PRODUCING NATURE LIKE SNOW IN A SUPERCOOLED CLOUD FOR LABORATORY EXPERIMENTS

Abstract

We here introduce our cloud chamber which enabled us to grow snow crystals from water vapour under conditions similar to those in natural clouds. The snow crystals grew as a result of water vapour supersaturation as well as due to the interaction of ice crystals and water droplets within the cloud chamber. The cloud chamber, of cylindrical shape with a volume of about 2.7 m 3 , was positioned in a cold room with regulated temperature ranging from −1 to −20°C. A fine mist of water droplets was fed into the cloud chamber and a short pulse of pressurized air triggered the nucleation. Observations of the size and shape distribution of the ice crystals as a function of temperature and supersaturation were consistent with literature values. Different crystal shapes – e.g. plates, columns, hollow columns, dendrites – were successfully formed at different conditions within the chamber. The produced snow was collected at the bottom of the cloud chamber and was used for further experiments and measurements.

Full text

PRODUCING NATURE LIKE SNOW IN A SUPERCOOLED CLOUD FOR LABORATORY
EXPERIMENTS
Ursula Enzenhofer1, 2, Michael Bacher2,*, Sergey Sokratov3, Christian Mahr2, Johannes Müller2, Ulrich
Worthmann1, Ingrid Reiweger1
1Institute of Mountain Risk Engineering, BOKU University of Natural Resources and Life Science,
Vienna, Austria
2NEUSCHNEE GmbH, Austria
3Faculty of Geography, Lomonosov Moscow State University, Russia
ABSTRACT: We here introduce our cloud chamber which enabled us to grow snow crystals from
water vapour under conditions similar to those in natural clouds. The snow crystals grew as a result of
water vapour supersaturation as well as due to the interaction of ice crystals and water droplets within
the cloud chamber. The cloud chamber, of cylindrical shape with a volume of about 2.7 m3, was
positioned in a cold room with regulated temperature ranging from −1 to −20°C. A fine mist of water
droplets was fed into the cloud chamber and a short pulse of pressurized air triggered the nucleation.
Observations of the size and shape distribution of the ice crystals as a function of temperature and
supersaturation were consistent with literature values. Different crystal shapes – e.g. plates, columns,
hollow columns, dendrites – were successfully formed at different conditions within the chamber. The
produced snow was collected at the bottom of the cloud chamber and was used for further
experiments and measurements.
KEYWORDS: ice crystals, snow formation, technical snow, supercooled cloud
1. INTRODUCTION
A number of techniques can be used to produce
technical or artificial snow. The most common
way to manufacture man-made snow e.g. for ski
runs, is to create a spray of fine water droplets in
sub-zero environment, which results in freezing
droplets (e.g. Fauve et al., 2002). This technique
produces ice droplets rather than snow crystals,
which are of limited use for certain scientific
purposes. For systematic studies of physical
processes in natural or nature-like snow cover,
artificial snow samples formed under regulated
and reproducible laboratory conditions as well as
independently of the season are very useful for
snow related scientific experiments in the cold
laboratory.
The most recent nature-like snow making devices
introduced by Schleef et al. (2014) and Bones
and Adams (2009) grow crystals by condensation
of water vapour based on the idea of Nakamura
(1978). Nakamura (1978) invented a machine
which was able to produce nature-like snow
grown on fixed nucleation points surrounded by
supersaturated air.
We used a slightly different approach which aims
at growing snow crystals in the air at conditions
mimicking those in a natural cloud. We therefore
built a cloud chamber (Schaefer, 1952 and Fig.1)
which was able to produce nature-like snow
crystals grown from the vapour phase under
controlled laboratory conditions. The crystal
growth process was similar to the process within
a natural cloud and was based on water vapour
supersaturation and interaction of neighbouring
crystals and water droplets at a regulated
temperature (e.g. Rauber and Tokay 1991).
Fig. 1: Cloud chamber inside cold room.
Our cloud chamber was capable of producing
crystals of different and predictable shapes – e.g.
plates, columns, hollow columns, dendrites – as
first depicted by Nakaya (1954).
* Corresponding author address:
Michael Bacher, NEUSCHNEE GmbH,
Hochstrasse 47, A-2380 Perchtoldsdorf, Austria
tel: +43 (0) 650 8090333;
email: michael.bacher@neuschnee.co.at

