the cloud 

 must fall with lower temperatures into the lower layer of the cloud, 

 and we thus perceive that we have to do with very complicated 

 processes that must produce remarkably different temperatures in 

 different portions of the same cloud. 



Moreover, the measurements of temperature made under such 

 difficult circumstances can only be completely conclusive when they 

 are made with the perfected apparatus that has lately been described 

 by Assmann. 9 



SUBCOOLING 



If the above given considerations have to a certain extent the 

 character of theoretical speculations, since we have not yet experi- 

 mentally demonstrated the existence in the free atmosphere of true 

 supersaturation, this is not true of the investigation to be presented 

 in the present chapter, on clouds containing subcooled particles of 

 water. Such clouds frequently occur and can exist for a long time 

 if the subcooling does not exceed certain limits. If, however, we 

 consider clouds of very great size, in whose highest parts the tem- 

 perature must be remarkably low, then a small external stimulus 

 will suffice to dissipate the subcooling and cause a sudden rise of 

 temperature and pressure. 



Let /, < o be the temperature of the mixture of dry air, vapor, 

 and subcooled water; y x grams the total quantity of water, partly 



9 Das aspirations-psychrometer. Abhandlungen d. Preussischen Met. 

 Institute, Vol. I, No. 5, Berlin, 1892. 



