surface facets compared to 31% for surface hoar and 6% for depth hoar. These are touchy in some spots and unreactive in others. The formation of depth hoar in Arctic or Antarctic firn can cause isotopic changes in the accumulating ice. Regional Differences: Temperature Gradient (TG) (but this is an outdated term) sugar snow, squares, sometimes incorrectly called “hoar frost” by old, rural geezers. Posted by Nathan Boyer-Rechlin on Dec 16, ... Chipmunks, mice, voles, and ermine spend their winter months tunneling through the loose depth hoar, rarely seen above the snowpack. The avalanche danger in the west central Montana backcountry is Moderate, with caution. April 20th, 2013 avalanche that killed 5 people that slide on a depth hoar layer from November 2012. image: CAIC. Please upload photos below. The snowpack around Mt Emma is generally heavily wind affected and thinner than when I was in the area 3 weeks ago, except in the most wind loaded spots. Good morning; this is Jeff Carty with the West Central Montana Avalanche Center’s avalanche advisory for January 2, 2021. Understanding failure initiation within weak snow layers is essential for modeling and predicting dry-snow slab avalanches. Weak layer fracture: facets and depth hoar I. Reiweger and J. Schweizer I. Reiweger and J. Schweizer . Contrary to popular belief, as long as the ground has an insulating blanket of snow, the ground is almost always warm–near freezing–even with very cold air temperatures. They have sharp angles and form from large temperature gradients within the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar, deeply-buried surface hoar, or facets surrounding a deeply-buried crust. Their message is one of warning, an early-season snowfall that will affect the layers of snow that will fall on top of this one for … Typically smaller in size than depth hoar, usually around 1-2mm. Depth hoar crystals are large, sparkly grains with facets that can be cup-shaped and that are up to 10 mm in diameter. If the faceting process continues, large, six - sided hollow or filled cup shaped grains called depth hoar are formed. Depth hoar crystals bond poorly to each other, increasing the risk for avalanches. The loading experiments were performed at a tempera- ture of −5 C with the apparatus shown in Fig. We therefore performed laboratory experiments with snow samples containing a weak layer consisting of either faceted crystals or depth hoar. An equally dangerous weak layer is surface hoar. The median persistence in compression tests in-creased to 78 days for facets larger than 2.3 mm. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs. Near surface facets are smaller in size than depth hoar (1-2 mm) but they are quite pesky and can persist in the snowpack for long periods of time. Fails both in collapse and in shear. Often makes up the entire snowpack until about February. Depth hoar forms when a shallow snowpack is exposed to an extended period of cold and clear weather. Depth Hoar in snow. This snow lurks silently at the base of our snowpack until activated by a larger snowfall event. Depth Hoar Summary: Looks like: Sparkly, larger grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be cup-shaped 4-10 mm. We therefore performed laboratory experiments with snow samples containing a weak layer consisting of either faceted crystals or depth hoar. Snowpack total depth 47" with 2" depth hoar at the ground. Extremely persistent in the snowpack from several days to several weeks, depending on temperature. We found hard wind slabs on NE aspects above 6500', where these were present we stayed on slopes 30 degrees or less. Distribution Pattern: Understanding failure initiation within weak snow layers is essential for modeling and predicting dry-snow slab avalanches. Surface hoar forms on cold clear nights - it is essentially frozen dew. The weak layer in the artificial samples was a thin layer of faceted crystals, whereas the natural samples contained a thick weak layer of large depth hoar crystals. Depth hoar is guilty until proven innocent. Carefully measure temperature gradients across the weak layer. Percolating melt water in spring often re-activates large-grained depth hoar. Understanding near-surface facet development will enable a deeper comprehension of an important aspect of the slab avalanche. Once this layer is buried by subsequent snowfalls, it is preserved, and can even deteriorate further to become a layer of depth hoar. ECTP27 on facets over 12/1 crust PST 38/100 End on facets 12/1 crust Multiple students got similar results in the vicinity on E aspect. The Colorado Avalanche Information Center released an early season snow update on their blog a couple of years ago, which still holds true today. Alain Duclos1, Stéphane Caffo1, Marc Bouissou1, Jane R. Blackford3, François Louchet2, Joachim Heierli3 1Data-avalanche.org, 15 Rue de la Buidonnière, 73500 Aussois, France. facets and depth hoar are more prone to failure in shear than. We toured in the Southern Whitefish Range to try and get a better idea of the distribution of the recently buried surface hoar. During these experiments the samples were loaded with different loading rates and at various tilt angles until fracture. Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. Depth hoar forms when a shallow snowpack, typically less than one meter deep, is exposed to a cold and clear weather pattern that persists for an extended period of time. Depth hoar forms because of large temperature gradients within the snowpack. These colder grains first develop sharp corners, then stepped facets. to failure in compression, as was previously shown for sam-ples containing a weak layer of surface hoar (Reiweger and. