INFLUENCE OF WEATHER ELEMENTS ON FREEZING NUCLEI 
concentration. In most cases the fluctuations of 
the three gases listed in Table 1 run parallel and 
are regulated by the actual weather situation. 
In an attempt to study the poisoning effect 
more in detail we have started a series of tests 
of the freezing nucleus concentration in an air 
sample to which a known volume of the trace 
gas is being added. In addition to each ice-nuclei 
count a second run in a sample of untreated out- 
side air is made to give information of the nor- 
mal conditions in the atmosphere at that time. 
It may be mentioned that in the course of these 
experiments the method of counting the ice 
erystals in a dish containing concentrated sugar 
solution was applied which was first invented by 
Bigg [1957]. The results with this counting 
method agree quite well with our usual pro- 
cedure of counting the ice crystals in a parallel 
light beam, the diameter of which can be nar- 
rowed by shutters when increasing ice-crystal 
numbers afford it. 
Figure 4 shows the results of a first series of 
20 inactivation tests using NH, as poisoning 
agent. In order to exclude the natural fluctua- 
tion of the absolute number of freezing nuclei 
on the different days when the samples were 
taken, the numbers found in normal outside air 
above —18°C and —21°C respectively were set 
equal 100. The numbers of freezing nuclei ac- 
tivated in an air sample containing 1500 ppm 
TaBLE 1—Relation of various gases to the 
activity of freezing nuclei 
onan te toe Activity quotient | Number of cases 
Ammonia 
0-8 93 6 
8-16 133 58 
16-32 143 8 
32448 210 3 
72-88 235 3 
Nitrogen dioxyd 
0-5 | 92 | 15 
5-10 135 60 
10-15 158 | 13 
15-20 175 | 4 
Sulfur dioxyd 
0-160 tosh |) a0 
160-320 134 34 
320-480 174 5 
237 
~ 
\ 
a b a 5b 
Fie. 4— Inactivation of 
freezing nuclei by adsorption 
of NH; within the tempera- 
ture ranges 0 to —18°C and 0 
to —21°C, respectively; (a) 
freezing nucleus concentration 
in normal outside air equal 100; 
(b) freezing nucleus concen- 
tration after addition of 1500 
ppm NH, to the air sample 
NH,, the ammonia concentration during these 
experiments, is a proportion of 100. It can 
clearly be recognized that the presence of this 
amount of ammonia reduces the number of ac- 
tive freezing nuclei remarkably. This first series 
of tests applying a rather high concentration of 
NH;, much higher than can be expected nor- 
mally in the atmosphere, was intended to present 
a rough estimate on the effect of NH, as surface 
poison. An extension of these measurements with 
lower concentration of NH, and with the addi- 
tion of other trace gases 1s under way. 
The results of these experiments can be con- 
sidered to be in this first stage as a confirmation 
of the investigations by Birstein [1954] on the 
inhibition of ice nucleation of silver iodide par- 
ticles by certain chemicals. Testing the effect of 
NH,, Birstein started with a partial pressure of 
0.1 mm increasing it in stages up to 7 mm. The 
ice-nucleation ability decreased consequently. At 
a partial pressure of 7 mm no ice crystals were 
observed above —20°C. The most effective chem- 
icals to inhibit ice nucleation were amines, 
methylated and ethylated amine. From Bir- 
stein’s results the conclusion must be drawn that 
