Recent Observations of Freezing Nuclei Variations at Ground Level 
Dwicut B. Kune 
U.S. Weather Bureau, Washington 25, D. C. 
Abstract—Daily observations of freezing nuclei in the vicinity of Washington, D. C., 
during the period January 11—-March 31, 1959, using a rapid-expansion technique, show 
indications of appreciable differences with respect to air-mass history. There was evi- 
dence of a pronounced tendency for the unusually high counts to occur in air with a 
recent marine fetch, while air with an extended continental trajectory characteristically 
showed lower concentrations of ice-forming nuclei in the subfreezing temperature ranges 
warmer than about —25°C. Some exploratory tests indicate that ocean water could be a 
source of freezing nuclei, as measured by the expansion technique. 
Introduction—Only a few sustained series of 
observations of the natural variations in freez- 
ing-nuclei concentrations in the atmosphere ex- 
ist. The tedious nature and other practical 
difficulties associated with obtaining reliable 
freezing nuclei observations together with un- 
certainties regarding current measuring tech- 
niques pose serious difficulties in this field. Pres- 
ent indications are that, although discrepancies 
of at least factors 2 to 4 in measured values 
may exist between different methods [Fenn and 
Weickmann, 1959], relative fluctuations are 
probably delineated by careful use of most 
techniques. There seems to be general agree- 
ment that variations in the number of natural 
freezing nuclei per unit volume span several 
orders of magnitude, that increases and de- 
creases seem fairly abrupt, and that the major 
anomalies may last from only a few hours to a 
day or so, seldom longer. 
The sources and nature of the responsible 
natural aerosols remain in doubt, as do their 
connection with meteorological parameters. Rau 
[1954] concluded that his measurements in Ba- 
varia showed appreciable differences with re- 
spect to airmass types, and that the more active 
nuclei were of polar-marine rather than recent 
continental origin. An analysis of an extended 
series of observations at Mt. Washington, N. H. 
[Schaefer, 1954], suggested that the higher 
counts were associated with air trajectories from 
the western semi-arid regions of the U. S., and 
Isono and others [1959] reported an association 
between abnormal counts and the arrival of 
loess dust particles from the Asiatic mainland 
and also voleanic eruptions in Japan. Mason 
and Maybank [1958] found a substantial amount 
of laboratory evidence that various forms of 
240 
siliceous materials were active nuclei. Some di- 
rect, but fragmentary, evidence of such particles 
at the center of snowflakes exists from electron 
microscopy techniques [Jsono, 1955]. The ef- 
fluents of certain industrial processes also ap- 
pear to contain active components [Soulage, 
1958]. The meteoritic-dust hypothesis advanced 
by Bowen [1953], although presenting seemingly 
insurmountable physical difficulties according to 
most existing astronomical and meteorological 
concepts, nevertheless continues to pose some 
interesting questions. A summary of January 
data [Kline and Brier, 1958] indicated some sta- 
tistical support for the hypothesized singulari- 
ties in freezing nuclei during this month, but 
anomalies in the succeeding two months at 
Washington, D. C. in a 1958 series of data 
could not be associated with any known meteor 
showers. There was a tendency for the abnormal 
counts with the mixing-chamber technique used 
to coincide with the intrusion of air with at 
least a limited marine fetch. Georgu and Met- 
nieks [1958] reported evidence of parallel trends 
in ice-nucleation activity and the concentrations 
of large and giant condensation nuclei, but there 
was an absence of direct evidence of a maritime 
origin in a limited summer series of data on the 
coast of Ireland. On the other hand, laboratory 
experiments by Birstein and Anderson [1953] 
yielded positive indications that sea salt in- 
duced nucleation to the ice phase beginning at 
—15°C, but the role of water-soluble substances 
in the range of typical natural cloud nucleation 
temperatures remains in a somewhat contro- 
versial status. 
The purpose of the following discussion is to 
outline the trends that appear to be emerging 
from a recent series of daily observations in the 
