NUCLEATION AT BLOCK ISLAND. 1 07 



The dry, cold weather of February gives a striking series of maxima 

 and minima, with the highest readings of the winter. It is interesting 

 to note the fall in nucleation on the 5th, accompanying rise in tem- 

 perature and vapor pressure, and east wind, although the sunshine 

 remains the same. Again, on the i3th, following rain in the night, 

 the vapor pressure and nucleation are at their previous value, although 

 there is no sun during that day. On the 2oth the nuclei persist 

 through cloud and snow, while on the 2 ad they disappear entirely with 

 cloudy weather. 



The ist, ad, and 3d of March, with steady sunshine and wind con- 

 stantly from the west, show well the agreement between the nuclea- 

 tion and vapor pressure. On the 6th, one notes again the lag of the 

 nucleation in building up, and that the light rain and fog of the suc- 

 ceeding days does not cut down the nucleatiou entirely. During the 

 rest of March and April there is a considerable amount of water vapor 

 and the temperature runs higher ; the nucleation, while it follows the 

 changes in the other curves, remains low, although there is at this 

 period plenty of sunshine. The rains of this time never wash out the 

 nuclei entirely, and readings of several thousand are often obtained in 

 thick fog. 



80. Tentative inferences. Perhaps the most striking result shown 

 by these observations is the variation of the nucleation with change 

 in wind direction. It is very probable that nuclei are brought by the 

 land breeze from towns over which it passes. Our winds from the 

 northeast to southwest, through the east, are pure sea breezes ; there 

 is no land in those directions for a great distance, and the readings 

 when such winds occur are usually low. High readings, on the other 

 hand, commonly accompany wind from the other quadrants. Such 

 winds (as seen in fig. 94) pass over towns and cities comparatively 

 near; Newport, R. I., is a little east of north 30 miles distant ; the 

 towns on the Connecticut shore lie between northwest and west at 

 distances from 20 miles up ; New York, from which one would expect 

 a vast number of nuclei, is a little south of west, distant about 100 

 miles. When the wind is from this quarter, however, the nucleation 

 is as a rule considerably less than for northwest winds, possibly because 

 the nuclei may not survive so long a journey. 



This wind effect, however, must not be overestimated. During the 

 winter months here, clear, cold weather, with small amountlof water 

 vapor, occurs quite regularly with northwest winds, and all these con- 

 ditions usually accompany high nucleation. It is difficult to deter- 

 mine the active factors or their relative influence from a limited series 



