Most of you have probably noticed how the wind is capable to shape and polish snow surfaces, make fascinating structures, often similar to ripples and sand dunes. Fascinating to look at, but a hustle to ski on. I guess many of you are also very capable to point out the latest wind-direction based on the shape of the structures. However, some of these structures are very complex and it can be difficult. Examples are sastrugi versus regular snow waves, they are very different features formed by wind blowing on top of snow. In this blog post we will touch a little bit into this, how they form in relation to wind direction, and how they are different from each other’s, and from ripples and dunes in sand.
Waves or ripples can form in the snow similarly to sand on a beach or underwater. The main mechanism for forming waves or ripples in snow or sand is the erosion effects of wind and water. When water waves pass over sand at a beach, grains of sand are picked up and re-deposited. The velocity of a (water) wave changes through a cycle, so the top of a wave moves at a different speed to the ‘sides’ of a wave, which means that some grains are able to be picked up by the fast moving part of the wave (stoss-side), but are redeposited when the wave slows down (at the lee-side). This mechanism creates the ripple lines that you feel under your feet when swimming at a beach. Dunes are generally created in the same manner, when wind picks up sand from a beach and re-deposits the grains when the wind slows down. Both dunes and ripples generally form long mostly straight lines because the wind or waves flow consistently in the same direction across the whole beach.
On a mountain, however, wind can be very variable in terms of the direction gusts come from. Therefore, ripples in snow are often very complex in morphology. The direction wind comes from is highly effected by the topography of the mountain itself. ‘Wind’, as a general rule, tries to flow all the time at a constant velocity in a constant direction. However, if there is an obstacle to wind flow the wind will speed up around the obstacle to ‘catch up’ as an air particle impeded by an obstacle had to move further than one not impeded. Eddies form at the back of the obstacle where wind is highly disturbed, and deposition of snow will favor the back of obstacles because the wind speed behind an obstacle is much slower than to the sides or on top. Eddies occur where the flow of air is disrupted and lower pressure, so wind can even reverse direction to ‘fill in’ the area behind the obstacle. Wind also speeds up in gullies, canyons or valleys where the flow of air is restricted and forced through a narrow passage:
All this essentially means that on a mountain, where there are many gullies, obstacles and so forth, the wind speed and direction can be very difficult to predict, therefore sastrugi may form complicated patterns and be punctuated by large snowdrifts. On large open faces, however they should form perpendicular to the prevailing wind direction. When the wind subsides these ripple features become ingrained in the surface due to re-crystallization of snow flakes – where the small pressure changes from a slightly larger amount of snow can cause the snow to become harder at the location of a ripple than around it (Figure 1). This makes them annoying for skiing because ripples often – and especially sastrugi`s do not give way under skis. A couple of more pictures from the weekend with different shapes of snow waves are presented below.
(And yes, the sled is a toboggan if you wonder, it worked surprisingly well).
Cornices are another phenomena that regular mountain goers may be familiar with. When wind travels over a sharp break in terrain, for example the crest of a mountain, snow builds up outwards, meaning there in no solid ground underneath. A small amount of pressure from above or below can cause a cornice to collapse and could trigger avalanches.