Glide snow avalanches are the hip and happening in the land of snow safety right now. And not without reason. Anyone who has been in the Alps in recent weeks could and can not ignore it. I have rarely seen so many glide snow avalanches (French: avalanche de glissement/avalanche de plaque de fond, Italian: Valanga per scivolamento di neve) as in the past few weeks. In Tirol the last month is No less than 90% of the reported avalanches in Tirol the last month are glide snow avalanches.
A unique situation and the problem is that they are very difficult to predict. But before we dive deeper into this subject, first of all an introduction to the three main types of avalanches (source: Mountain Academy):
The main type slab avalanche is the so-called dry slab avalanche which usually originates on slopes steeper than 30 degrees and with a sharp, horizontal fracture line. As many as 90 percent of all fatal avalanches involving skiers and snowboarders are slab avalanches triggered by the victim or members of the group. You'll need four ingredients for this kind of avalanche: a slope of at least 30°, a bonded layer, a weak layer and a trigger. You'll read more about it in this paragraph of Mountain Acacemy.
The loose snow avalanche is generally not as dangerous for freeriders, but do not underestimate these slides. A loose snow avalanche starts at one point, usually on a slope steeper than 40 degrees and fans outward as it descends. Essentially, a small amount of snow becomes unsettles and starts a chain reaction—the quintessential snowball effect. Loose snow avalanches can be both dry (sometimes they are called sluffs) and wet. Though they are unlikely to bury you, a loose snow avalanche can knock you off your feet, and is particularly worrisome if you are above another hazard, such as a cliff, exposed rocks or a crevasse. You'll read more about it in this paragraph of Mountain Acacemy.
The winter of 18/19 is the winter of the glide snow avalanche and they just behave completely different. This is a purely sliding process in which the entire snow cover can slide off a slope with sometimes fatal consequences. Glide snow avalanches are the result of a melting processes on the boundary between the snow cover and the soil. The more humid the surface of the soil, the lower the friction and the greater the chance of a glide snow avalanche. Essential here is a smooth surface (for example slippery rocks and alpine meadows, but also roofs of houses). Glide snow avalanches can - in contrast to loose snow or plate avalanches - also occur on slopes below 30°.
During snowy winters, a lot of panic arises at the time of the long, intense and especially constant snowfall. A high avalanche danger, with a high possibility of loiose snow avalanches. Often good to predict and that's the reason why roads are closed for example.
During heavy snowfall in combination with wind, there is also warning for slab avalanches. Rightly so, because the new snow in combination with the wind creates a temporary unstable situation. But because of the (often) lack of "old snow" and the fact that there is a lot of snow, results in a snow cover that settles fast. The slab avalanche problem is therefore often a temporary problem.
Why do you see so much glide snow avalanches during snowy winters? For the following two reasons:
The combination of a lot of mass, moisture and a smooth ground (smooth rocks and alpine meadows) thus ensures all the glide snow avalanches.
Winters are getting up and running later more and more often. According to Rudi Maier, chief of the avalanche service of Tyrol, this is a major cause of the many glide snow avalanches. Autumn is often mild for a long time, and if the first major snowfalls follow later in the year, the ground is often not frozen. If there is a lot of snow, the heat is isolated and a permanent problem arises for the rest of the winter. There is no solution to the problem.
Glide snow avalanches are seen more often during a period of high temperatures because the snow cover then becomes more humid (and therefore even heavier), but can also occur on the coldest day of the year in the middle of the night. Another disadvantage of glide snow avalanches is that it's really hard to trigger them by purpose. A lot of dynamite creates a considerable gap, but the snow cover is not necessarily sliding down (as you can see with slab avalanches and loose snow avalanches). Finally, they prove very difficult to predict. We never know where and when they go.
With a further global warming of the climate there is a realistic chance that the number of glide snow avalanches will increase and the bad news is that they are very difficult to predict and really hard to artificially trigger. The myth that frosty nights solve the problem is just, well, a myth. With a thick snow cover, the cold air does not reach far into the snow cover. Annoying, but true.