Benjamin Walter resembles a yellow and blue cross between a Teletubby and the Michelin man. And no wonder, as whenever the scientist from the Snow Physics group opens the door to his laboratory, he is met by ice-cold air. At between -20 and -25 °C, anyone working in this lab needs to wear a protective suit like the one Benjamin is currently sporting. He is studying the properties of snow in a wind tunnel in the cold laboratory of the WSL Institute for Snow and Avalanche Research (SLF). His results show, for example, the wind conditions under which cornices can build up on mountain ridges and how dangerous weak layers of snow can form.
The basic principle is similar to that applied in the automotive and aviation industries; the only difference is that these industries strive to determine and optimise air resistance and the objects being studied are fixed in place.
At the SLF, however, the wind always blows the snow in circles. "This allows us to simulate infinite expanses," explains Benjamin, pointing to a sight glass. The crystals whiz by constantly at speeds of up to 25 kilometres per hour and take just one second to spin around the interior of the wooden tunnel.
New findings about snow cornices
Benjamin checks cable connections that transmit data to a computer. A camera films the snow in flight while sensors measure the wind speed, the temperatures of the air and snow as well as the humidity.
"This is the first time that a circular wind tunnel has been used in a lab to study how snow is compacted by wind," says Benjamin. The density depends heavily on the wind speed and air temperature, he explains. Benjamin is using the wind tunnel to simulate how snow cover builds up in strong winds, which allows him to replicate the snowpack in places where snow and weather are never measured.
"The wind causes fine crystals to break, creating smaller fragments and compacting the snowpack," explains Benjamin. As part of a recent study, he also investigated how cornices are formed. To this end, he modelled a mountain ridge of snow in the tunnel, where snow is deposited over time and a cornice forms. The results were surprising, with medium wind speeds being found to be the most conducive to cornice formation. Too little wind provides for too little snow, while too strong a wind blows the snow away.
On the trail of surface hoar
Behind Benjamin, his colleague Sonja Wahl opens a cover in the wind tunnel. She uses a kind of scoop with a bag attached rather than a net to collect snow crystals from the wind. After a few minutes, she finally has half a handful. Sonja then disappears into a side room to analyse her sample.
Before Christmas, Benjamin's group had investigated another aspect that is particularly relevant for skiers and snowshoe hikers. By passing water vapour over a very cold copper plate in moderate wind, they were able to form surface hoar. The goal of this research? "We want to know the conditions under which this weak snow layer, typically associated with avalanches, is most likely to develop," clarifies Benjamin.
This article first appeared in the Davoser Zeitung on 28th February 2023.