FREEZING NUCLEI AT GROUND LEVEL 
Washington, D. C., area and some related test 
results using a rapid expansion technique. Be- 
cause of existing uncertainties regarding the 
quantitative interpretation of measurements in 
this field, the data should tentatively be con- 
sidered as relative rather than absolute in na- 
ture. The time factor in the activation of ice 
nuclei is such that the rapid-expansion tech- 
nique may result in the detection of fewer nu- 
cle: than mixing-chamber or constant-tempera- 
ture methods. However, the rapid-expansion 
technique has the extremely desirable feature 
of permitting replicated observations over a 
wide range of cloud-chamber temperatures with- 
out unreasonable time requirements. The data 
presented here were collected primarily to gain 
further insight into the observational problem, 
and were obtained with a prototype model of 
a refrigerated expansion chamber intended for 
a sustained observational program at a number 
of sites (planned by the U.S. Weather Bureau 
as a joint effort with and supported in part by 
the National Science Foundation under Grant 
Gl-29 with the assistance of a number of co- 
operating groups). 
Observational procedure—The observations 
during the period January 11—March 31, 1959, 
were made about eight miles west of Washing- 
ton, D. C. in an area removed from any known 
local sources of industrial pollution. At least 
two sets of data consisting of replicated tem- 
perature-spectrum runs from the threshold 
value (the appearance of one crystal per ten 
liters) to about —30°C were obtained on most 
days. The equipment used is similar in basic de- 
sign to that described by Warner [1957], the 
primary difference being the addition of an 
electrically operated air pump for both purging 
and pressurization. The technique depends on 
the creation of a supercooled fog during an 
adiabatic temperature drop on expansion from 
a normal operating wall temperature of —10° 
to —12°C. Resulting ice crystals settle into a 
removable tray coated with a thin layer of sugar 
solution which is placed at the bottom of the 
ten liter chamber. The crystals subsequently 
grow to visible size in 30 to 60 sec, and the 
total number is then counted visually. Estimat- 
ing procedures were used when the number of 
crystals exceeded 150 to 200 per ten liters by 
limiting exact counts to known fractional areas 
of the trays. All observations were made with 
the equipment located out-of-doors whenever 
practicable to reduce the risk of misleading re- 
241 
sults caused by possible contamination with un- 
representative inside air. Counts at —20°C (the 
primary ‘reference temperature’ used here) were 
augmented to some extent by extra readings, 
particularly during variable conditions. The 
data from each set of observations covering a 
range of expansion temperatures were plotted 
and mean-temperature spectrum curves con- 
structed. At least 10 to 15 separate observations 
were usually considered necessary to define a 
temperature-spectrum curve with reasonable 
confidence. 
Results and discussion—Approximately 2950 
individual measurements comprising 172 sets 
(mean temperature spectrum runs) were ob- 
tained during the 80-day period under con- 
sideration. The variety of weather patterns 
together with the abruptness with which airmass 
changes frequently occur during the winter 
months in the Washington, D. C., area offers 
an opportunity to evaluate the trends in freez- 
ing-nuclei observations in relation to synoptic 
weather features with perhaps less ambiguity 
than at locations farther inland or in more uni- 
form climatic regimes. 
Typically, the number of nuclei activated with 
decreasing temperature shows an exponential 
type of relationship such that a tenfold increase 
occurs with roughly a 5°C drop in temperature. 
However, as shown by several examples in 
Figure 1 there are considerable variations in 
3-4,000 
a 
th 
/ t 
Lae 26MARCH,1959 
Ti 0804-0913 EST iA 
rey 
? 27MARCH,1959 
i 823.1934 EST 
i 24MARCH,1959 
j (1813-1917 N 
28MARCH,1959 
1726-1805 EST 
NUMBER OF CRYSTALS IN 10 LITERS 
= 2 —25 
TEMPERATURE °C 
—35 
Fie. 1—Examples of freezing nuclei data ob- 
tained in the Washington, D. C., area; ordinate 
scale is VV + VN + 1, where N is the number 
of ice erystals 
