A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 213 



One may note that the tendency of rain to change the normal air potential 

 from positive to negative values would thus be accompanied by relative ab- 

 sence of nuclei. In other words minimum nucleation exists here cotempo- 

 raneously with maximum negative ionization. 



30. Snow effect. — Wet snow usually acts similarly to rain, but less power- 

 fully. The effect is particularl}' noticeable during a thaw. There are cases, 

 however, in which the nucleation is high with a fresh fall of wet snow. Dry 

 snow may even increase the nucleation. 



The present experiments, moreover, sljow that ordinary terrestrial dust, 

 properly so-called, has no bearing on the nucleation; for this is frequently a 

 maximum immediately after a rain, when the earth is blanketed with water, 

 or after the earth has been covered with snow or with sleet to the exclusion of 

 all dissemination of dust as such. 



31. Cloud effect. — Another interesting feature are the cloud minima as 

 seen on October 15, 17, 21, 23, 1902; March 19, 25, 27, April i. May 20, August 

 27, September 27, October 18, 19, 26, December 15, 1903, etc. These observa- 

 tions are incidental, and a larger number would have been fotmd if systemati- 

 cally sought for. Usually a higher nucleation is again established after the 

 cloud train has passed the sky, the phenomenon beginning and ending in periods 

 of clear weather. At other times, however, the minimum remains, as on 

 October 17, when manv observations were made with this end in view. The 

 explanation of this residt is at hand: the air has mov^ed Ixidily with the cloud, 

 the whole constituting a region of deficient niicleation. The nuclei may have 

 been ]:)recipitated by rain elsewhere and the cloud may even have vanished 

 from the region. 



32. Solar nucleating effect absent. — Since the nuclei cannot re-enter a re- 

 gion by diifusion as quickly as is tisualh' observed, one may be tempted to 

 infer that solar radiation is the cause by which the nucleation of a deficient 

 region is re-established. There is no evidence of this in the chart and much 

 against it. Thus remarkably low minima are frequently maintained through- 

 out the day in full sunlight. The minimum may be part of a cloud region with 

 which the day closed, but sunlight is powerless to replenish it. Similar refer- 

 ences mav be made to the notched midday minima which so often occur. By 

 contrast high nucleation develops in s])ite of an overcast skv'. Finally night 

 observations show neither increase nor decrease of nucleation, but are usually 

 normal in character. Hence there is no evidence, so far as these observations 

 go, that ultra-violet light or other solar radiation has any potency in producing 

 the nucleations here immediately in question, and cloud effects have therefore 

 been explained as ptu-eh' convecti\'e. Indeed, compatibly with the final sum- 

 mary (JJ37) of mean monthly nucleations, the sun must be regarded rather 

 in the light of a denucleating agency. 



33. Temperature effect. — An important general result is the frequent oc- 

 currence of maxima of nucleation, cotemporaneously with the sudden fall of 

 atmospheric temperattu-e in cold weather. So far as the drop of temperature 



