A flash of lightning, followed after some seconds 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 768 mph (1,236 km/h) at 20 °C (68 °F). However, this figure is only an approximation, as air is hardly dry in a thunderstorm.
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 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 traveled the same distance as circling the globe 37 times. Thunder is seldom heard at distances over 20 kilometers (12 mi). 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.
Read more about this topic: Thunder
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