Design and Operation of the Swiss Hail Tunnel 
Rouanp List 
Swiss Federal Snow and Avalanche Research Institute, 
Weissfluhjoch, Davos, Switzerland 
Abstract—An acceptable theory on hail formation should be based on the general 
physical conditions prevailing within thunderstorm clouds and in particular on the proc- 
esses of nucleation and growth of ice particles. An attempt has been made to approach 
this problem from the experimental side by means of a vertical wind tunnel offering a 
wide range of airconditions. In this paper the Swiss Hail Tunnel as it has been built in 
the Laboratories of the Federal Snow and Avalanche Research Institute, Weissfluhjoch, 
Davos is described and discussed. It is shown that in the tunnel stationary and variable 
conditions can be reproduced similar to those to be expected in a natural hail-pro- 
ducing atmosphere. 
Introduction—The hail tunnel was designed and 
built in order to grow hailstones experimentally 
in the laboratory. For it was felt that compara- 
tive observations of these with natural hailstones 
would enable conclusions to be drawn as to the 
conditions in which hail forms in nature. Experi- 
ence gained over a number of years at the Swiss 
Federal Snow and Avalanche Research Institute 
in investigating thin sections, when applied to 
soft hail (graupel) and hailstones [List and de 
Quervain 1953, List 1958ab], showed that here 
too the key to any explanation of how these nat- 
ural ice particles arise must lie in an interpreta- 
tion of their physical structure. It may be ex- 
pected that specific structural zones in iced-up 
particles of precipitation are a direct consequence 
of specific growth conditions in a cloud: that the 
structure of a particle is essentially the product 
of the conditions in which it originates. 
The experiment has thus to clarify the connec- 
tion between conditions of growth and the re- 
sultant ice build-up on a nucleus particle. The 
hail tunnel is the plant which permits this experi- 
mental aim to be carried out. It consists basically 
of a wind tunnel which is vertical in the section 
where measurements are made, has a closed cir- 
cuit and an adjustable climate. 
When formulating the problem it must be 
clear that, as a result of the many parameters of 
the experiment, it is not possible in every case to 
classify a certain ice formation under a particular 
set of growth conditions. This is especially true 
of different growth phases built up in time at the 
same place. The final conclusions are also com- 
plicated by the multiplicity of forms which occur 
in natural hailstones [List, 1959a]. 
Before designing a hail tunnel, it is important 
to elucidate the extent to which the conditions 
concerning temperature, humidity, air speed, air 
pressure, impurities in the air, electrical effects, 
etc., such as may be found in a hailcloud, can in 
fact be imitated in the experiment. Enquiry into 
these factors gives a positive answer on all counts. 
It would in itself be desirable to reproduce vari- 
ations in pressure, but here the financial expense 
is disproportionately high, and so experiments 
have to be designed at constant pressure and the 
results transformed by suitable rules of similarity 
[List, 1959b]. In addition it was decided when 
planning our wind tunnel not to try to check and 
influence the electrical conditions, on the prelim- 
inary assumption that they are only of secondary 
importance. The possibility still remains, how- 
ever, of elaborating the plant in this respect or 
of otherwise improving its capacity. 
The construction of the hail tunnel—The basic 
construction of the hail tunnel can best be seen 
in Figure 1. The blower a generates the necessary 
air-speed relative to the object b suspended or 
floating in the measuring section. (The direction 
of the air stream is counterclockwise). In the 
position ¢ next to the blower is the air cooler or 
vaporizer. Ammonia vaporizes in its ribbed tube 
system, is compressed in the refrigerating com- 
pressor d (or Fig. 2) and is then cooled and con- 
densed in the condenser e by means of air from 
the atmosphere (since the plant stands at 2665 m 
above sea level there is no supply of cooling wa- 
ter). A heater f with a capacity of 0 to 19 kw, 
which can be engaged immediately at any level, 
makes possible rapid periodic changes of tempera- 
ture as well as the eventual warming of the ex- 
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