212 



A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 



the clear regions usually but by no means always accompanying rain or lying 

 under clouds. 



28. Wind effect. — Changes of wind velocity were not observed. The 

 nucleation consistently follows certain changes in the directions of the winds, 

 as will appear from a detailed examination of the charts as a whole. The 

 following table, which is a summary of this kind, shows the distribution of 

 maxima above n = 60000 with the wind direction. 



TABLE II. 

 Wind Directio.xs for M.\xim.\ Exceeding « = 60000. 



* 8 after March, 2 after May. 



By far the greater number of these maxima occur for nearls' westerly 

 winds and but few for easterly winds, if stress is laid on the winter nucleations. 

 In 1904, where high maxima occurred in the spring and even in the summer, 

 the distribution is more southerly, but in the main like the preceding. It by 

 no means follows, however, that nucleation is inherently associated with these 

 ]mrticular winds, inasmuch as the winds observed are merely those prevailing 

 in Providence. Moreover, the density of population, etc., is far greater towards 

 the southwest than towards the northwest of the University, whereas the 

 northwest winds prevail during the period of maxima. 



Considered as a whole, therefore, it is improbable that an\- real variation of 

 the nucleation with the direction of the winds is in question. What has been 

 observed is the c)l)\-iousl>^ more frequent occurrence of maxima during the 

 prevailing winds. 



29. Rain effect. — The observation next in importance is the occurrence of 

 pronounced minima during rain, of which the charts contain examples in great 

 abundance. There is rarely an exception to this rule. It implies a faster 

 removal of nuclei by precipitation in a saturated atmosphere (the result of fall 

 of temperature probably) than the supply of nuclei to the same region by 

 either diffusion, or subsidence, or convection, or other more occult causes. 

 Whether this deficiency is eventually made up from the lower air strata in 

 contact with the surface of the earth or from the higher air strata is at the 

 outset left open. 



Rain minima never fall quite down to the zero of micleation, in cities 

 rarely below « = iooo, and they are themselves quite variable. Thus the 

 summer minimum is as a mle much lower than the winter minimtmii. 



Minima quite as low as the rain minima are sometimes observed in clear 

 weather, but they are very rare. 



