On the power spectrum and mechanism of thunder

Abstract

Forty thunder events from intracloud and cloud‐to‐ground lightning were analyzed by analog and digital power spectrum methods. Thunder exhibits a low Q(0.5 to 2) spectrum with peak power observed at frequencies from <4 to 125 Hz. Significant differences are noted between thunder from intracloud and cloud‐to‐ground lightning. Intracloud thunder spectrums show a mean peak value of power at 28 Hz with a mean total acoustic energy of 1.9×1013 ergs. Cloud‐to‐ground spectrums show a mean peak value at 50 Hz with a mean total acoustic energy of 6.3×1013 ergs. The mean total acoustic efficiency for discharges to ground is calculated to be 0.18%. The thunder power spectrum is time varying. The mechanism of thunder production by a thermally driven expanding channel as described by A. A. Few appears to account for the dominant frequencies observed from many cloud‐to‐ground flashes, but cannot explain the high‐energy low‐frequency peaks of some cloud‐to‐ground and most intracloud discharges. The electrostatic mechanism of C. T. R. Wilson and of S. A. Colgate is proposed to explain the large energy peaks at frequencies less than 10 Hz.

NOx from lightning: 1. Global distribution based on lightning physics

“…To do this, assumptions have to be made regarding the above parameters. Furthermore, since it appears that intracloud (IC) flashes have significantly lower energies than cloudto-ground (CG) discharges [Holmes et al, 1971; Kowalczyk and Bauer, 1981], it is incorrect to divide the total NO• observed over a certain period by the total number of flashes.…”

Cross Spectral Analysis of Acoustic Signals

“…its time dependence) Holmes, et al (1971) and McCrory (1971), found that the power spectrum had peaks at low frequencies and that these peaks were inconsistent with the theory that thunder was caused totally by the acoustic mechanism described above. As a possible explanation, these investigators put forth the theory that a second mechanism was in operation --this mechanism was proposed originally by Wilson (1920) and studied further by Colgate (1967) and Colgate and McKee (1969).…”

“…Various studies of the power spectra of thunder have been carried out by Few (1968), Few, et al (1967), McCrory (1971) and Holmes, et al (1971). The earlier studies of Few, et al…”

“…(1967), indicated a peak in the spectrum at about 200 HZ, but more accurate estimates of the spectrum by McCrory (1971) 6 and Holmes, et al (1971) show that the peaks are lower than this --for ground flashes they measured peaks (after correction for wind-noise) in the 40 to 80 HZ range. Some attenuation occurs due to propagation through the medium, with the higher frequency portions of thunder suffering the greatest attenuation.…”

“…Holmes, et al (1971), McCrory (1969), and Few (1968and Few ( , 1969, among others, have analyzed the acoustic signal generated by thunder. Although the shape of the spectrum is not completely known, various aspects of the spectrum are discussed in the papers mentioned above.…”