Thunder is the

sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' b ...

caused by

lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an avera ...

. 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 Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

and hence

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

caused by the lightning produces rapid expansion of the air in the path of a lightning bolt. In turn, this expansion of air creates a sonic 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''.

Etymology

The ''d'' in Modern English ''thunder'' (from earlier

Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlers in the mid-5th c ...

''þunor'') is

epenthetic In phonology, epenthesis (; Greek ) means the addition of one or more sounds to a word, especially in the beginning syllable ('' prothesis'') or in the ending syllable (''paragoge'') or in-between two syllabic sounds in a word. The word ''epent ...

, and is now found as well in Modern Dutch ''donder'' (cf.

Middle Dutch Middle Dutch is a collective name for a number of closely related West Germanic dialects whose ancestor was Old Dutch. It was spoken and written between 1150 and 1500. Until the advent of Modern Dutch after 1500 or c. 1550, there was no overarc ...

''donre''; also

Old Norse Old Norse, Old Nordic, or Old Scandinavian, is a stage of development of North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia and their overseas settlemen ...

þorr Thor (; from non, Þórr ) is a prominent god in Germanic paganism. In Norse mythology, he is a hammer-wielding god associated with lightning, thunder, storms, sacred groves and trees, strength, the protection of humankind, hallowing, and ...

'',

Old Frisian Old Frisian was a West Germanic language spoken between the 8th and 16th centuries along the North Sea coast, roughly between the mouths of the Rhine and Weser rivers. The Frisian settlers on the coast of South Jutland (today's Northern Fri ...

''þuner'',

Old High German Old High German (OHG; german: Althochdeutsch (Ahd.)) is the earliest stage of the German language, conventionally covering the period from around 750 to 1050. There is no standardised or supra-regional form of German at this period, and Old High ...

''donar'', all ultimately descended from

Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed proto-language of the Germanic branch of the Indo-European languages. Proto-Germanic eventually developed from pre-Proto-Germanic into three Germanic bran ...

*''þunraz''). In

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

the term was ''tonare'' "to thunder". The name of the Nordic god

Thor Thor (; from non, Þórr ) is a prominent god in Germanic paganism. In Norse mythology, he is a hammer-wielding god associated with lightning, thunder, storms, sacred groves and trees, strength, the protection of humankind, hallowing, an ...

comes from the

word for thunder. The shared

Proto-Indo-European root The roots of the reconstructed Proto-Indo-European language (PIE) are basic parts of words that carry a lexical meaning, so-called morphemes. PIE roots usually have verbal meaning like "to eat" or "to run". Roots never occurred alone in the lan ...

is *''tón-r̥'' or ''*'', also found in Gaulish ''

Taranis In Celtic mythology, Taranis (Proto-Celtic: *''Toranos'', earlier ''*Tonaros''; Latin: Taranus, earlier Tanarus) is the god of thunder, who was worshipped primarily in Gaul, Hispania, Britain, and Ireland, but also in the Rhineland and Danube r ...

''.

Cause

The cause of thunder has been the subject of centuries of speculation and scientific inquiry.Vavrek, R. J., Kithil, R., Holle, R. L., Allsopp, J., & Cooper, M. A. (n.d.). The science of thunder. Retrieved from http://lightningsafety.com/nlsi_info/thunder2.html Early thinking was that it was made by deities, but the ancient

Greek philosophers Ancient Greek philosophy arose in the 6th century BC, marking the end of the Greek Dark Ages. Greek philosophy continued throughout the Hellenistic period and the period in which Greece and most Greek-inhabited lands were part of the Roman Empire ...

attributed it to natural causes, such as wind striking clouds ( Anaximander,

Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...

) and movement of air within clouds (

Democritus Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek pre-Socratic philosopher from Abdera, primarily remembered today for his formulation of an atomic theory of the universe. No ...

).Heidorn, K. C. (1999). Thunder: Voice of the heavens. Retrieved fro
http://www.islandnet.com/~see/weather/elements/thunder1.htm
The Roman philosopher

Lucretius Titus Lucretius Carus ( , ; – ) was a Roman poet and philosopher. His only known work is the philosophical poem ''De rerum natura'', a didactic work about the tenets and philosophy of Epicureanism, and which usually is translated into En ...

held it was from the sound of hail colliding within clouds. In the mid 19th century, the accepted theory was that lightning produced a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

and that the collapse of that vacuum produced what is known as thunder. In the 20th century a consensus evolved that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma (physics), plasma in the lightning channel., The temperature inside the lightning channel, measured by Spectroscopy, spectral analysis, varies during its 50 Microsecond, μs existence, rising sharply from an initial temperature of about 20,000 Kelvin, K 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. In close proximity to the source, the Sound pressure, sound pressure level of thunder is usually 165 to 180 Decibel, dB, 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.

Consequences

The shock wave in thunder is sufficient to cause property damage and injury, such as internal contusion, to individuals nearby. Thunder can rupture the eardrums of people nearby, leading to permanently hearing loss, impaired hearing. Even if not, it can lead to temporary deafness.

Types

Vavrek et al. (n.d.) reported that the sounds of thunder fall into categories based on loudness, duration, and Pitch (music), pitch. ''Claps'' are loud sounds lasting 0.2 to 2 seconds and containing higher pitches. ''Peals'' are sounds changing in loudness and pitch. ''Rolls'' are irregular mixtures of loudness and pitches. ''Rumbles'' are less loud, last for longer (up to more than 30 seconds), and of low pitch. Inversion thunder results when lightning strikes between cloud and ground occur during a temperature inversion; the resulting thunder sounds have significantly greater acoustic energy than from the same distance in a non-inversion condition. In an inversion, the air near the ground is cooler than the higher air; inversions often occur when warm moist air passes above a cold front. Within a temperature inversion, the sound energy is prevented from dispersing vertically as it would in a non-inversion and is thus concentrated in the near-ground layer. Lightning 14

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.

Perception

The most noticeable aspect of lightning and thunder is that the lightning is seen before the thunder is heard. This is a consequence of the speed of light being much greater than the speed of sound. The speed of sound in dry air is approximately or at . This translates to ; 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). 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.

References

External links

*
The Science of Thunder
- National Lightning Safety Institute

by Keith C. Heidorn, PhD, ACM * b:Engineering Acoustics/Thunder acoustics, Wikibooks: Engineering Acoustics/Thunder acoustics
Storm: Thunder sounds in binaural audio
{{Authority control Lightning Noise Severe weather and convection Meteorological phenomena ja:雷#雷鳴 fi:Ukkonen sv:Åska