It didn't snow in the Alps for quite a while. The nights were cold and there were no clouds. That resulted in a layer of surface hoar on lots of north faces, which can become dangerous after the next snowfall.
Surface hoar is formed on top of the snow cover and consists of large crystals, often in the form of feathers, wedges, or a kind of oversized chocolate flakes. Surface hoar is not a form of precipitation, but rather a result of a nights without clouds. During those nights the snow cover is able to radiate a part of its energy. Moisture is released in this process, that moisture condenses and sticks to the snow pack in the form of surface hoar. That effect is even close to creeks and rivers that don't get to much sun. Considerable amounts of surface hoar can form during long periods without any clouds. If it is cloudy or if there's a closed pack of trees above the snow cover, it's not possible for the energy to radiate and it's impossible for the surface hoar to form.
I saw a lot of surface hoar when I was ski touring in the Hautes Alps yesterday. Sometimes one or a few centimeters thick, but sometimes as much as 15 centimeters thick. Surface hoar feels just as light and dry as powder and it's great skiing. When you ski a bit faster it can hurt a bit when the crystals hit your face, but that's worth it.
Surface hoar itself isn't dangerous and the skiing is good. But only if it's the top layer of the snowpack. It becomes an immediate thread when fresh snow falls on top the surface hoar. A layer of surface hoar is extremely weak and a typical sliding layer for avalanches. Obviously the layer is not visible with the naked eye anymore. The biggest problem is that it could take weeks or sometimes even months before the snowpack is stable again. Let's hope that the next dump will be a big one that will put the layer of surface hoar deep into the snow cover.
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