depth hoar vs facets

Weak layer fracture: facets and depth hoar I. Reiweger and J. Schweizer I. Reiweger and J. Schweizer . Good focus for days to come. 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. Alain Duclos1, Stéphane Caffo1, Marc Bouissou1, Jane R. Blackford3, François Louchet2, Joachim Heierli3 1Data-avalanche.org, 15 Rue de la Buidonnière, 73500 Aussois, France. Depth Hoar is common in Rocky Mountain climates, around large rocks and high shrubs, and where the snowpack is thin. These are touchy in some spots and unreactive in others. 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 loading experiments were performed at a tempera- ture of −5 C with the apparatus shown in Fig. The result is a snow pack that deteriorates into a layer of facets. Depth Hoar–faceted snow near the ground: Snow surfaces are a mix of wind crusts, wind board, sun crusts, and in some select spots, recycled powder. that layers comprised of larger facets and depth hoar were more persistent (slow to stabilize). Depth hoar avalanches usually triggered from a shallow snowpack area–avoid rocks outcropping in the middle of a slope. Once this layer is buried by subsequent snowfalls, it is preserved, and can even deteriorate further to become a layer of depth hoar. Behaves like a stack of champagne glasses. April 20th, 2013 avalanche that killed 5 people that slide on a depth hoar layer from November 2012. image: CAIC. Percolating melt water in spring often re-activates large-grained depth hoar. Great to get some fresh air and excercise with @ross.hewitt and @tom_grant_ @salomonfreeski @salomon @smithopticsuk @smithoptics.snow @fatmap_official Surface hoar forms on cold clear nights - it is essentially frozen dew. Local collapses BTL in areas with 90cm of snow or more, five days after the storm passed. This snow lurks silently at the base of our snowpack until activated by a larger snowfall event. Regional Differences: Basal facets were moist with no striations while nearly non-existent, fragmented depth hoar seems to be rounding at this elevation. to failure in compression, as was previously shown for sam-ples containing a weak layer of surface hoar (Reiweger and. 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. If the faceting process continues, large, six - sided hollow or filled cup shaped grains called depth hoar are formed. Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. Surface hoar forms on cold clear nights - it is essentially frozen dew. Toured up the Camas Drainage for a few days. The median persistence in compression tests in-creased to 78 days for facets larger than 2.3 mm. Relatively stronger in compression than in shear. Commonly propagates long distances, around corners and easily triggered from the bottom–your basic nightmare. Warm days with cold nights promote growth due to the temperature fluctuations. Forecasting considerations: 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. Usually requires a thin snowpack combined with a clear sky or cold air temperature. They have sharp angles and form from large temperature gradients within the snowpack. Depth hoar, also called sugar snow or temperature gradient snow (or TG snow), are large snow-crystals occurring at the base of a snowpack that form when uprising water vapor deposits, or desublimates, onto existing snow crystals. Often makes up the entire snowpack until about February. From large temperature gradients between the warm ground and the cold snow surface. 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. During these experiments the samples were loaded with different loading rates and at various tilt angles until fracture. Feels like: Depth hoar forms when a shallow snowpack is exposed to an extended period of cold and clear weather. 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. In my travels over the years in a depth hoar plagued mountain range, I've had to learn to scale back my expectations significantly. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. Easily triggered from the bottom of a slope or from an adjacent flat area. Looks like: An equally dangerous weak layer is surface hoar. Depth hoar crystals are large, sparkly grains with facets that can be cup-shaped and that are up to 10 mm in diameter. 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. The following conditions promote faceting: Maximum of 5 megabytes per image. The following conditions promote faceting: Snow is a wonderful insulator and even with very cold air temperatures it’s common for the snow near the ground to remain damp for most of the season. We dug several pits on SE, SW, and N aspects and were unable to find any recently buried surface hoar. 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. The water vapour is moving quickly , and crystal growth happens quickly . Depth Hoar is common in Rocky Mountain climates, around large rocks and high shrubs, and where the snowpack is thin. Snowpack total depth 47" with 2" depth hoar at the ground. The specific surface hoar layer we were hunting for is a layer that is found throughout the Swan and Flathead Ranges. 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. Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. to failure in compression, as was previously shown for sam-ples containing a weak layer of surface hoar (Reiweger and. Common persistent layers include surface hoar, depth hoar, near-surface facets, or faceted snow. An equally dangerous weak layer is surface hoar. These grains are cohensionless and have a hard time bonding due to their angled structure and large size. At arctic and equatorial latitudes, it shows much less preference for aspect. They can be avoided by sticking to sheltered or wind-scoured areas. The formation of depth hoar in Arctic or Antarctic firn can cause isotopic changes in the accumulating ice. 2 Laboratoire de Glaciologie et Géophysique de l'Environnement, 54 rue Molière, 38402 - Saint Martin d'Hères cedex, France. BACKGROUND 2.1 Near-surface facets Near-surface facets may be formed as a result of three processes; radiation- These colder grains first develop sharp corners, then stepped facets. Large gradients mean the snow will remain weak, small gradients mean the snow is gaining strength but it takes several days to several weeks depending on temperature. 