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Old English þunor, from Proto-Germanic *thunraz (cf. Old Norse þorr, Old Frisian thuner, Middle Dutch donre, Dutch donder, Old High German donar, German Donner "thunder"), from PIE *(s)tene- "to resound, thunder" (cf. Sanskrit tanayitnuh "thundering," Persian tundar "thunder," Latin tonare "to thunder"). Swedish tordön is literally "Thor's din." The intrusive -d- is also found in Dutch and Icelandic versions of the word.
Old English þunrian, from the source of thunder (n.). Figurative sense of "to speak loudly, threateningly, bombastically" is recorded from mid-14c. Related: Thundered; thundering.
The explosive noise that accompanies a stroke of lightning. Thunder is a series of sound waves produced by the rapid expansion of the air through which the lightning passes. Sound travels about 1 km in 3 seconds (about 1 mi in 5 seconds). The distance between an observer and a lightning flash can be calculated by counting the number of seconds between the flash and the thunder. See Note at lightning.
The noise created when air rushes back into a region from which it has been expelled by the passage of lightning.
often referred to in Scripture (Job 40:9; Ps. 77:18; 104:7). James and John were called by our Lord "sons of thunder" (Mark 3:17). In Job 39:19, instead of "thunder," as in the Authorized Version, the Revised Version translates (ra'amah) by "quivering main" (marg., "shaking"). Thunder accompanied the giving of the law at Sinai (Ex. 19:16). It was regarded as the voice of God (Job 37:2; Ps. 18:13; 81:7; comp. John 12:29). In answer to Samuel's prayer (1 Sam. 12:17, 18), God sent thunder, and "all the people greatly feared," for at such a season (the wheat-harvest) thunder and rain were almost unknown in Palestine.
sound caused by a lightning discharge. Lightning heats the air in its path and causes a large over-pressure of the air within its channel. The channel expands supersonically into the surrounding air as a shock wave and creates an acoustic signal that is heard as thunder. The loudest thunder heard after a flash to the ground is actually produced by the return stroke that follows the path forged by the initial stroke, or stepped leader. The return stroke is louder because it contains a larger and faster-rising electric current than either the leader or a discharge within a cloud. Because the path of a lightning channel is usually branched, tortuous, and very long, sound waves from more distant portions arrive later than those from nearer portions, accounting for the duration of thunder and for the characteristic claps and rumbles. The distance to a flash can be estimated by measuring the time delay between the flash of light and the thunder-the formula being about three seconds for each kilometre (or five seconds for each mile). Thunder is seldom heard at distances greater than about 20 km (12 miles). See also thunderstorm.