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From Nordan Symposia
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==Origin==
[http://nordan.daynal.org/wiki/index.php?title=English#ca._1100-1500_.09THE_MIDDLE_ENGLISH_PERIOD Middle English] thoner, thunder, from [http://nordan.daynal.org/wiki/index.php?title=English#ca._600-1100.09THE_OLD_ENGLISH.2C_OR_ANGLO-SAXON_PERIOD Old English] thunor; akin to Old High German thonar thunder, [[Latin]] tonare to thunder. The name of the Germanic god [http://en.wikipedia.org/wiki/Thor Thor] comes from the Old Norse [[word]] for thunder.
*[http://en.wikipedia.org/wiki/12th_century before 12th Century]
==Definitions==
*1: the [[sound]] that follows a flash of [[lightning]] and is caused by sudden expansion of the [[air]] in the path of the [[electrical]] discharge
*2: a loud utterance or threat
*3: bang, rumble <the thunder of big guns>
==Description==
'''Thunder''' is the [[sound]] made by [http://en.wikipedia.org/wiki/Lightning lightning]. Depending on the [[nature]] of the lightning and distance of the listener, thunder can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in [[pressure]] and [[temperature]] from lightning produces rapid expansion of the [[air]] surrounding and within a bolt of lightning. In turn, this expansion of air creates a [http://en.wikipedia.org/wiki/Sonic_boom sonic shock wave] which produces the [[sound]] of thunder, often referred to as a clap, crack, or peal of thunder. The distance of the lightning can be [[calculated]] by the [[listener]] based on the time [[interval]] from when the lightning is seen to when the sound is heard.
==Cause==
The [[cause]] of thunder has been the subject of centuries of [[speculation]] and [[scientific]] [[inquiry]]. The first recorded [[theory]] is attributed to the [[Greek]] philosopher [http://en.wikipedia.org/wiki/Aristotle Aristotle] in the third century BC, and an early speculation was that it was caused by the collision of [http://en.wikipedia.org/wiki/Clouds clouds]. Subsequently, numerous other theories have been proposed. By the mid-19th century, the [[accepted]] [[theory]] was that lightning produced a [[vacuum]]. In the 20th century a [[consensus]] evolved that thunder must begin with a [http://en.wikipedia.org/wiki/Shock_wave shock wave] in the air due to the sudden thermal expansion of the [[plasma]] in the lightning channel. The [[temperature]] inside the lightning channel, measured by [http://en.wikipedia.org/wiki/Spectroscopy spectral analysis], varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K. The [[average]] is about 20,400 K (20,100 °C; 36,300 °F). This heating causes it to expand outward, plowing into the [[surrounding]] cooler air at a [[speed]] faster than [[sound]] would travel in that cooler air. The outward-moving [[pulse]] that results is a shock wave, similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft. More recently, the [[consensus]] around the cause of the shock wave has been eroded by the [[observation]] that measured overpressures in simulated lightning are greater than could be achieved by the amount of heating found. Alternative proposals rely on [http://en.wikipedia.org/wiki/Electrodynamic electrodynamic] [[effects]] of the massive current acting on the [[plasma]] in the bolt of lightning. This shockwave is sufficient to cause [[injury]], such as internal [http://en.wikipedia.org/wiki/Contusion contusion], to [[individuals]] nearby.
==Calculating Distance==
A flash of lightning, followed after some seconds by a rumble of thunder is, for many people, the first [[illustration]] of the [[fact]] that [[sound]] [[travels]] significantly slower than [[light]]. Using this [[difference]], one can estimate how far away the bolt of lightning is by timing the [[interval]] between seeing the flash and hearing thunder. The speed of sound in dry air is approximately 343 m/s or 1,127 feet per second or 768 mph (1,236 km/h) at 20°C (68 °F). However, this figure can only be used as an approximation of the speed of a thunder-clap, as you are unlikely to find dry air in a thunderstorm.
The [[speed of light]] is high enough that it can be taken as [[infinite]] in this [[calculation]] because of the [[relatively]] small distance involved. Therefore, the lightning is approximately one kilometer distant for every 2.9 seconds that elapse between the visible flash and the first sound of thunder (or one mile for every 4.6 seconds). In the same five seconds, the light could have traveled the same distance as circling the globe 37 times. Thunder is seldom heard at distances over 20 kilometers (12 miles). A very bright flash of lightning and a [[simultaneous]] sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike was very near.
