214 A CONTINUOUS RECORD OP ATMOSPHERIC NUCLEATION. 



is concerned, one would have the conditions for the formation and growth of 

 water nuclei, remembering that persistent nuclei may be made in an atmos- 

 phere not too far from saturation, from solutions, or in the presence of a solute. 

 Without actual accession to their numbers, however, there could be no in- 

 crement of nuclei in the condensation chamber at about 20 centigrade. In 

 other words, an actual rise and fall of the number of nuclei per cubic centimeter 

 must occur in the atmosphere at the place of observation. 



This temperature or cold-weather effect is very striking in winter where 

 both the daily and the periodic effects frequently coincide. In warm weather 

 the temperature effect is vague or may be even quite reversed. In relation to 

 this one may note that rains in winter usually correspond to rising temperatures, 

 but in summer to falling temperatures. This would accentuate the winter 

 effect, which is otherwise quite independent of rain, but would obscure or even 

 reverse the summer effect, if it exists. 



If a polar air current or a current from the upper atmosphere is associated 

 with the cold waves in question, a part of the region where there is continual 

 production of nuclei may be bodily transported to the place of observation. 



34. Local effect. The prevailing westerly winds sweeping over a part of 

 the city of Providence carry the products of combustion and other local im- 

 purities along with them. A large part of the nucleation at the place of ob- 

 servation must therefore be of artificial origin. The local effect should be 

 greater as the wind velocity is smaller and as the temperature is lower. In this 

 way the winter temperature effect is in a manner explained, seeing that fuel 

 will be consumed more rapidly during the cold waves. Observations of wind 

 velocity are inadequate. 



The difficulty with this view is that it does not consistently explain all of 

 the observed facts. Thus cold waves occur in winter without maximum nuclea- 

 tion, as, for instance, on November n, December 8, 1903. The Sunday nuclea- 

 tions during clear weather in summer are not uniformly low. Maxima during 

 relatively warm weather are often quite as high as during very cold weather, 

 the temperature effect is often reversed even in winter, etc. But the most 

 direct reason for caution will be given in the summary of monthly nucleations 

 where it appears that the highest nucleations do not occur in midwinter so far 

 as cold weather is concerned. Speculation as to the origin of the nuclei is thus 

 premature. Whether it is the residue of the ionized products of combustion, 

 or whether ultra-violet light or other radiation at the boundary of the atmos- 

 phere is the efficient and preponderating source will only be made clear in a 

 series of correlative observations obtained in a wilderness remote from cities or 

 on isolated mountains. Work of this character is now actively under way. 



SUMMARY AND CONCLUSIONS. 



35. Mean daily nucleations. Meanwhile it is expedient to deduce from 

 the above data, both the daily and the monthly average nucleations or number 



