Backcountry
Avalanche
In backcountry skiing, avalanches are on our minds almost all the time. Avalanches are unpredictable, powerful, and destructive. A huge part of safety in the backcountry is about identifying and mitigating avalanche risk, and selecting terrain is one of the best ways to do that. Slab avalanches are the most dangerous and talked about avalanche types, but did you know that you can help mitigate slab avalanche risk by selecting particular slope aspects? Let’s talk about that.
There are nine types of avalanches, each of which forms with different weather conditions and snowpack. The most destructive, and often most discussed, are slab avalanches. Slab avalanches happen when a strong, cohesive slab of snow forms on top of a weaker layer. This weak layer can be poor interface bonding between two slabs, unstable loose snow, and more. The formation and persistence of these weak layers depend a lot on solar radiation.
Why is the aspect of a slope important for stability? The theory behind this revolves mostly around solar loading.
The sun loads south aspects, melting and breaking down surface weak layers (faceted snow grains, surface hoar, and graupel, particularly) before they become buried and turn into persistent instabilities. Sun crust and MFcr (melt-freeze crust, when the sun repeatedly warms a snow surface that re-freezes overnight) layers generated by solar loading increase the shear strength of the snowpack, making a collapse less likely to occur. These same crusts increase the overall tensile strength of slabs, meaning if a collapse were to occur, the risk that the slab above it fractures and escalates into an avalanche remains low.
North facing slopes, on the other hand, do not have the benefit of the sun to break down weak layers before they are buried. Not only this, but north-facing slopes also host colder surface snow temperatures, as the sun does not warm up the snow surface as much due to a lower or even nonexistent sun angle. This means that a vapor pressure gradient causes near-surface faceting, causing the snow to lose strength and when buried, turning into persistent weak layer instabilities.
How does faceting work? When a vapor pressure gradient forms, it begins to suck moisture from the bottom (warmer) parts of the snowpack to the upper parts, which are generally cooler on shaded, north-facing slopes. On south aspects or in the springtime when the surface of the snow heats up more than the ground due to air temp and sun exposure, the reverse happens. The moisture percolates from the surface of the snowpack down to the bottom until the entire snowpack reaches the same amount of moisture content and temperature. At this point, the snowpack becomes what we call “isothermal.” Once the snow has become isothermal, there’s nowhere in the snowpack for the moisture to travel to, so it runs out of the snowpack as water.
As the vapor pressure gradient sucks moisture from the bottom to the top of the snowpack, it has to “jump” from crystal to crystal to travel. The moisture, unfortunately, can’t just travel through the air pockets of snow. What ends up happening is the moisture deposits on the downward side of the crystal and leaves from the uphill side, creating an extremely angular snow crystal known as a facet. The only word you need to remember when thinking of facets is “bad.” Facets are loose, unpredictable, and the culprit of many slab avalanches.
Therefore, due to the sun, south aspects typically don’t see as much formation of weak layers and therefore, there are typically fewer slab avalanches on southern slopes. However, this is not always the case. There are other avalanche types that are more common on south slopes. As always, every snowpack is different, and assumptions and “rules of thumb” are always incorrect unless proven correct through the use of a localized snowpit or other advanced snowpack analysis.
