PowderPeople | SLF snow researcher Jürg Trachsel

A frontal system is pushing moist air from the northwest towards the Alps. Water molecules that once swam in the Atlantic are swirled around, along with fine dust, pollen and other small particles, which are carried upwards by wind and turbulence.

The air flowing in hits the edge of the Alps and has to move upwards, cooling down in the process. This causes the relative humidity to rise to saturation point. Individual water molecules begin to freeze to the existing dust particles and gradually form a large, gray, PowderAlert-triggering north jam cloud.

The ice particles are not arranged randomly, but form a regular grid. At normal pressure, this lattice has a hexagonal structure. In the compound, the atoms of a water molecule react to the charge of the atoms of the neighboring molecules and arrange themselves in the most energetically favorable way possible. Six water molecules thus form a tiny, hexagonal prism - the basic building block of every snowflake.

Due to the conditions in the cloud (supersaturation), molecules that are still flying around freely tend to join the hexagonal mini-crystals. This means that the hexagonal base prism gradually grows arms from the corners (dendritic growth), which in turn can develop branches. Depending on the temperature and humidity, the classic snowflake stars from the picture book or other hexagonal structures, such as small platelets or needles, are formed.

The proverbial and actual uniqueness of snowflakes results from minimal differences in the environmental conditions in the cloud and major fluctuations in temperature, pressure and humidity that a crystal passes through on its way through the cloud. Even the smallest changes result in differently shaped crystals and each flake takes a different path through the cloud - turbulence causes individual flakes to rise again and guide them through different layers of air.

Snow as a subject of research

Many questions about the formation of snowflakes and the physics of snow have already been answered in recent decades. However, not everything has been clarified. Today, attention is focused on material-specific properties such as fracture and flow mechanics, which play an important role in avalanche forecasting, for example, or exchange processes between the snowpack and the atmosphere, which are relevant in climate research.

Potting the snow sample with the chemical diethyl phthalate in the field is one way of protecting such structures during transportation. This fills the cavities between the snow crystals and thus supports them. The chemical hardens and when the ice evaporates in the laboratory, a negative image of the structure remains, which can be examined further. However, this method is only suitable for subsequently analyzing the physical structure of the snow. As soon as chemical substances are to be analyzed, any external contamination must be prevented.

Transporting his samples is also a challenge for Jürg Trachsel. He carries out some of his measurements and experiments at the SLF measuring site on the Weissfluhjoch (2550 m above sea level). The great advantage of this is that it is equipped with a large number of sensors. Data on weather, radiation balance, surface and ground temperatures are thus available to him and other researchers without having to be collected individually.

Jürg records the entire snow profile at the measuring field every month, which in his case is somewhat more complex than a "skier profile". Dressed in a white protective suit, he fills each layer of snow individually into a plastic tube. The tubes are sealed airtight and have to be sent to PSI in Villigen AG, a distant location, promptly and without melting. For this purpose, they are carefully packed with cooling elements and multiple layers of insulation. The crate is then transported down to the valley on skis. And thanks to the (usually) excellent public transport system, onwards to PSI by train.

Powder from the lab

Natural snow is not suitable for all experiments, however! In basic research in particular, it can be advantageous to have snow that is as well defined and uniform as possible as a starting material. Not all snow for Jürg's experiments in the laboratory therefore comes from wild-growing northern snow clouds. Some samples come from a precisely adjustable, artificial "cloud" that lives permanently in one of the SLF cold laboratories - the so-called SnowMaker. In the temperature-regulated climate chamber that houses the SnowMaker, it is -24°C in both summer and winter. Instead of a lab coat, Jürg wears a thick down expedition suit.

The first thing that catches the eye behind the heavy insulated door are the white polystyrene boxes containing snow samples from all over the world. The rest of the room is taken up by the SnowMaker. The artificial cloud hums away in the cold and looks more like a bulky closet than a cloud. The concept was originally devised by a Japanese snow researcher in the 1970s. SLF snow researcher Martin Schneebeli then optimized the artificial cloud over the years. The basic principle is still the same - cold air flows over warm water and becomes supersaturated in the process.

Similar to the polar air, which is enriched with moisture as it passes over the comparatively warmer North Atlantic, the air from the SnowMaker fan also absorbs moisture as it passes over a basin of warm water: the water is heated to temperatures of around 30°C and the air that the fan draws in has the same temperature as the ambient air in the cold laboratory, i.e. -24°C. The strong temperature difference promotes the evaporation process.

Laboratory snow helps to understand nature

The sample that has just been produced is placed in a smaller climate chamber. Together with samples from the measurement field, it is exposed to a temperature gradient. This means that the temperature below the sample is a few degrees higher than on the surface. Exactly the same conditions can be found in nature: while the ground beneath the snow cover is constantly at zero degrees throughout the winter, the surface is colder due to the weather. This temperature difference causes the snow crystals to transform and completely change their shape without melting. This recrystallization can not only lead to weak layers that can be problematic for skiers, it also influences the distribution of the impurities contained in the snow - Jürg's field of research. Some substances are transported to the surface, others are firmly trapped in the snow crystals. With the experiments in the laboratory, the observations from nature can be verified and the individual processes studied in great detail.

Artificial powder for skiing too?

Jürg's enthusiasm for snow is also evident outside of work: in his free time, he can be found in the snow just as often, whether privately on ski tours, as a leader with the JO Brugg or simply on the slopes. Of course, his affinity for the "white gold" plays a role in his choice of job and working at the SLF, where almost everything revolves around snow, is twice as exciting if you enjoy being out in the snow yourself.

When asked whether it will one day be possible to produce powder snow for the slopes, Jürg waves it off. The capacity is too small and the energy consumption of the artificial cloud is too high. But he is actually quite happy about it: Nature is not so easy to trick! This makes it all the more important to him that we all continue to protect our environment. Because only if it stays cold enough in the coming decades can the northern thaw cloud bring us not only rain, but also real powder.