An avalanche (also called a snowslide or snowslip) is a rapid flow of snow down a sloping surface. Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche). After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current.

Slides of rocks or debris, behaving in a similar way to snow, are also referred to as avalanches. The remainder of this article refers to snow avalanches.

The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches. Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger the failure in the snowpack and avalanches. A popular myth is that avalanches can be triggered by loud noise or shouting, but the pressure from sound is orders of magnitude too small to trigger an avalanche.

Although primarily composed of flowing snow and air, large avalanches have the capability to entrain ice, rocks, trees, and other material on the slope, and are distinct from mudslides, rock slides, and serac collapses on an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack. Avalanches are most common during winter or spring but glacier movements may cause ice and snow avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective natural hazards to life and property, with their destructive capability resulting from their potential to carry enormous masses of snow at high speeds.

There is no universally accepted classification of avalanches - different classifications are useful for different purposes. Avalanches can be described by their size, their destructive potential, their initiation mechanism, their composition and their dynamics.

Most avalanches occur spontaneously during storms under increased load due to snowfall. The second largest cause of natural avalanches is metamorphic changes in the snowpack such as melting due to solar radiation. Other natural causes include rain, earthquakes, rockfall and icefall. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work.

Avalanche initiation can start at a point with only a small amount of snow moving initially; this is typical of wet snow avalanches or avalanches in dry unconsolidated snow. However, if the snow has sintered into a stiff slab overlying a weak layer then fractures can propagate very rapidly, so that a large volume of snow, that may be thousands of cubic meters, can start moving almost simultaneously.

A snowpack will fail when the load exceeds the strength. The load is straightforward; it is the weight of the snow. However, the strength of the snowpack is much more difficult to determine and is extremely heterogenous. It varies in detail with properties of the snow grains, size, density, morphology, temperature, water content; and the properties of the bonds between the grains. These properties may all metamorphose in time according to the local humidity, water vapor flux, temperature and heat flux. The top of the snowpack is also extensively influenced by incoming radiation and the local air flow. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time. A complicating factor is the complex interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack. Read more

In the News ...

Is there a connection between activity at CERN (LHC) and ongoing earthquakes - and the resulting Avalanche - in Italy? Are the experiments stressing the fault lines? The Large Hadron Collider lies in a tunnel 27 kilometres (17 mi) in circumference, as deep as 175 metres (574 ft) beneath the Franco-Swiss border near Geneva, Switzerland. I wonder if the LHC will open a wormhole or black hole to the source of origin or another time stream.

Many feared dead as avalanche buries Italian hotel after earthquakes   CNN - January 19, 2017

At least one person has died and many others are feared dead after an avalanche buried a hotel in central Italy following a series of earthquakes. At least 22 guests and several staff members were in the Hotel Rigopiano, at the foot of the Gran Sasso mountain, when it was hit by the avalanche on Wednesday, Antonio Crocetta, a rescue group leader in the area told Italian state media.

  Video inside the hotel   Independent - January 19, 2017

Italy avalanche: 'Many missing' in Rigopiano hotel in Abruzzo   BBC - January 19, 2017
The roof on the four-star spa hotel, close to the Gran Sasso mountain in the Abruzzo region, partly collapsed on Wednesday night. Rescuers said at least 20 tourists and seven staff were inside at the time. Crews had been calling out to survivors but there was no response. Local residents in Farindola alerted emergency services.

  Rome metro evacuated as 5.3-magnitude earthquake rocks Italy   CNN - January 18, 2017
4 tremors 20 minutes apart ... The metro system and some schools in Italy's capital, Rome, were being evacuated Wednesday following an earthquake that struck the country's heart. A 5.3-magnitude quake hit central Italy 7 kilometers (4.3 miles) west-northwest of Amatrice, a town devastated by a series of powerful temblors last year, the US Geological Survey reported. It struck at 10:25 a.m. (4:25 a.m. ET) at a shallow depth of 10 kilometers (6.2 miles), it reported. It was followed by a more powerful 5.4-magnitude aftershock. There are no immediate reports of casualties or damage. Nearly 300 people were killed in central Italy in an August quake and a series of aftershocks that reduced much of Amatrice's center to rubble.

4 Quakes hit snow-bound central Italy   BBC - January 18, 2017
Central Italy has been struck by a series of earthquakes, as the region shivers in freezing temperatures. The biggest quake was 5.4 in magnitude and struck an area devastated by earthquakes in August. The tremors hit between 10:25 (9:25 GMT) and 11:25 and were felt as far away as Rome. Almost 300 people died in the mountainous region around Amatrice last year. There are no reports so far of any injuries in the latest quakes.

'Smart boulders' record huge underwater avalanche   Live Science - December 13, 2016

Scientists have had a remarkable close-up encounter with a gigantic underwater avalanche. It is the first time researchers have had instruments in place to monitor so large a flow of sediment as it careered down-slope. The event occurred in Monterey Canyon off the coast of California in January. The mass of sand and rock kept moving for more than 50km, as it slipped from a point less than 300m below the sea surface to a depth of over 1,800m. Speeds during the descent reached over 8m per second.

  What causes an avalanche?   Telegraph - November 3, 2016
In this video we examine how an avalanche is triggered, resulting in a dramatic collapse of snow from the mountainside. There are three main elements that make them more likely to occur; the gradient of a slope, the quantity of snowfall and the climate conditions. Avalanches occur most commonly in areas with repeated, heavy snowfall.