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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 donre, and Old Norse þorr, Old Frisian þuner, Old High German donar descended from Proto-Germanic *þunraz). In Latin the term was tonare "to thunder". The name of the Germanic god Thor comes from the Old Norse word for thunder.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

The shared Proto-Indo-European root is *tón-r̥ or *tar-, also found Gaulish Taranis and Hittite Tarhunt.

Calculating distance

A flash of lightning, followed after some time by a rumble of thunder illustrates 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 ft/s or {{safesubst:#invoke:convert|convert}} at 20 °C (68 °F).<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> This translates to 0.213 miles per second or about 5 seconds per mile; however, this figure is only an approximation. In countries that use the metric system, an approximation of 3 seconds per km is often used.<ref>http://www.dmi.dk/laer-om/temaer/vejr/lyn-og-torden/lynhurtige-facts/</ref>

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 3 seconds that elapse between the visible flash and the first sound of thunder (or one mile for every 5 seconds). In the same five seconds, the light could have traveled the Lunar distance four times. (In this calculation, the initial shock wave, which travels at a rate faster than the speed of sound, but only extends outward for the first 30 feet, is ignored.)<ref>http://www.lightningsafety.com/nlsi_info/thunder2.html</ref> Thunder is seldom heard at distances over {{safesubst:#invoke:convert|convert}}.<ref name="britannica"/> 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.



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