New Snow on Surface Hoar

12 January 2021, Portes du Soleil, France

In this article we look at the real life formation of a typical avalanche problem.  Storm slabs can form when we get significant new snow on top of a weak layer (like surface hoar).  Below we will look at:

  1. Overview video of storm slabs
  2. The state of the pack leading up to the 12th of Jan
  3. Incoming weather and the implication on avalanche risk

First have a look at the video below to get an overview of the problem.

It's one thing to know the theory, but how do we recognise this situation as it develops around us?  Below we look at a concrete example by examining the state of the pack, and the change in avalanche danger level for the Portes du Soleil region in France on 12 January 2021.

Snowpack overview & history

The French Meteo avalanche risk bulletin valid until the 11th of January had the avalanche risk at level 2 (Moderate) for upper and lower mountain.  The last proper snow was on the 1st of January, and since then most of the existing avalanche problems had largely consolidated.  As can be seen in the avalanche bulletin extract below, the days leading up to the 10th it was fairly cold.

Isotherme is the altitude of freezing level (0 degrees).

To get an idea of the temperatures on the mountain, look at the freezing level in the graph above, and then keep in mind that dry air (i.e. not in cloud) typically cools at 3 degrees celsius for every 100m of altitude gain (called the Dry Adiabatic Lapse Rate)

If you've been out and about at all in the days leading up to the 12th of January you would have seen absolutely stunning and prolific surface hoar.  Surface hoar is essentially frost forming on top of snow and it results in beautiful shimmering crystals as can be seen in the photo below.

photo credit: float backcountry

Surface hoar is beautiful, but it should set your spider senses on alert when you see it. The extract below is from this great article, and highlights the nature of this beautiful beast.

Surface hoar is a fancy name for frost. When you have to scrape your windshield in the morning, surface hoar grows on the surface of snow—hence its name. It grows during clear, humid and calm conditions and once buried, it is a particularly thin, fragile and persistent weak layer in the snowpack, which accounts for a number of avalanche deaths each season.  Surface hoar is an especially tricky weak layer because it can form very quickly. One calm, clear night—sometimes just a few hours—is enough time to deposit a thin layer on the snow surface. And once buried, it is very thin and difficult to detect, yet very weak. Also, it tends to form in a complex, hard-to-predict distribution pattern on the terrain. For instance it might form only above a certain elevation where the mountain rises above the clouds. It might form below a certain elevation where cold, humid air pools... The best way to detect surface hoar is to carefully pay attention to the snow surface each day

To summarise, the snowpack was stable and consolidated but it was very cold leading up to the 12th, and conditions were ideal for widespread surface hoar formation.

New weather on the 12th of January

The second piece of the puzzle is the arrival of significant new snow.  On the morning of the 12th of January the forecast was for new snow in the region of 45 - 75cm (depending on the forecast model you looked at)  to fall between the 12th and 15th of Jan.

Wind at altitude is also a crucial part of the picture. Winds at 2500m was forecasted to be 40 - 60km/h from the NW / W. This is helpfully included in the french meteo avalanche bulletin. You can also get fantastic air profile forecasts that show winds at different altitude - our favorite source is the meteoblue air forecast

When taking into account wind strengths, be sure to look at the wind at altitude, and not the value forecasted for the town in the valley.

Now you have the full picture of the state of the pack and the incoming weather.  Before moving on, have a think about the avalanche implications.  Where do you think think the main problems will form?  How would you adjust your plans for the coming days?

Avalanche risk implications

Lucky for us the highly dedicated and skilled staff of the Meteo France and their partners on the ground saw this coming a mile off. In their bulletin issued on 4pm of the 11th of January, they already warned of an increase in avalanche risk from level 2 (moderate) to level 3 (considerable) for the 12th. (Understanding avalanche danger levels is crucial! To learn more see our article on it here). Below is the bulletin issued on the afternoon of the 12th, where they raised the Avalanche risk to level 4 for the first time in the season.

As can be seen the risk has shot up, and they warned of even further increase as the storm snow accumulates. The bulletin at 4pm on 13th of January reinforced this with level 4 for all aspects and at all elevations. It also stated:

The preventative trips made this Wednesday often give positive results, setting in motion the snowpack 40/50 cm thick.  Woufs have been reported by several observers

Float Backcountry headed out on the 13th (staying in low angled terrain) , and all around we saw the signs of instability and natural slides.

cracks and natural slides abound on the 13th of January. photo credit: Float Backcountry

For several days after the 13th, the few Ski Touring runs that were open in the French Portes du Soleil area were closed while they performed extensive avalanche control blasting.  The internet was also abound with videos of massive avalanches all through the alps.  Unfortunately there were also some lives lost.

The avalanche bulletin kept the risk at level "4 - High" for 6 days while the snow pack consolidated, and on the 19th of January it was reduced to level 3.

The moral of the story:  our understanding of the risk will depend on how much attention we pay and how much effort we put in.  The take-aways here are:

  • Pay attention to the state of the pack throughout the season
  • Stay aware of the weather on the mountain and add that to your mental picture
  • Make checking the avalanche bulletin and weather forecast part of your daily routine
  • When the avalanche risk is high, adjust your plans - stick to terrain with less than 30 degree slope, and stay out of avalanche run out zones. For some examples take a look at our Avy Savvy Route Choices post.
  • Forming these simple habits might just save your life one day

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Happy Trails!