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Infrasound from Tornados: Theory, Measurement,
and Prospects for Their
Use in Early Warning Systems
Tornados may produce a low-frequency signature that could be used in automatic warning systems.
Introduction
Infrasound is propagating sound waves with frequencies below the range of human hearing. A practical frequency range for infrasound that will propagate over long distances is 0.01-20 Hz. The lower frequency cut off for propagating infrasound arises because the buoyancy of parcels of air becomes comparable to the pressure gradient forces of acoustic waves at sufficiently low frequencies. The precise fre- quency for this cut off occurs at the Brunt–Väisälä frequency (Stull, 1995), the exact value of which depends on details of the vertical profile of the atmosphere.
One of the more famous examples of long-range propagating infrasound is the ex- plosive eruption of Krakatoa Volcano in Indonesia. Audible sounds were heard as far away as 5,000 km, and low-frequency infrasound signals with periods greater than one minute were propagated around the Earth at least seven full times (e.g., Fee and Matoza, 2013).
Email:
rwax@olemiss.edu
Carrick Talmadge
Postal:
National Center for Physical Acoustics University of Mississippi University, MS 38677 USA
Email:
clt@olemiss.edu
Roger Waxler
Postal:
National Center for Physical Acoustics University of Mississippi University, MS 38677 USA
Figure 1. Annual average death rate from tornados. Also shown are the geographical re- gions referred to in the text. Note that these geographical regions correspond approximately to tornado climatological zones rather than to traditional geographical regions. Adapted from original figure by The National Oceanic and Atmospheric Administration (NOAA).
Because infrasound propagates for long ranges, it can be used for hazard moni- toring. For volcanic warnings, atmospheric blasts can be detected by infrasound stations even when the peak of the volcano is obscured by clouds so that optical observations are not possible. Another important infrasound source is hurricanes, which produce characteristic tonelike signals called “microbaroms” produced by
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