Some science behind the scenes

Avalanches

An avalanche starts when an unstable mass of snow breaks away from a mountainside and moves downhill. It picks up speed as it rushes down the mountain, and may also pick up ice, soil, rocks and uprooted trees. Some reach speeds of 245 mph.  And in doing so they often create infrasound.

Avalanches produce infrasound, but clearly an avalanche does not generally last for long, thus the duration of the experience – if a spiritual experience is invoked – can be very short.  So short a person may not realise they are hallucinating for example.  If the avalanche lasts long enough and produces the right frequency then there is no reason why we might not see all sorts of hallucinatory objects – buildings that aren’t there and also figures in the landscape – like abominable snowmen!

One useful source of measurements about avalanches and their frequencies is available from the monitoring stations in Alaska.  Infrasound that is created by snow and ice avalanches on the glacier-hung peaks in the Alaska Range is frequently observed in Fairbanks, where many of the monitoring stations are. Mt. McKinley rises to an elevation of 20,300 feet to the southwest of Fairbanks at distance of 236 km at a bearing of 222 degrees. Avalanches on the great north face of McKinley drop 19,000 feet from the summit to the Peters glacier at base of the mountain. This means they have plenty of time to generate a long sustained level of sound.

A very large infrasonic signal from an azimuth of 221 degrees that came from a McKinley avalanche at 10:18 UT on April 8, 2005 is shown below. The peak-to-peak amplitude of the avalanche infrasound wave train was 0.9 Pa. The trace-velocity of the signal was 0.339 km/sec.  As you can see the signal lasted about 2 minutes reaching a peak amplitude after about a minute.

Chart of very large infrasonic signal from a McKinley avalanche at 10:18 UT on April 8, 2005

Because infrasound is capable of travelling great distances, it is possible for an avalanche hundreds of miles away to be ‘experienced’.  For example, a large rock and ice avalanche occurred 540 km southeast of Fairbanks, Alaska on September 14, 2005. According to the Alaska Earthquake Information Center in Fairbanks, approximately 50 million cubic meters of rock and ice plummeted 2.6 km from the south face to the base of 3100 meter Mt Steller.

It is estimated that the debris run-out extended 9.5 km from the base of the mountain. [This event is described by Kenneth M. Arnoult Jr. et al, “Infrasound Associated with Mt. Steller Avalanche”, in the on-line journal Inframatics, Number 12, December 2005 pages 4 to 7].

Alaska and the mountains of the Arctic are not the only high mountains to produce infrasound.  The mountains on Ross Island, to the north of the monitoring array at Windless Bight, Antarctica, are a frequent source of avalanche infrasound. In particular there is an ice fall named Vee Cliffs on Mt. Terror 15 kilometers to the north-northeast of one monitoring station  that produces ice avalanches.  

The figure below shows a large amplitude avalanche signal observed at I55US from Vee Cliffs on April 6, 2004.

Vee Cliffs avalanche signal at Windless Bight. April 4, 2004.

References

  • McClung, David and Schaerer, P.; "Avalanche Handbook" - A classic reference which is thorough but quite technical..
  • Gray and Male, ed.; "Handbook of Snow"; Pergamon Press - This is a more detailed and technical book which covers most aspects of snow science, not just avalanches.
  • LaChapelle, Edward; "Field Guide to Snow Crystals"; Univ. of Wash. Press - Listed because it has many good photos of different types of snow crystals. available through the International Glaciological Society
  • Frankenfield, James; "Snow and Avalanche Physics"; USU Thesis 1989 - This is a technical treatment of the subject and includes a large bibliography of scientific journal articles.