Thunder
Overview
 
Thunder is the sound made by lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

. 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
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave
Sonic boom
A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion...

 which produces the sound of thunder, often referred to as a clap, crack, or peal of thunder.
Unanswered Questions
Encyclopedia
Thunder is the sound made by lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

. 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
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave
Sonic boom
A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion...

 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
Inquiry
An inquiry is any process that has the aim of augmenting knowledge, resolving doubt, or solving a problem. A theory of inquiry is an account of the various types of inquiry and a treatment of the ways that each type of inquiry achieves its aim.-Deduction:...

. The first recorded theory is attributed to the Greek philosopher Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 in the third century BC, and an early speculation was that it was caused by the collision of clouds. Subsequently, numerous other theories have been proposed. By the mid-19th century, the accepted theory was that lightning produced a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

. In the 20th century a consensus evolved that thunder must begin with a shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

 in the air due to the sudden thermal expansion of the plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 in the lightning channel. The temperature inside the lightning channel, measured by spectral analysis
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

, varies during its 50 μs
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...

 existence, rising sharply from an initial temperature of about 20,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about 20400 K (20,126.9 °C; 36,260.3 °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
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

, or at the front of a supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

 aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

.
More recently, the consensus around the cause of the shock wave has been eroded by the observation that measured overpressure
Overpressure
The term Overpressure is applied to a pressure difference, relative to a "normal" or "ambient" pressure, in various circumstances:* In engineering: the pressure difference over the wall thickness of a pressure vessel...

s in simulated lightning are greater than could be achieved by the amount of heating found. Alternative proposals rely on 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 contusion, to individuals nearby.

Etymology

The d in Modern English thunder (from earlier Old English þunor) is epenthetic, and is now found as well in Modern Dutch donder (cp Middle Dutch
Middle Dutch
Middle Dutch is a collective name for a number of closely related West Germanic dialects which were spoken and written between 1150 and 1500...

 donre, and Old Norse
Old Norse
Old Norse is a North Germanic language that was spoken by inhabitants of Scandinavia and inhabitants of their overseas settlements during the Viking Age, until about 1300....

 þorr, Old Frisian
Old Frisian
Old Frisian is a West Germanic language spoken between the 8th and 16th centuries in the area between the Rhine and Weser on the European North Sea coast. The Frisian settlers on the coast of South Jutland also spoke Old Frisian but no medieval texts of this area are known...

 þuner, Old High German
Old High German
The term Old High German refers to the earliest stage of the German language and it conventionally covers the period from around 500 to 1050. Coherent written texts do not appear until the second half of the 8th century, and some treat the period before 750 as 'prehistoric' and date the start of...

 donar descended from Proto-Germanic *þunraz). In Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 the term was tonare "to thunder". The name of the Germanic god Thor
Thor
In Norse mythology, Thor is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing, healing, and fertility...

 comes from the Old Norse word for thunder.

The shared Proto-Indo-European
Proto-Indo-European language
The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

 root
Proto-Indo-European root
The roots of the reconstructed Proto-Indo-European language are basic parts of words that carry a lexical meaning, so-called morphemes. PIE roots always have verbal meaning like "to eat" or "to run", as opposed to nouns , adjectives , or other parts of speech. Roots never occur alone in the language...

 is *tón-r̥ or }, also found Gaulish Taranis
Taranis
In Celtic mythology Taranis was the god of thunder worshipped essentially in Gaul, the British Isles, but also in the Rhineland and Danube regions amongst others, and mentioned, along with Esus and Toutatis as part of a sacred triad, by the Roman poet Lucan in his epic poem Pharsalia as a Celtic...

and Hittite Tarhunt.

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
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 travels significantly slower than light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

. 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
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

 in dry air is approximately 343 m/s or 1,127 feet per second or 768 mph (343.3 m/s) 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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

 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 travelled the same distance as circling the globe 37 times. Thunder is seldom heard at distances over 20 kilometres (12.4 mi). A very bright flash of lightning and a simultaneous sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike was very near.

See also

  • Thunderstorm
    Thunderstorm
    A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

  • Thunderbolt
    Thunderbolt
    A thunderbolt is a discharge of lightning accompanied by a loud thunderclap or its symbolic representation. In its original usage the word may also have been a description of meteors, or, as Plato suggested in Timaeus, of the consequences of a close approach between two planetary cosmic bodies,...

  • Lightning
    Lightning
    Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

  • Brontophobia (fear of thunder)
  • Mistpouffers
    Mistpouffers
    Mistpouffers are unexplained reports that sound like a cannon or a sonic boom. They have been heard in many waterfront communities around the world such as the banks of the river Ganges in India, the East Coast and inland Finger Lakes of the United States, as well as areas of the North Sea, Japan...

  • The Castle Thunder sound effect
    Castle thunder (sound effect)
    Castle thunder is a sound effect that consists of the sound of a loud thunderclap during a rainstorm. It was originally recorded for the 1931 film Frankenstein, and has since been used in dozens of movies, Disney and Hanna-Barbera cartoons, and TV series, and television commercials...

  • List of thunder gods
  • Thursday
    Thursday
    Thursday is the fourth day of the week according to the ISO 8601 international standard adopted in most western countries. In countries that use the Sunday-first convention and in the Judeo-Christian calendar it is the fifth day of the week. It falls between Wednesday and Friday...

    (day of Thor)
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