The Swiss Randomized Hail Suppression Project in the Tessin 
RAYMUND SANGER 
Swiss Federal Institute of Technology, Zurich, Switzerland 
Abstract—The first two years of the Swiss randomized antihail project does not yet 
allow any statement having statistical weight about the effect of the AgI seedings to 
prevent hail, neither in a positive, negative, nor indifferent sense. However, the pre- 
cipitation patterns display indications for a strong increase of the rain as a result of the 
AglI seeding by ground generators. According to the general design the project will be 
carried through for the time of five years in the expectation to arrive until then to 
some definite decisions as to the effects of the silver iodide on hail formation. 
Since 1957 the Swiss Federal Commission for 
the Study of Hail Formation and Prevention 
[Sanger and others, 1957, 1958] has been carry- 
ing out its Third Hail Suppression Project in the 
Tessin. The test area hes in the southernmost 
part of the Alps and covers a region of about 
3000 km* having a pronounced orographic char- 
acter. The project is financed jointly by a Fed- 
eral Grant from the Department of Public 
Economy (Division of Agriculture), by the Can- 
ton Tessin, and by two private firms which have 
cooperated in the project: Essagra 8S. A., Ba- 
lerna, and the Oerlikon Machine-Tool Works, 
Biihrle and Co., Ziirich, the latter maintaining 
a large agricultural estate in the test area. 
The seeding of the atmosphere is done from 
20 silver iodide ground generators which sur- 
round the test area at distances ranging from 
less than one up to 30 km; seven of these are 
on Italian soil. The output of the generators is 
in the order of 10"/see of active ice-forming 
nuclei, measured at a temperature of —10°C, 
which is in keeping with the figures usual in 
such experiments. The generators have been 
made available by the Oerlikon Machine Tool 
Works, Ziirich, until now two different types 
have been used, the first a copy of the model de- 
veloped by North American Consultants, Inc., 
Goleta, Calif., and the second a new type of con- 
struction in which the AgI solution is sucked in 
by the flow of the fuel gas. After some initial 
trouble the second type well proved its worth, 
so that from the beginning of the third annual 
experimental period this year it alone will be 
used. The experimental period runs from the 
middle of May to the beginning of October. 
A strict process of randomization governs the 
occasions when seeding is to be carried out. 
Every day at 16h 30m a weather forecast is re- 
ceived from the meteorologists at the Osserva- 
torio Ticinese in Locarno-Monti; if they con- 
sider that there is a danger of thunderstorms on 
the following day, then this is considered to be 
a test day. A randomization experiment then 
determines whether seeding is to be done or not. 
In this way two groups of test days are ob- 
tained, for which the observational results can 
be compared with one another on a statistically 
satisfactory basis. 
Seeding always takes place between the hours 
of O7h 30m and 21h 30m, with the generators 
working to the rhythm of a ten-minute interval 
between five-minute operational periods. In ac- 
cordance with the seeding time the observational 
or test period runs from OSh 00m to 22h 30m; 
only hailfalls occurring within this observational 
period are included in the later statistical evalu- 
ation. A special network of 29 observation posts, 
distributed over the entire area of southern 
Switzerland, has been set up in order to collect 
the necessary data in sufficiently local detail. 
In the first two years of the project there 
were S85 test days, 42 seeded and 48 not. Hail 
was observed on 15 of the seeded days and on 13 
of the unseeded days, but this statement takes 
no account either of the recorded intensity or 
of the geographical extent of the hailfalls. It 
only means that, during the test period, hail was 
reported at some place in the test area. It must 
further be noted that thunderstorms were ac- 
tually observed only on 33 of the 42 seeded days 
and on 26 of the unseeded days. This allows us 
to see that the group of seeded days was more 
heavily burdened with storms than the un- 
seeded group, so that the latter benefited from a 
higher rate of false forecasts. Generally speak- 
ing the amount of hail which has fallen during 
the first two years has been within modest limits, 
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