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs. The ones most likely to fail currently are buried surface hoar and near surface facets. Shady mid elevation slopes, areas of frequent wind scour, rocky outcroppings, and the bottom of basins where cold air pools remain suspect. Warm days with cold nights promote growth due to the temperature fluctuations. This can influence analysis of ice cores in scientific research. Relatively stronger in compression than in shear. Tests: 2,000 ft, W , 21° Slope, STE 40cm down on N/O Interface CT11 SP (Simultaneous fractures) 20cm down within New interface 32cm down on buried surface hoar The following conditions promote faceting: Formed: The top of the snow surface, on the other hand, can become extremely cold–especially when exposed to a clear sky–thus creating one of the most common temperature gradient conditions. During these experiments the samples were loaded with different loading rates and at various tilt angles until fracture. Strong winds tonight (Dec 22) and snow fall will increase the danger. Image: Agricultural Research Service, U.S. Department of Agriculture Understand and Managing Depth Hoar: By: Eric Trenbeath When the first significant storm paints the peaks white and we’re getting stoked for winter, it’s time to start thinking about how that first layer will affect snowpack stability during the upcoming season. Depth hoar, also called sugar snow[1] or temperature gradient snow (or TG snow),[2] are large snow-crystals occurring at the base of a snowpack that form when uprising water vapor deposits, or desublimates, onto existing snow crystals. Download PDF: Sorry, we are unable to provide the full text but you may find it at the following location(s): https://doi.org/10.5194/tcd-7-... (external link) Most noticable was a crown on an east aspect above 8000', nearby a wind pillow was seen on a NE aspect at the same elevation. Depth Hoar is common in Rocky Mountain climates, around large rocks and high shrubs, and where the snowpack is thin. BACKGROUND 2.1 Near-surface facets Near-surface facets may be formed as a result of three processes; radiation- There is a soft slab about 8-10 inches thick on top of these facets. Persistent, Deep-Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. This advisory does not apply to operating ski areas, expires at midnight tonight, and is the sole responsibility of the U.S. Forest Service.... Read more » Persistent, Deep-Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. Especially in the early winter, cold temperature often combines with a thin snowpack making the perfect breeding conditions for the dreaded faceted snow near the ground, which we call depth hoar. At arctic and equatorial latitudes, it shows much less preference for aspect. Weak layer fracture: facets and depth hoar I. Reiweger and J. Schweizer I. Reiweger and J. Schweizer . Faceted crystals, or facets, are produced when a strong vertical temperature gradient exists. Large cup-shaped facets form and may reach 4-10 mm in size. Surface hoar forms on cold clear nights - it is essentially frozen dew. Depth Hoar–faceted snow near the ground: Our limited data for facet layers with average grain size less than or equal to 0.7 mm suggests such layers usually do not remain weak for long Facets– Facets, or squares as they are sometimes referred to, are associated with most the avalanche fatalities. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. Sparkly, larger grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be cup-shaped 4-10 mm. The only exception to this is in permafrost areas (very high elevations at mid latitudes or arctic latitudes) or in areas with a very thin snow cover combined with very cold temperatures. Good focus for days to come. • Continental climates: extremely common throughout the season. 20" consolidated snow over 2" persistent facets on top of 12/1 rain crust. Great to get some fresh air and excercise with @ross.hewitt and @tom_grant_ @salomonfreeski @salomon @smithopticsuk @smithoptics.snow @fatmap_official Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. The process in reverse, however, occurs much slowly because it takes so much energy to create a faceted crystal that when we take the energy source away (the strong temperature gradient) it take a lot of time for the crystal to return to its equilibrium state (rounds). Fascinating Facets of Eagle County Snow Country. The following conditions promote faceting: Large-grained, faceted, cup-shaped crystals near the ground. Areas with slightly thinner snow - sometimes at mid elevations or thinner for other reasons - have the weakest facets which have become striated depth hoar crystals. Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. It is possible to trigger avalanches on this layer. They can be avoided by sticking to sheltered or wind-scoured areas. Depth Hoar is common in Rocky Mountain climates, around large rocks and high shrubs, and where the snowpack is thin. Behaves like a stack of champagne glasses. Makes large and scary avalanches. The most common persistent weak layers involved in deep, persistent slabs are depth hoar, deeply-buried surface hoar, or facets surrounding a deeply-buried crust. Grows best at snow temperatures from -2 deg C to -15 deg C. Mechanical Properties: that layers comprised of larger facets and depth hoar were more persistent (slow to stabilize). 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