THERMODYNAMICS OF ATMOSPHERE VON BEZOLD 273 



ing to the observations of Reichmann. 28 It is therefore desirable 

 that special attention be given to the occurrence of low-lying thunder- 

 storm clouds of slight power. 



In reference to subcooling we are in a more favorable position than 

 in reference torsupersaturation. Here we have to do with demon- 

 strated facts and it is only necessary to give precision to our ideas 

 as to the formation of clouds by adiabatic expansion and especially 

 the formation of thunderstorm clouds. 



When an active ascending current exists condensation will occur 

 when the dew-point is attained, insofar as the necessary nuclei 

 are present. If now the expansion continues, more and more water 

 will collect on these nuclei while presumably the number of fog 

 particles is not increased. 



Consequently the individual fog particles become larger and 

 larger, and by this means as well as by the union of many into one, 

 they develop into small drops that may be visible to the unaided eye. 



But in a sufficiently active ascending current these droplets will 

 by no means sink but be carried up to great altitudes, so long as 

 their magnitudes do not exceed a certain limit, which of course 

 depends on the velocity of the ascending current and on the density 

 of the air. 



If this process did not proceed in this manner and if the water par- 

 ticles at once fell down as rain, then a progressive increase in the 

 size of rainless clouds would be impossible. 



Moreover, the clouds could never attain that appearance that we 

 are accustomed to see in the cumulus clouds which reminds us of 

 compact masses, but they could only produce the impression of 

 streaks of fog or mist, which would be thinner, and more transparent 

 as the altitude increased without having any sharp boundary on the 

 upper side. 



Since all this is not the case in nature we must assume that the 

 fog particles formed in the lower part of the cloud are at least to 

 some extent carried up to the upper boundary, after which, by falling 

 through the lower layers of the cloud, they grow larger. 



If in this ascension the particles pass through the isotherm of 

 o°C. or freezing, still it does not follow that they will freeze into ice, 

 but it is quite possible that they may retain their fluid condition 

 while being carried up into regions where the temperature is far 

 below freezing. 



Now the ascending current of air has by no means ceased at that 



28 Met. Zeit., 1886, III, pp. 249 et seq. and 1887, IV, pp. 164 et seq. 