[[Category: Physics]]
==Origin==
[http://nordan.daynal.org/wiki/index.php?title=English#ca._1100-1500_.09THE_MIDDLE_ENGLISH_PERIOD Middle English] thoner, thunder, from [http://nordan.daynal.org/wiki/index.php?title=English#ca._600-1100.09THE_OLD_ENGLISH.2C_OR_ANGLO-SAXON_PERIOD Old English] thunor; akin to Old High German thonar thunder, [[Latin]] tonare to thunder. The name of the Germanic god [http://en.wikipedia.org/wiki/Thor Thor] comes from the Old Norse [[word]] for thunder.
*[http://en.wikipedia.org/wiki/12th_century before 12th Century]
==Definitions==
*1: the [[sound]] that follows a flash of [[lightning]] and is caused by sudden expansion of the [[air]] in the path of the [[electrical]] discharge
*2: a loud utterance or threat
*3: bang, rumble <the thunder of big guns>
==Description==
'''Thunder''' is the [[sound]] made by [http://en.wikipedia.org/wiki/Lightning lightning]. Depending on the [[nature]] of the lightning and distance of the listener, thunder can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in [[pressure]] and [[temperature]] from lightning produces rapid expansion of the [[air]] surrounding and within a bolt of lightning. In turn, this expansion of air creates a [http://en.wikipedia.org/wiki/Sonic_boom sonic shock wave] which produces the [[sound]] of thunder, often referred to as a clap, crack, or peal of thunder. The distance of the lightning can be [[calculated]] by the [[listener]] based on the time [[interval]] from when the lightning is seen to when the sound is heard.
==Cause==
The [[cause]] of thunder has been the subject of centuries of [[speculation]] and [[scientific]] [[inquiry]]. The first recorded [[theory]] is attributed to the [[Greek]] philosopher [http://en.wikipedia.org/wiki/Aristotle Aristotle] in the third century BC, and an early speculation was that it was caused by the collision of [http://en.wikipedia.org/wiki/Clouds clouds]. Subsequently, numerous other theories have been proposed. By the mid-19th century, the [[accepted]] [[theory]] was that lightning produced a [[vacuum]]. In the 20th century a [[consensus]] evolved that thunder must begin with a [http://en.wikipedia.org/wiki/Shock_wave shock wave] in the air due to the sudden thermal expansion of the [[plasma]] in the lightning channel. The [[temperature]] inside the lightning channel, measured by [http://en.wikipedia.org/wiki/Spectroscopy spectral analysis], varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K. The [[average]] is about 20,400 K (20,100 °C; 36,300 °F). This heating causes it to expand outward, plowing into the [[surrounding]] cooler air at a [[speed]] faster than [[sound]] would travel in that cooler air. The outward-moving [[pulse]] that results is a shock wave, similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft. More recently, the [[consensus]] around the cause of the shock wave has been eroded by the [[observation]] that measured overpressures in simulated lightning are greater than could be achieved by the amount of heating found. Alternative proposals rely on [http://en.wikipedia.org/wiki/Electrodynamic electrodynamic] [[effects]] of the massive current acting on the [[plasma]] in the bolt of lightning. This shockwave is sufficient to cause [[injury]], such as internal [http://en.wikipedia.org/wiki/Contusion contusion], to [[individuals]] nearby.
==Calculating Distance==
A flash of lightning, followed after some seconds by a rumble of thunder is, for many people, the first [[illustration]] of the [[fact]] that [[sound]] [[travels]] significantly slower than [[light]]. Using this [[difference]], one can estimate how far away the bolt of lightning is by timing the [[interval]] between seeing the flash and hearing thunder. The speed of sound in dry air is approximately 343 m/s or 1,127 feet per second or 768 mph (1,236 km/h) at 20°C (68 °F). However, this figure can only be used as an approximation of the speed of a thunder-clap, as you are unlikely to find dry air in a thunderstorm.
The [[speed of light]] is high enough that it can be taken as [[infinite]] in this [[calculation]] because of the [[relatively]] small distance involved. Therefore, the lightning is approximately one kilometer distant for every 2.9 seconds that elapse between the visible flash and the first sound of thunder (or one mile for every 4.6 seconds). In the same five seconds, the light could have traveled the same distance as circling the globe 37 times. Thunder is seldom heard at distances over 20 kilometers (12 miles). A very bright flash of lightning and a [[simultaneous]] sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike was very near.
[[Category: Physics]]
