Abstract
Every day, about one thousand thunderstorms occur around the world, producing about 45 lightning flashes per second. One prominent infrasound station of the International Monitoring System infrasound network of the Comprehensive Nuclear-Test-Ban Treaty Organization for studying lightning activity is in Ivory Coast, where the lightning rate of this region is relatively high. Infrasound defines acoustic waves with frequencies below 20 Hz, the lower limit of human hearing. Statistical results are presented in this paper based on infrasound measurements from 2004 to 2019. One-to-one association between infrasound detections from 0.5 to 5 Hz and lightning flashes detected by the World Wide Lightning Location Network within 500 km from the infrasound station is systematically investigated. Most of the infrasound signals detected at IS17 in this frequency band are due to thunder, even if the thunderstorms are located up to 500 km away from the station. A decay of the thunder amplitude with the flash distance, d, is found to scale as d−0.717 for flashes within 100 km from the station, which holds for direct propagation. Interestingly, the stratospheric detections reflect a pattern in the annual azimuth variation, which is consistent with the equatorial stratospheric semi-annual oscillation.
Highlights
Do elephants hear the thunder from far away well enough to make their way to recently watered regions? This possibility has been evaluated, taking into account the lowfrequency hearing capacity of these pachyderms and the infrasonic thunder propagation range, by several authors (e.g., [1])
For a detailed analysis of the association of infrasound detections with thunderstorm activity and lightning flashes, we show the comparison of the lightning activity and the infrasound detections for a typical case
We looked for the number of World Wide Lightning Location Network (WWLLN) detections that are in good agreement with its azimuth within a 30 min interval
Summary
Do elephants hear the thunder from far away well enough to make their way to recently watered regions? This possibility has been evaluated, taking into account the lowfrequency hearing capacity of these pachyderms and the infrasonic thunder propagation range, by several authors (e.g., [1]). Infrasonic waves, or infrasound, are acoustic waves with frequencies lower than 20 Hz, which is approximately the lower limit of human hearing. Elephants are capable of hearing sounds with frequencies of 10 Hz and possibly lower. From a historical point of view, thunderstorm activity over different geographical regions was evaluated through an empiric index named the “keraunic level”. The first estimations of global lightning activity were based on thunder hearing by humans [2]. Lacroix et al [3] have pointed out that arrays having an aperture in the kilometer range prevent precise 3D reconstruction; Farges et al [4] showed that beyond 20 km it is not possible to reconstruct in 3D the lightning channels
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