Thunder

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For other uses, see Thunder (disambiguation).

Thunder is, even today, not completely understood by modern science. The word usually describes a sonic shock wave caused by the rapid heating and expansion of the air surrounding and within a bolt of lightning. The bolt changes the air into plasma and it instantly explodes causing the sound known as a thunder clap.

This phenomenon occurs at the same time as a lightning flash but a thunderclap is usually heard after lightning is seen because light travels faster (186,000 miles / 299,338 kilometers per second) than sound (around 700 miles / 1,126 kilometers per hour but varies depending on temperature, humidity and air pressure.) In very close proximity to the lighting strike, sound and light can be heard and seen almost simultaneously.

Fear of thunder is called astraphobia.

1 Theories of the cause of thunder
2 Etymology
3 Calculating distance
4 See also
5 References
6 External links

[edit] Theories of the cause of thunder

A rolling thunderstorm (Cumulonimbus arcus) photographed on July 17, 2004 in Enschede, The Netherlands by John Kerstholt.

The cause of thunder has been the subject of centuries of speculation and scientific enquiry. The first recorded theory is attributed to Aristotle 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 along its path, and that thunder was caused by the subsequent motion of air rushing to fill the vacuum. Later in the 19th century it was believed that thunder was caused by an explosion of steam when water along the lightning channel was heated. Another theory was that gaseous materials were created by lightning and then exploded. 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 in the lightning channel.

Experimental support for this theory came from spectroscopic temperature measuring up to 36000 K. Arc photography then proved that the plasma did not expand thermally in all directions, but preferentially at right angles to the electric current. According to Graneau, the mechanism for this is not yet completely understood in terms of magnetohydrodynamics.

[edit] Etymology

The d in thunder is epenthetic, and is now found in Modern Dutch donder, from earlier Old English þunor, Middle Dutch donre, together with Old Norse þorr, Old Frisian þuner, Old High German donar descended from Proto-Germanic *þunraz, built on a PIE stem *(s)tene- also found in Sanskrit tana-itnu- "thundering", Latin tonare "to thunder" (see also tornado). The Germanic god Thor was named after the thunder.

[edit] Calculating distance

Since sound and light travel at different speeds through the atmosphere, one can estimate how far away the bolt of lightning is by timing the interval between seeing the flash and hearing the thunder. The speed of sound in air is approximately 332 m/s (742.6 mph). The speed of light is so fast that it can be ignored in this calculation. Therefore, the lightning is approximately one kilometer distant for every three seconds (or one mile for every five seconds). Thunder is seldom heard at distances over 15 miles.