If an avalanche occurs in the mountains and there is no one to hear it, does it make a sound?

Yes it does. And that sound can be used to count, measure and map avalanches. Leighton Watson from the University of Oregon in the USA, with colleagues, has tested a system of using microphone arrays to detect and characterise avalanches falling on to the Milford Road. They installed two sets of “infrasound” sensors near the Homer Tunnel in September 2020. The very next day the Waka Kotahi’s Milford Road Alliance triggered seven large avalanches in the area by dropping explosives from helicopters onto the mountain snow pack. Leighton’s microphones clearly recorded the sound of the avalanches. In fact the sounds were stronger than had been recorded anywhere else in the world. Through cunning trigonometry of the arrival time of the sound at the different microphones, Leighton was able to track and map where the avalanches were occurring. His maps were validated by the observations of the Milford Road Alliance avalanche team. The microphones were left in place for a month and continued to collect avalanche sound data, including naturally occurring night time avalanches. In each case the avalanche location was mapped. Current avalanche risk assessment relies on incomplete avalanche occurrence data, as observation techniques rely on human observation of events or debris. This is problematic in remote regions or during low visibility in storms or at night. Leighton and his colleagues have demonstrated a new approach which could improve avalanche frequency knowledge and make our mountains safer.

Watson, L.M., Carpenter, B., Thompson, K., Johnson, J.B., 2021. Using local infrasound arrays to detect plunging snow avalanches along the Milford Road, New Zealand (Aotearoa). Nat Hazards. https://doi.org/10.1007/s11069-021-05086-w

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