Predits before snow avalanche, surface hoar makes perhaps the perfect weak layer. It’s thin, very weak, notoriously persistent, and it commonly forms on hard bed surfaces, which are also slippery. Escape snow avalanche. When they are critically loaded, just one thump will catastrophically collapse all the columns, like the old college trick of standing on an upright empty beer can without crushing it, then one tap of a finger and crunch! Ready for the recycle bin.

Surface hoar, avalanche

This is, in fact, the most common scenario for surface hoar, as well as other persistent weak layers: often the first storm on top of a surface-hoar layer does not weigh enough to overload it, but the second or third storm finally adds up to the critical weight. Whamo!

Just like the college beer-can experiment.

Most importantly, surface-hoar avalanches occur on gentler slope angles than other types of weak layers which often takes people by surprise—including me. My partner and I once stood at the top of a mostly 25-degree slope, which had just one very small section of 30-degree slope. I told him, “I’ve never seen this slope slide.”

And, of course, he jumped in and promptly triggered a small soft slab on surface hoar that slowly washed him a couple hundred feet (thirty or so meters) down the gentle slope, where he ended up on the surface. It seems like the universe always knows when pride needs a spanking.

And then there was that time when I was backcountry skiing on a day off with a few friends, including my friend the then mayor of Salt Lake City, and I had been warning everyone all day long not to go over 30 degrees in steepness because of the widespread surface hoar problem. But late in the day, someone skied over a rollover close to 30 degrees and triggered a shallow, slow-moving soft slab that oozed down a short distance toward where most of the group had gathered.

I envisioned the breaking news scroll, “Avalanche Expert Buries Salt Lake Mayor in an Avalanche,” or something worse. But luckily, the slow-moving debris oozed slowly down the 25-degree slope and stopped short of people below. Everyone learned a cheap lesson about why surface hoar is a different animal from most avalanches.

Forecasting Considerations

Surface-hoar crystals are notoriously persistent in the snowpack, and they slide on gentler slope angles. Instabilities commonly last for a week or two and up to months in cold snowpacks. Surface hoar on a hard, slick bed surface is especially dangerous because it lasts longer; is easier to trigger; and has little friction, so it slides on surprisingly gentle slopes.

And remember, surface hoar can be very sneaky. You may go to bed during a snowstorm, and it might still be snowing and cloudy when you wake up. But during the night, unbeknownst to you, the winds died down, the sky cleared for a few hours, and a thin layer of surface hoar formed.

The next day you notice sensitive soft slab avalanches within the new snow and expect them to calm down after a day, as usual, but instead, they last for several days. You dig to investigate and find the culprit. Darn that sneaky surface hoar!

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