FREEZING NUCLEI AT GROUND LEVEL 243 
100 crystals per ten liters at —20°C, all but 
one occurred during such airflow regimes while 
67 out of 84 observations in the range of 0 to 
10 crystals per ten liters were in continental 
air masses. Figure 5 shows a more detailed 
presentation of this trend. The incidence of the 
unusually high counts in excess of 500 per ten 
liters on February 9, March 14, and March 26 
was in air with an overwater fetch from the 
northeast. 
The sparsity of meteorological observations 
a 
i=] 
Ss 
a 
So 
i=] 
T 
@ CONTINENTAL TRAJECTORY 
@ RECENT MARINE TRAJECTORY 
@ MARITIME TROPICAL AIR MASS 
NUMBER OF CRYSTALS IN 10 LITERS AT -20°C 
(afi Keren ea ee 
S 30 
20 
JANUARY ,1959 
600}- 
400- 
iL, e 
jc are VS (hr ees sl Bs 
Cota 15 10 
n Aidt 
15: 20 25 30 
FEBRUARY 1959 
1000; 
600; 
NUMBER OF ICE CRYSTALS IN 10 LITERS AT -20 °c 
400;- 
200}- 
ee 
A Fac 2 St a eae ar eee ree es Za 
y 10 15 
MARCH 1959 
Fic. 4—Mean daily values and variations in 
freezing nuclei counts at —20°C with respect to air 
trajectory 
BO}- 
60}-- 
oO! 
0-10 11-100 
NUMBER OF OBSERVATIONS. 
| 
Fic. 5—Number of cases in 
which specified values of freez- 
ing nuclei active at —20°C oc- 
curred with respect to airmass 
categories 
101-1000 
NUMBER OF ICE CRYSTALS/10 LITERS AT -20 °C 
off-shore prevents a detailed examination of 
possible upwind factors. There was no obyious 
correlation with wind speed or probable extent 
of over-water fetch. However, there were some 
indications that low-level instability and wide- 
spread precipitation were associated with the 
freezing nuclei anomalies. If so, this may provide 
a clue regarding the conditions most favorable 
for the generation and transport of the respon- 
sible aerosols. The existence of a recent marine 
history alone does not appear to represent both 
a necessary and sufficient condition, since, as 
shown in Figure 5, there were several instances 
of low concentrations in such flow regimes. 
In view of the highly suggestive nature of 
these empirical results, samples of ocean water 
were obtained from the Rehoboth, Del., coastal 
area. On the assumption that the bursting of 
bubbles at the sea surface would be the most 
likely mechanism for the natural production of 
aerosols from oceanic sources, the expansion 
chamber was purged and pressurized with air 
ingested from a few inches above the surface of 
agitated ocean-water samples. These tests have 
been conducted a number of times with similar 
results. Two examples are shown in Figure 6 
in which the background counts were compara- 
tively low. While there is considerable scatter 
in the results, there appears to be little doubt 
of a positive response with the expansion cham- 
ber technique. Whether the responsible particles 
are solely the soluble components of ocean 
water or substances in colloidal suspension can- 