2. METHODS
The main component of our snow-making device
was a chamber which was built from an
aluminium supporting structure which was
covered by a stretched polyethylene canvas, with
a volume of 2.7m3 (Fig. 1). This setup allowed
free air circulation at atmospheric pressure. The
temperature was regulated via the temperature
regulation system of the old room.
A fog consisting of a fine spray of water droplets,
created inside an atomizer box, was blown into
the cloud chamber via bended tubes, using a
small fan. The fan controlled fog density and wind
speed and provided cold air from the
environment. During operation the fog was
continuously introduced into the cloud chamber
forming a supercooled cloud which followed a
spiral trajectory due to the tangent injection of the
air flow.
For producing the fog water was fed into the
atomizer box from a low-lying reservoir (20 litres).
The atomizer contained an ultrasonic atomizer,
with an atomization rate of approximately one
litre per hour. The water level above the atomizer
was kept constant with a spillway.
The formation of ice particles within the
supercooled fog in the cloud chamber was
triggered by a short burst of pressurized air which
was directed into the cloud chamber. These ice
particles acted as condensation nuclei to
encourage condensation of water vapour in order
to form snow crystals.
3. MEASUREMENTS INSIDE THE CLOUD
CHAMBER
We installed four type K thermocouples to derive
a vertical temperature profile inside the cloud
chamber. In addition we measured the relative
humidity of the air within the cloud chamber as
well as water temperature.
After different time periods of crystal growth the
ice crystals were collected at the bottom and at
the top of the cloud chamber and were
immediately photographed under a microscope.
We used an optical microscope with an adapter
for a single lens reflex camera. By applying a
microscopic scale we were able to derive the size
of the individual ice crystals. We used standard
classifications of ice crystals (Nakaya, 1954) to
organize and classify our observations. Snow
density was measured by weighing a known
volume of collected snow.
Fig. 3: Snow crystals from our cloud chamber as
a function of measured temperature and
estimated supersaturation (Lettner, 2012).
4. RESULTS AND DISCUSSION
A summary of the produced crystals is shown in
Fig. 3. The diverse crystal shapes were formed
successfully at different controlled conditions
within the cloud chamber. Measurements of snow
crystal sizes showed that the largest snow
crystals were found at temperatures between
−12°C and −16°C.
First systematic observations of the size and
shape distribution of the ice crystals as a function
of temperature and supersaturation were
consistent with the other literature data. (e.g.
Libbrecht, 2005)
We could determine a vast difference in the
shape of the snow crystals growing in our cloud
chamber (Fig.3). We observed large ice crystals
inside the cloud chamber when we performed
longer test runs and took samples at the end of
the test period. At the start of the test run we
measured crystal sizes of about 0.001mm and
0.002. Afterwards the crystals were growing
slowly, but in the final growth phase the crystal
size was increasing fast (Figs.4 and 6). Fast
increase of snow crystal sizes might be explained
by the fact that close to the end of the test run
only a small amount of ice crystals remained in
the cloud but still the same amount of water
vapour was available. On the other hand the
increasing size of the individual ice particles
provided more surface for free water molecules
to deposit on the crystalline surface.

Fig. 4: Snow crystals taken from the cloud
chamber 3, 6, and 11 min after injection of
compressed air. The temperature within the cloud
chamber was −14°C. The width of each image
corresponds to 1.2 mm (Lettner, 2012).
Even though the snow crystals grew fast at the
end of the growth period, we observed that the
air temperature had a greater impact on the
crystal size.
Results from the tests carried out in early 2016
showed that producing exactly reproducible
homogenous snow samples had only limited
success. It was still not possible to fully control
the ventilation and supersaturation inside the
cloud chamber over a period of more than one
hour.
The production rate of lightweight snow in our
cloud chamber set-up depended on the amount
of atomized water per time unit and laboratory
temperature. For a cold room temperature of
-15°C and usage of one atomizer box it was
possible to produce about 10 litres of snow per
hour.
5. OUTLOOK
In the future we aim to test our cloud chamber
also for outdoor conditions (Fig. 5). We therefore
built a larger version of our laboratory cloud
chamber and installed it in a ski resort in
Obergurgl, Tyrol, Austria. In a future project we
want to test the feasibility of using a cloud
chamber to produce artificial snow for ski runs.
REFERENCES:
Bones, J., Adams, E, 2009. Controlling Crystal Habit in a Small
Scale Snowmaker. In: Proceedings ISSW 2009.
International Snow Science Workshop, Davos,
Switzerland, 27 September - 2 October 2009. 2009. Swiss
Federal Institute for Forest, Snow and Landscape
Research WSL.
Burkart, Julia, 2012: Internal report about crystal growth in a
supercool cloud. Boku, Vienna, Austria.
Fig.5: Outdoor testing of the cloud chamber in a
in the ski resort in Obergurgl, Tyrol, Austria.
Fauve, M., Hansueli R., Schneebeli, M.,2002. Preparation and
maintenance of pistes: handbook for practitioners. Swiss
Federal Institute for Snow and Avalanche Research SLF.
Lettner A., 2012. Growth of snow crystals in an artificial cloud:
effect of air temperature on crystal growth. M.S. Thesis
Boku, Vienna, Austria.
Libbrech, K.G. , 2005. The physics of snow crystals. Reports
on Progress in Physics, 68(4), 855-895..
Nakamura, H,1978. A new apparatus to produce fresh snow.
Report of the National Research Institute for Earth
Science and Disaster Prevention 19, 229-238.
Nakaya, U., 1954. Snow crystals: natural and artificial. Harvard
University Press, Cambridge, MA.
Rauber, R. M., Tokay, A. 1991. An explanation for the
existence of supercooled liquid water at the top of cold
clouds. Journal of Atmospheric. Sciences, 48: 1005–
1023.
Schaefer, V.J., 1952. Formation of ice crystals in ordinary and
nuclei-free air. Industrial and Engineering Chemistry
44(6): 1300-1304.
Schleef, S. Jaggi, M., Löwe, H., Schneebeli, M., 2014. An
improved machine to produce nature-identical snow in the
laboratory. Journal of Glaciology, 60(219): 94-102.
Techel, F., Jarry, F., Kronthaler, G., Mitterer, S., Nairz, P.,
Pavšek, M., Valt, M. and Darms, G., 2016. Avalanche
fatalities in the European Alps: long-term trends and
statistics. Geographica Helvetica, 71(2): 147-159.

APPENDIX
Fig. 6: Observed crystal sizes (blue stars), temperature within cloud chamber (purple line), ambient
temperature (res line), and relative humidity (yellow dashed line) over time (Burkart, 2012).