World of Science | Avalanche triggered by sound?

The Austrian army is currently practising flying its so-called Eurofighters. These are the infamous Typhoon fighter planes from Eurofighter Jagdflugzeug GmbH, which were purchased for the army by the Schüssel government in an "opaque process possibly accompanied by bribes amounting to 100 million" (Wikipedia)

Flight maneuvers are planned until 17 November, during which supersonic speeds are to be reached twice a day. These flights were supposed to take place in the Salzburg area, but have now been moved to the lowlands due to the snow conditions. "There is so much snow in Salzburg, Tyrol, Vorarlberg and Carinthia that avalanches could be triggered by a sonic boom," Major Martin Baierer from the airspace surveillance authority is quoted as saying. ORF report.

Can sound trigger avalanches?

Avalanches usually either occur on their own, for example due to the ever-increasing load of fresh snow during a precipitation event, or they are triggered by external forces on the surface, for example by skiers or avalanche blasts.

An SLF study (Reuter, B, Schweizer, J, 2009. Avalanche triggering by sound: myth and truth. International Snow Science Workshop Davos, Processdings) deals with the question of whether avalanches can also be triggered by loud shouting, airplanes or even a sonic boom:

Any kind of noise produces waves in the air that propagate at the speed of sound (approx. 340m/s). Sound waves are longitudinal waves, which means that they oscillate in the direction in which they propagate. (When we think of a wave, we usually think of transverse waves, which oscillate perpendicular to the direction of propagation). Sound waves are pressure waves - air of different densities moves from the source of the sound to our ear.

The force effect on a snowpack caused by loud shouting, airplane noise, sonic booms and pressure waves produced by blasting can be physically described as the impact and penetration of the respective waves into the snowpack. (A skier on the snow surface, on the other hand, is usually approximated as a near-surface force acting on an elastic half-space. Note: a skier is not a wave.)

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Avalanche blasting

The pressure waves produced by noise or blasting therefore penetrate the snow cover and influence it. However, some wave energy is lost during the transition from the air to the snow. Explosives are best detonated 1 to 2 meters above the snow surface, as this allows the waves to spread over a larger area before they reach the snow, in which they can then no longer spread far. This increases the effective radius of the blast. If the explosive device were detonated in the snow, the snow would virtually swallow it up and the shock wave would hardly spread at all.

In the case of avalanche blasting, the "effective range" is defined as the area in which the blast exerts at least as much force at a depth of 50 cm as a skier would exert. Measurements have shown that skiers exert a dynamic load of around 200 to 1200Pa on the snow. To be on the safe side, values of around 1500Pa are assumed when calculating the blast range - the blast pressure of the explosion must therefore reach at least this value. If a 2.5kg explosive device is detonated at a height of one meter, this results in an effective range of around 40m.

A sound pressure of 2Pa is specified for a loud scream and 20Pa for a jet plane, i.e. much less than a skier can manage. A landing helicopter only causes a pressure of around 10Pa in the snowpack.

Supersonic boom

The term supersonic boom is generally used to describe the shock wave that an airplane drags behind it at supersonic speed" or the audible part of the shock wave. At a certain position you only hear the bang once, but the wave is there as long as the aircraft is flying at supersonic speed. It therefore reaches the ground not just at one point, but over a relatively large area and for the entire duration of the supersonic flight.

Shock wave caused by supersonic flight. The observer hears the bang when the cone touches his position for the first time.

Wikipedia CC

Old studies from the 1960s and 70s on the subject of sonic booms and avalanches contain measured values of 200 to over 500 Pa sound pressure on the ground with a supersonic aircraft flying 900m above the ground (according to the army, the Eurofighters fly at over 12500m above sea level during exercises in order to reduce noise pollution on the ground). During the charmingly titled "Operation Bangavalanches", a few small loose snow avalanches combined with above-average sound pressure were recorded on only one of the 20 test flights. The flights were carried out over a period of 7 days in a "critical" avalanche situation. In 1965, in a similar test, no avalanches were triggered at all, albeit with a generally low avalanche danger. In a further study in 1972, the force of the sonic boom was simulated using explosives. Here it was possible to trigger avalanches in 3 out of 4 cases, but only at load values of well over 500Pa.

In the more recent study, Reuter and Schweizer assume that loads of 200-500Pa are required to produce a fracture in a weak layer with low stability. Correspondingly greater loads are required for more stable snow cover.

Summary:

Exposures due to different pressure waves according to SLF study:

Loud scream: 2Pa
Jet plane: 20Pa
Supersonic blast: 200Pa (up to 500Pa depending on the study)
Burst: >1500Pa
Applicable for fracture in weak layer in very unfavorable avalanche situation: 200-500Pa.

As far as we know, there are no confirmed examples of avalanches that were definitely triggered by a sonic boom. The results of practical tests in the 1960s and 70s, in which avalanches were actually triggered by sonic booms, indicate that avalanches triggered by sonic booms are very unlikely. The possibility of triggering avalanches with supersonic flights is therefore very theoretical.

The fact that the Austrian Armed Forces still want to play it safe is of course legitimate. However, avalanche expert and mountain guide trainer Paul Mair, who brought this topic to our attention, criticizes the rather sensationalist media coverage in view of the very low probability of avalanches being triggered by the Eurofighter exercise:

"What remains and is reinforced by this is the general avalanche hysteria