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July 26, 2024: Surface Hoar Misconceptions
This is the second topic which was covered in the media with misconceptions last winter. The first was Creep and Glide, the topic of the last post.
There were three things portrayed incorrectly. The first was how surface hoar forms, and how it is distinguished from faceting due to kinetic metamorphism. The second was how it eventually stabilizes. And the third, related largely to the second, was a quote on why it's not a common problem in maritime climates.
Lets take a look at these things.
Formation
The first issue was in an article which described faceting and surface hoar as more or less the same thing, referring to the movement of water vapor within the snow. (One article lumped these together in one paragraph but later did distinguish them. Which is inconsistent in their own writing and description.)
Faceting occurs when water vapor moves through the snowpack from higher concentrations (generally from higher temperatures) to lower concentrations. This is a process of change within the snowpack, which we call metamorphism. (Kinetic metamorphism in this case since it is driven by this vapor gradient and does not result in rounded equilibrium forms.)
Surface hoar is a deposition process. Vapor in the atmosphere deposits onto the cold snow surface as a solid, in particular crystal forms. It is often compared to condensation, which is the phase change from vapor to liquid rather than vapor to solid. Condensation can be seen on cold windows, or cold eyeglasses upon entering a warm room. This condensation does not come from within the material but rather from the air.
The results and implications of the two different processes differ. Faceting results in a layer within the existing snow becoming weak and sugary. Deposition results in crystals on the surface which are weak when buried. The shapes of the crystals are much different.
A Level 1 course should describe these processes in more detail, as well as the implications. So further details are left for your course or course review.
Stabilization
Most weak layers, when buried, stabilize largely through equilibrium metamorphism (which occurs in the absence of a vapor/temperature gradient) which results in rounding and bonding of the grains of snow. Compression due to the overburden also plays a roll. However, surface hoar is a bit different.
Surface Hoar crystals are typically large and flat, growing upwards at various angles and in branches towards the atmosphere. When a layer of surface hoar is buried within the snowpack a close-up photo of the layer often looks like a truss bridge holding up the layer(s) above. These crystals are not very conducive to equilibrium metamorphism and rounding, for thermodynamic reasons.
Since the buried surface hoar is not particularly prone to rounding and bonding it primarily stabilizes when the overburden eventually pushes it into the layer below. For this reason it is especially problematic and persistent when it forms on a hard ice or crust layer. Regardless of the layer below it stabilization can take a long time in a continental type snowpack where less frequent low density snow is the rule.
Maritime Snowpacks
One article quoted a professional from California concerning the reasons surface hoar is not a persistent problem there. They speculated that it was a result of melting.
Surface hoar melts in any climate on sunny/warm slopes when it is on the surface. This is not unique to California or to maritime climates. In fact maritime climates generally have less sun than continental regions. On benched slopes you can sometimes observe surface hoar on the flat benches where the sun hits obliquely and note its absence on the rolls and short slopes between where the solar angle of incidence is more direct.
The reason surface hoar generally does not persist in maritime climates has more to do with misconception #2 above concerning stabilization. Maritime climates are characterized by heavy wet snow and frequent large storms. Slides do occur on surface hoar as well as facets early in storm cycles when they are initially buried. But by the time the cycle ends the weaknesses have been compressed and crushed (and subject to equilibrium metamorphism in the case of facets).
Summary
So now you know a bit more about surface hoar, which is an especially tricky and problematic layer. It can form with very erratic distributions, making it hard to reliably assess specific slopes. It stabilizes slowly resulting in less activity on many slopes while some remain questionable. And it persists in continental and transitional climates but rarely in maritime climates.
As a note on the difference in snowpack climates, this is very important to understand. In our Avalanche Institute Level 1 this is emphasized (as it was in the predecessor traditional versions). The climate affects the weather (or is defined by that), the snowpack, your terrain assessments, and your final plans decisions. We have received positive feedback on this, as well as comments that other courses downplay or overlook this. It's important that your instructor(s) have experience in as well as complete knowledge of the various climates.
