Blueprint: Laminar Near-Surface Faceting & Faceting Around a Laminar Crust

Purpose: A field-usable formation + identification + forecasting playbook for (1) near-surface facets (NSF) and (2) facets that form on/under/within a thin, laminar crust (sun crust, wind skin, rain crust, melt-freeze, freezing drizzle).

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The Ingredients

A. For Near-Surface Faceting (NSF)

• Cold, clear nights → strong radiative cooling at the snow surface.
• Low wind (or sheltered micro-terrain) → prevents mixing/warming of the surface.
• Thin snow cover / shallow new snow → steep temperature gradient in the top few cm.
• Time: even 1–3 nights can start it; 3–10 days can make it significant.

Physics shortcut: a strong temperature gradient in the top 1–5 cm drives vapour transport upward, growing angular grains fast.

B. For Faceting Around / Within a Laminar Crust

• A thin, dense, smooth crust (sun crust, rain crust, melt-freeze, wind-skin, freezing drizzle crust).
• Then cold, clear weather (or cold air over warmer snow) → a gradient develops across the crust interface.
• Often amplified when the crust is relatively tight/laminar compared to the snow above/below (lower permeability).

Why it becomes dangerous: the crust can act as a bed surface and the facets become the weak layer. Or the crust becomes a stiff bridge that allows propagation once failure finally occurs.

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Canonical Formation Sequences (You’ll Recognize These)

Sequence 1: “Classic NSF → Burial”

1. Snowfall ends.
2. Clear, cold, calm nights follow.
3. Feathery / sugary near-surface facets form in the top 0–3 cm.
4. Next storm buries them → a persistent weak layer (PWL).

Common terrain: sheltered bowls, below ridgelines, treeline glades, convexities that clear quickly.

Sequence 2: “Sun Crust → Facets → Burial”

1. Short warming or sun creates a thin melt-freeze / sun crust (often on solar aspects).
2. Then cold high pressure moves in.
3. Facets grow on top of the crust (and sometimes just below it).
4. Next loading event (snow/wind) forms a slab → classic storm-on-crust persistent problem.

Common terrain: E/SE/S/SW aspects (but don’t assume—wind and brief warming can crust “weirdly”).

Sequence 3: “Wind Skin / Thin Hard Slab → Facets Under It”

1. Wind event forms a thin wind board / wind skin (laminar dense layer).
2. Cold gradient drives faceting beneath that dense cap.
3. Stiff cap over sugar → propagation potential once loaded.

Common terrain: near ridges, cross-loaded gullies, sub-ridges, behind terrain breaks.

Sequence 4: “Rain / Wetting Crust → Cold Snap”

1. Wetting (rain, drizzle, warm snow) produces a crust.
2. Rapid cold snap follows.
3. Facets develop at the crust boundary and can persist for weeks to months.

Common terrain: lower elevations, maritime-influenced storms, shoulder season.

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Field Identification Checklist (Fast, No Microscope Needed)

Visual + Feel

• Near-surface facets: sparkly, loose “sugar,” angular feel; doesn’t pack well.
• Crust-interface facets: sharp change: hard/slick crust + dry sugary layer immediately adjacent (above and/or below).

Hand Hardness Pattern

• Often shows Fist (F) or 4F facets near a Knife (K) / 1F crust or wind skin.
• Hard-over-soft is a red flag: easier propagation once a cohesive slab forms.

Quick Tests

• Hand shear focused on the crust interface: does it slide cleanly on the crust? Any collapse sensation?
• CT / ECT focus:
  • Sudden planar results on the crust interface → take it seriously.
  • ECTP on the facet/crust interface → propagation likely.

Where to Dig (So You Actually Find It)

• NSF hunting: shaded, sheltered pockets; treeline glades; below terrain breaks.
• Crust hunting: solar aspects (sun crust), lower elevation bands (rain/wetting crust), wind-affected start zones (wind skin).
• In most cases, inspect the top 50 cm (deeper if older crusts were buried early).

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Forecasting & Behaviour Blueprint

Stage A: Formation (Clear + Cold = Setup Phase)

• Instability may be minimal today, but the snowpack is being primed.
• Think: a trap being laid for the next loading event.

Stage B: First Burial / First Slab (Often 24–96 Hours After Loading)

• Often the most reactive window once a cohesive slab forms.
• Potential signs: cracking, small collapses, remote triggers in connected terrain (when slabs are “just right”).
• Highest risk when the slab is:
  • stiff enough to propagate, but
  • not yet so thick that it becomes stubborn/unreactive.

Stage C: Persistent Period (Days to Weeks+)

• These layers can linger as a low-likelihood / high-consequence problem.
• Spatially variable: may “switch off” in obvious places but remain in cold, shaded, sheltered pockets.

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Terrain Blueprint: Where It’s Most / Least Likely

Most Likely

• Shaded + sheltered: north-ish aspects, treeline glades, below rollovers.
• Solar crust zones: where sun crust forms, then cold preserves facet growth at the interface.
• Wind-structured start zones: thin stiff slabs over weak sugary snow.

Less Likely (But Not Impossible)

• Frequent wind mixing, cloudy nights, repeated small snowfalls that disrupt radiative cooling.
• Very warm snowpacks dominated by melt-freeze cycles (still: crust boundaries can facet after a cold snap).

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Operational If–Then Rules (Guide-Mode)

• If you had 2–10 days of clear, cold, calm weather after a storm, then assume NSF exists somewhere.
• If you find a thin crust and you’ve had a cold snap since, then suspect facets forming at that interface (above and/or below).
• If the next storm brings wind + cohesive slab, then manage it as a persistent slab problem until you have evidence otherwise.
• If you get ECTP on a crust interface, then avoid connected steep terrain and manage for propagation (thin spots, convexities, unsupported features).

One-Page Pit Workflow (For This Specific Problem)

1. Locate likely crust zones by aspect/elevation (or known wind event layers).
2. Dig to capture: surface → slab → crust → snow immediately above/below the crust.
3. Record:
   • Crust thickness + hardness
   • Facet layer thickness + feel
   • Slab character (storm slab vs wind slab; soft vs stiff)
4. Test:
   • One hand shear at the crust interface
   • One ECT targeting that interface
5. Map variability: one sheltered pit + one wind-affected pit in the same band often reveals the real pattern.

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Practical takeaway: laminar crusts and near-surface faceting are often “quiet” while they form. The problem appears when a slab arrives. Track setup → burial → persistence, and assume the weak layer exists until you can disprove it in the terrain you want to ski.