~ National Lightning Safety Institute ~
The Thunder Mechanism
By Richard Kithil, President & CEO, NLSI
Thunder is the pressure wave of the lightning event. When there is lightning there is thunder, and when there is thunder there is lightning. There is audible thunder and there is inaudible thunder. Thunder may cause personal injuries and damage to property at very close ranges.
Readers are directed for further reading to three excellent studies of thunder: Rakov & Uman, Lightning Physics and Effects (2003), pp. 373-393; Uman, The Lightning Discharge (1987), pp. 281-312; and Hill in Goldes Lightning (1977) pp.385-406. What follows in this paper are extractions and summaries taken from the above citations as well as other works. The author thanks Mssrs. Mousa (BC Hydro), Rakov (Univ Florida), and Tobias (US Army) for helpful comments.
2. Thunder Definitions
Claps are sudden loud sounds lasting 0.2 to 2 seconds. Peals are sounds changing in frequency or amplitude. Rolls are irregular sound variances. Rumbles are of long duration but relatively weak frequencies. Close-in lightning has been described as first with a clicking or cloth-tearing sound, then a cannon shot sound or loud crack, followed by continuous rumbling. Malan (1963) described these in more technical terms as: click is the upward leader(s); crack is the return stroke; rumble originates from the upper regions of the channel. Viemeister (1961) said the rumble, in part, is reflection of sound off of buildings, clouds and hills.
3. The Science Behind Thunder
3.1 Lightning is an event which heats air to 30,000 K in less that 10 u-seconds (Uman, 1987). A sudden increase in pressure and temperature causes surrounding air to expand at a rate faster than the speed of sound. Thunder is exploding air occurring along the whole length of the lightning channel. Thunder can be heard easily up to 20 km distance. (Viemeister, 1961).
3.2 Rarely is thunder heard beyond 30 km. Sound velocity is proportional to the square root of temperature. Temperature typically decreases with height. Thus thunder sound will be deflected upward. Thunder audibility also is influenced by humidity, wind velocity and wind sheer, temperature inversions, terrain features, clouds, etc. (Uman, 1987 and Fleagle, 1949).
3.3 Thunder contains a roughly cylindrical initial pressure shock wave at the lightning channel in excess of 10 atmospheres. The shock wave decays to a sound wave rapidly, within meters. When thunder is heard about 100 m distant it consists of one large bang, yet hissing and clicking may be heard just prior to the bang (upward streamers?). When heard about 1 km from the lightning, generally thunder will rumble with several loud claps sometimes being reported. (Uman, 1982).
3.4 The dominant frequency of thunder energy is in the 100 Hz range. Thunder occurring in the infrasonic range (below 20 Hz) is inaudible to humans. Infrasonic thunder is considered to be related to sound conversion of energy stored in the thundercloud electrostatic field. Thus, thunders expanding hot gas channel produces an over-pressure (we hear it) while the electrostatic-pressure release mechanism produces an under-pressure or negative pressure change (we do not hear it) (Rakov and Uman, 2003). Of infrasonic sound, Viemeister (1961) speculated those lightnings were of low current/long duration (small amplitude/positive flashes?) which heated up the air so slowly that explosive air rupture did not occur. Sonic and infrasonic thunder can be observed in the same thunderstorm (Holmes et al, 1971). Both cloud-to-cloud and cloud-to-ground lightning can generate infrasonic thunder (Rakov, personal communication).
3.5 Distant thunder can be detected and its source localized by use of spatially separated and networked microphones. Several researchers have studied this. (Few and Teer, 1974. Winn et al, 1978. MacGorman et al, 1981.)
4.0 Other Thunder Factoids
4.1 Flash-to-Bang measurements consider the time from seeing lightning to hearing the associated thunder. Lightning light signals travel at the speed of light, arriving at the observer in about 10 u-seconds if the strike point is 3 km distant. The sound wave, with air temperature being about 20 C and at normal atmospheric pressure, arrives much more slowly in about 10 seconds. A time interval from Flash-to-Bang of 10 sec. = 3-4 km; 15 sec. = 4-6 km ; 20 sec. = 6-8 km can be approximated. This simple method does not work for infrasonic thunder since it cannot be heard.
4.2 The pressure wave - shock propagation sometimes may cause exterior and interior damage to structures. Popping of nail-supported drywall away from horizontal and vertical wooden studs inside houses has been noted. Glass windows have been broken by thunder. Information here is mostly anecdotal.
4.3 Lightning victims sometimes exhibit rupturing of the tympanic membrane (e ardrum) from thunder. Covering ones ears when nearby lightning threatens is advisable.