410 
6 Se 2 He piss Girl Sy mee 
32 75 69 47 34 20 " 6 7 
2 Jt ay Ea 2 of 2 u 8 
NS 36 7 72 38 25 " 12 | 8 
100 Veeco 
— Seeded (320 Clouds) \ 
--- Not S. (297 Clouds) oi 
gt 
Ht 
80 q H 
' 
es 4 
wn 
x) nee | Cnn nn I I 2 crs = 
2 60 
oO 
WwW 
se 
= 
= 
= 40 
= 
WwW 
oO 
a 
WwW 
a , 
20 Note: On seeded 
7 days 19 clouds and 
on not-seeded days 
13 clouds with 
T<-42°C. All had 
echoes. 
+6 0 -6 -I2 -I8 -24 -30 -36 -42 
CLOUD TOP TEMPERATURE °C 
Fic. 1—Fraction of clouds observed to extend 
to the indicated temperature levels which con- 
tained precipitation echoes; data on which the 
curves are based are shown at the top of the 
diagram; ten clouds, five in each sample, had 
temperatures above +6°C and have not been 
plotted 
thunderstorms is to take radar observations 
every 30 min and note whether there is at least 
one cloud extending above any particular alti- 
tude. When this was done, it was found that dur- 
ing the seeded days there were about twice as 
many echoes extending above 30,000, 35,000 ft, 
and 40,000 ft. A sign-rank test in this instance 
showed that the probability that the differences 
in the number of clouds extending above 30,000 
ft occurred by chance was 0.05. 
Lightning—Lightning observations were not 
taken in 1957. However, in 1958, it was found 
that on the seeded days there were about nine 
times more lightning strokes than on the not- 
seeded days. A sign-rank test revealed that the 
probability of chance occurrence of the observed 
ranking of the differences of strokes on pairs of 
days was about 0.015. It was interesting to find, 
that notwithstanding the large difference in 
lightning frequency, there was little or no dif- 
ference in the number of lightning-caused forest 
fires. One might offer the explanation that the 
higher lightning frequency was offset by more 
BATTAN AND KASSANDER 
rain which reduced the likelihood of the forma- 
tion and spread of fires. 
Initiation of precipitation—By means of the 
cloud camera and radar data, it was possible to 
note the vertical extent of clouds (and thus 
cloud-top temperatures) and whether or not 
they contained precipitation. When a sufficient 
number of clouds have been examined it be- 
comes possible to speak of the ‘probability of 
precipitation’ in clouds whose summit tempera- 
tures are between —12 and —18°C, or any other 
temperature interval. Figure 1 shows a summary 
of the observations made during 1957 and 1958. 
The smoothed solid and dashed curves were 
drawn in by eye. It is quite obvious that on the 
seeded days, the likelihood of precipitation was 
greater than on the non-seeded days. The fairly 
uniform shift of the curve towards the left lends 
support to the belief that the effect is real, and 
that in fact, the AgI seeding caused the forma- 
tion of precipitation in clouds which would not 
have precipitated naturally. 
If the nearly straight parts of the curves are 
extended to the abscissa (dotted lines) it is 
found that the ‘not-seeded curve’ intercepts the 
abscissa at about —17°C. It might be argued 
that this result is reasonable because observa- 
tions of ice nuclei in the atmosphere show that 
in general, the concentrations at temperatures 
above —15°C are small. As the temperature is 
reduced the concentration increases. It appears 
reasonable to assume that the dotted-dashed 
curve represents clouds in which the ice-crystal 
mechanism was effective in causing precipitation. 
An extension of the ‘seeded curve’ shows that 
it intercepts the abscissa at about —9°C, a tem- 
perature just below the value at which AglI 
crystals can be expected to become effective as 
ice-crystal nuclei. 
If the interpretations of the significance of the 
dotted curves are correct, then one is led to the 
assertion that those precipitating clouds which 
fall to the left of the projected curves produced 
the precipitation by the condensation-coalescence 
process. 
Summary—In view of the fact that this re- 
search is still in progress, the authors feel that 
they are still not ready to draw final conclu- 
sions. Although the results to date pomt towards 
the conclusion that seeding produced important 
effects, it is vital that more data be compiled in 
order to be sure that the results have not been 
