This article originally appeared in The Powder Cloud, March 25, 2024. Minor revisions here.

Hey, Northerners! Have you ever skied soft snow, weeks after the last storm, that has been preserved by the mid-winter bitter cold? Some call it recycled powder or settled powder, but it’s actually an undocumented type of snow. Colloquially known as urnal near-surface facets. These facets may preserve good skiing while on the surface, but once buried, they often create a persistent weak layer for avalanches.

Urnal faceting is unique to the wintery north, such as central Alaska, where the sun doesn’t shine or its oomph has been filtered out by its low angle. This consistent cold, combined with clear skies, causes the snowpack’s top layer—30 cm—to maintain a steep temperature gradient that’s conducive to faceting.

Near-surface faceting produces a layer of small (up to 1 mm) faceted crystals on the top of the snowpack. In 1998, snow scientist Karl Birkeland presented three processes that form near-surface facets, depending on the source of the steep temperature gradient. 1) Diurnal near-surface faceting occurs from a daily cycle of warm days and cold, clear nights. 2) Radiation near-surface faceting occurs from a balance between incoming solar radiation and outgoing longwave radiation. 3) Melt-layer near-surface faceting occurs when new cold snow is deposited on a melted-layer and is followed by clear skies.

The different near-surface faceting processes are driven by changing the energy balance inputs; temperature, sun, or a melt-layer. It’s a continuum of processes. Birkeland also mentions a fourth category that occurs at high latitude, which is what we’re calling urnal facets, a term possibly coined by Alaska helicopter ski guide Henry Munter. It’s like diurnal faceting, but without the daily cycle. While urnal faceting is a specific term, it’s okay to simply call these near-surface facets, or surface facets. They often form together with surface hoar—the frozen equivalent of dew that forms a weak and persistent layer—producing a double whammy weak layer once buried.

The snowpack temperature profile during urnal faceting remains consistent during the cold and clear weather. It keeps a steep temperature gradient in the top 20-30 cm of the snowpack. At about 30 cm depth is a distinct kink in the temperature profile where it shifts to a shallow temperature gradient to the ground.

So, the big question is, why is the temperature profile kinked? It other words, why doesn’t the temperature profile become a straight line as a shallow temperature gradient throughout? I took this question to the world authority of near-surface faceting, Karl Birkeland.

Birkeland says that a number of influences keep the temperature profile kinked: 1) snow is a very effective emitter of longwave radiation, 2) space is really cold, and 3) when the sky is clear so much longwave is emitted that the snow surface just cools and cools. “Our sense is that the end result is that the rest of the snowpack is just unable to keep up with that heat loss, especially since conduction within the snowpack is not as efficient for energy transfer as longwave losses from the surface.”

The original diurnal faceting diagram from Birkeland’s 1998 paper. Urnal faceting is effectively the night portion (right side) of his diagram in which the snow surface is losing longwave radiation both night and day, while the temperature around 30 cm depth remains relatively warm.

January 2022 at Turnagain Pass was a good example of an urnal faceting and avalanche cycle. From January 1-7 the skies were clear, the air temperature was around 0°F, and the wind was light (Schauer and others 2023). A storm on January 8-13 brought snow, wind, and avalanches releasing on the buried layer of urnal facets.

This chart shows the snow temperature at the Tincan study site at Turnagain Pass, Alaska during the January urnal faceting cycle. The vertical temperature profile (red line) shows a distinct turn in the temperature profile at 300 cm which is the snow surface. From about 300 cm to 270 cm the temperature gradient is steep—over 10°C per 10 cm—and well beyond the 1°C per 10 cm required for faceting metamorphism. At 270 cm the temperature profile kinks between the steep temperature gradient above and the shallow temperature gradient below.

Small faceted crystals about 0.5 mm across formed by urnal near-surface faceting at Turnagain Pass, Alaska.

For recreational skiers, it’s good to keep urnal faceting in perspective. It is yet another fascinating attribute of the snowpack. Having a keen interest in snowpack, and nature in general, is a good thing. But understanding the nuances of urnal faceting doesn’t necessarily improve your backcountry safety. What’s important here is that facets form a persistent weak layer once buried. Persistent weak layers are unpredictable and kill the most skiers. It doesn’t matter how they’re formed.

References

Birkeland, Karl. 1998. Terminology and Predominant Processes Associated with the Formation of Weak Layers of Near-Surface Faceted Crystals in the Mountain Snowpack. Arctic and Alpine Research 30(2):193.

Schauer, Johnston-Bloom, McKee and Kennedy. 2023. Crusts and Facets: A Case Study of a Season with Deep Issues Near Girdwood, AK. Proceedings of the International Snow Science Workshop.