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Thunder is the sound caused by lightning. Depending upon the distance from and nature of the lightning, it can range from a long, low rumble to a sudden, loud crack. The sudden increase in temperature and hence pressure caused by the lightning produces rapid expansion of the air in the path of a Lightning strike. In turn, this expansion of air creates a Shock wave, often referred to as a "thunderclap" or "peal of thunder". The scientific study of thunder is known as brontology and the irrational fear (phobia) of thunder is called brontophobia.
The shared Proto-Indo-European root is , also found in Gaulish Taranis.Matasovic, Ranko. Etymological Dictionary of Proto Celtic. Leiden, The Netherlands: Brill. 2009. p. 384.
Scientists have agreed since the 20th century that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel. (2025). 9780521035415, Cambridge University Press. ISBN 9780521035415, The temperature inside the lightning channel, measured by Spectroscopy, varies during its 50 Microsecond existence, rising sharply from an initial temperature of about 20,000 Kelvin to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about . This heating causes a rapid outward expansion, impacting the surrounding cooler air at a speed faster than sound would otherwise travel. The resultant outward-moving pulse is a shock wave, similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft. Near the source, the Sound pressure of thunder is usually 165 to 180 Decibel, but can exceed 200 dB in some cases.
Experimental studies of simulated lightning have produced results largely consistent with this model, though there is continued debate about the precise physical mechanisms of the process. Other causes have also been proposed, relying on electrodynamic effects of the enormous current acting on the plasma in the bolt of lightning.
Inversion thunder results when lightning strikes occur between the cloud and ground during a temperature inversion. The resulting thunder sounds have significantly greater acoustic energy than those produced from the same distance in non-inversion conditions. In a temperature inversion, the air near the ground is cooler than the air higher up. Inversions often happen when warm, moist air passes above a cold front. Within a temperature inversion, sound energy is prevented from dispersing vertically as it would in non-inversion conditions, and is thus concentrated in the near-ground layer.
Cloud-to-ground lightning (CG) typically consists of two or more return strokes, from ground to cloud. Later return strokes have greater acoustic energy than the first.
This translates to approximately ; saying "one thousand and one... one thousand and two..." is a useful method of counting the seconds from the perception of a given lightning flash to the perception of its thunder (which can be used to gauge the proximity of lightning for the sake of safety). To estimate the distance from the lightning strike, divide the counted seconds by five for miles, or three for kilometers.
A very bright flash of lightning and an almost simultaneous sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike was very near.
Close-in lightning has been described first as a clicking or cloth-tearing sound, then a cannon shot sound or loud crack/snap, followed by continuous rumbling. The early sounds are from the leader parts of lightning, then the near parts of the return stroke, then the distant parts of the return stroke.
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