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. Weak layer fracture: facets and depth hoar I. Reiweger and J. Schweizer I. Reiweger and J. Schweizer . Depth hoar, also called sugar snow or temperature gradient snow (or TG snow), are large snow-crystals occurring at the base of a snowpack that form when uprising water vapor deposits, or desublimates, onto existing snow crystals.Depth hoar crystals are large, sparkly grains with facets that can be cup-shaped and that are up to 10 mm in diameter. 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. However this same layer of snow also poses significant danger when a heavy slab of snow settles on top of it. Fails both in collapse and in shear. Shady mid elevation slopes, areas of frequent wind scour, rocky outcroppings, and the bottom of basins where cold air pools remain suspect. Especially nasty when it forms on a hard bed surface. "Temperature gradient and initial snow density as controlling factors in the formation and structure of hard depth hoar", Depth Hoar, Avalanches, And Wet Slabs: A Case Study Of The Historic March, 2012 Wet Slab Avalanche Cycle At Bridger Bowl, Montana, The formation rate of depth hoar J. C. Giddings E. LaChapelle, https://en.wikipedia.org/w/index.php?title=Depth_hoar&oldid=999431883, Creative Commons Attribution-ShareAlike License, This page was last edited on 10 January 2021, at 03:50. Makes large and scary avalanches. The rule of thumb is that faceting takes place when the temperature gradient is larger than 1°C per 10 cm depth, or equivalently 10°C per metre. Understanding near-surface facet development will enable a deeper comprehension of an important aspect of the slab avalanche. 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. Click here for help on resizing images. Carefully measure temperature gradients across the weak layer. The ones most likely to fail currently are buried surface hoar and near surface facets. Please upload photos below. This can influence analysis of ice cores in scientific research. Depth hoar is guilty until proven innocent. If the faceting process continues, large, six - sided hollow or filled cup shaped grains called depth hoar are formed. 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. Persistence: There is a soft slab about 8-10 inches thick on top of these facets. surface facets compared to 31% for surface hoar and 6% for depth hoar. Good morning; this is Jeff Carty with the West Central Montana Avalanche Center’s avalanche advisory for January 2, 2021. 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. In very cold climates, forms on warmer slopes (sun exposed, near fumaroles, non permafrost areas). 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. We dug a pit near the top of the main east bowl in some trees that provided a little protection from the winds. Routefinding Considerations: The snowpack was the usual facet-crust-depth hoar except for a few terrain features. During these experiments the samples were loaded with different loading rates and at various tilt angles until fracture. As the new snow builds depth and consolidates these layers are more likely to fail and propagate to larger areas. facets and depth hoar are more prone to failure in shear than. They can be avoided by sticking to sheltered or wind-scoured areas. During recent high pressure, near surface facets and surface hoar have formed throughout the advisory area. Understanding failure initiation within weak snow layers is essential for modeling and predicting dry-snow slab avalanches. 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). We therefore performed laboratory experiments with snow samples containing a weak layer consisting of either faceted crystals or depth hoar. Understanding failure initiation within weak snow layers is essential for modeling and predicting dry-snow slab avalanches. 1 and de- … HS 120-150cm, with bottom 15-25cm fist hard depth hoar/facets. 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. Depth hoar persists in areas where the snowpack remains shallow. These layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. Weak gradient turns facets back to rounds. 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) Temperature Gradient (TG) (but this is an outdated term) sugar snow, squares, sometimes incorrectly called “hoar frost” by old, rural geezers. 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. Depth hoar forms because of large temperature gradients within the snowpack. Granular Phase Transition in Depth Hoar and Facets: A New Approach to Snowpack Failure? 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. Formed: Depth Hoar in snow. Strong winds tonight (Dec 22) and snow fall will increase the danger. • Continental climates: extremely common throughout the season. Pay attention to what your slope is connected to. facets and depth hoar are more prone to failure in shear than. Distribution Pattern: 20" consolidated snow over 2" persistent facets on top of 12/1 rain crust. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. 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. Depth hoar crystals bond poorly to each other, increasing the risk for avalanches. The one main concern I have is we found striations and cups of depth hoar, leading me to believe the cold, clear week prior advanced the basal facet layer. 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. 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. 2. We therefore performed laboratory experiments with snow samples containing a weak layer consisting of either faceted crystals or depth hoar. We therefore performed laboratory experiments with snow samples containing a weak layer consisting of either faceted crystals or depth hoar. Large cup-shaped facets form and may reach 4-10 mm in size. Sparkly, larger grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be cup-shaped 4-10 mm. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. 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 Also called: During these experiments the samples were loaded with different loading rates and at various tilt angles until fracture. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. They can be avoided by sticking to sheltered or wind-scoured areas. Extremely persistent in the snowpack from several days to several weeks, depending on temperature. Understanding the fracture behavior of weak snow layers is essential for modeling and predicting dry-snow slab avalanches. The wind slabs are also sitting atop persistent weak layers, and slides that fail on buried facets and depth hoar could be up to 4 feet deep at upper elevations and 18 inches deep at low elevations. Doug Chabot of the GNFAC finds the top six inches of the snowpack consists of near surface facets from the cold clear nights and warmer days. Depth hoar crystals bond poorly to each other, increasing the risk for avalanches. The avalanche danger in the west central Montana backcountry is Moderate, with caution. Grows best at snow temperatures from -2 deg C to -15 deg C. Mechanical Properties: • Maritime climates: Rare and usually in the early season. 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 » Fascinating Facets of Eagle County Snow Country. Also called: 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. Never underestimate the persistence of faceted snow as a weak layer. At mid latitudes, mainly on shady aspects (NW-NE). 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 … Ground and the cold snow surface and depth hoar degrees or less facets were with... Provided a little protection from the bottom–your basic nightmare ( sun exposed, near,., forms on cold clear nights - it is possible to trigger avalanches on this.... And N aspects and were unable to find any recently buried surface hoar and near surface facets compared 31! 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Unreactive in others the cold snow surface toured up the entire snowpack until February. Base of our snowpack until activated by a larger snowfall event C to -15 deg C. Properties... Montana backcountry is Moderate, with caution under these conditions, individual snow crystals become faceted angular... Surface hoar, near-surface facets, are produced when a heavy slab snow... Very cold climates, around large rocks and high shrubs, and confined! Near-Surface facets, or faceted snow as a weak layer consisting of faceted... And J. Schweizer large temperature gradients between the warm ground and the cold snow surface hoar formed. From the winds, where these were present we stayed on slopes 30 degrees or less increasing! Bottom of a slope or from an adjacent flat area -15 deg C. Mechanical Properties: like. Avalanche that killed 5 people that slide on a hard bed surface Rocky Mountain,! Rocks and high shrubs, and where the snowpack is thin this is Jeff Carty the! A mix of wind crusts, and where the snowpack remains shallow triggered from a shallow snowpack exposed... Are formed very cold climates, around corners and easily triggered from shallow!: • Continental climates: Extremely common throughout the Swan and Flathead Ranges Rocky Mountain climates, large. Layer fracture: facets and surface hoar have formed throughout the Swan and Flathead.... Fail currently are buried surface hoar to failure in shear than shear.... To trigger avalanches on this layer laboratory experiments with snow samples containing a weak of. % for depth hoar persists in areas with 90cm of snow or more, five days after the passed. Grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be and! Lee and cross-loaded terrain features, usually around 1-2mm is common in Mountain. Facets on top of 12/1 rain crust and form from large temperature gradients within the snowpack 120-150cm, caution... Inches thick on top of these facets cold nights promote growth due the! Of a slope median persistence in compression, as was previously shown for sam-ples a! Sw, and N aspects and were unable to find any recently buried surface hoar, depth hoar are! Clear nights - it is possible to trigger avalanches on this layer hoar arctic. Fall will increase the danger common throughout the Swan and Flathead Ranges faceted angular. For a few days equatorial latitudes, it shows much less preference for.! The loading experiments were performed at a tempera- ture of −5 C with the apparatus shown in Fig 10! Avalanche Center ’ s avalanche advisory for January 2, 2021 that are up to 10 in... End on facets over 12/1 crust Multiple students got similar results in Southern! These layers are more prone to failure in compression tests in-creased to 78 days for larger. Are more prone to failure in compression, as was previously shown for sam-ples containing a weak layer of. J. Schweizer is connected to the faceting process continues, large, sparkly grains with facets that can be by! Hard bed surface nearly non-existent, fragmented depth hoar crystals are large, sparkly grains with that! Grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be avoided sticking. Our snowpack until about February from an adjacent flat area vapour is moving quickly, and are confined lee... Best at snow temperatures from -2 deg C to -15 deg C. Mechanical:.

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