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 travels more slowly 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 feet per second or 768 mph (1,236 km/h) at 68°F (20 °C). The speed of light is high enough that it can be taken as infinite in this calculation. Therefore, the lightning is approximately one kilometer distant for every 2.9 seconds (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 kilometers (12 miles). A flash of lightning and a simultaneous sharp "clap!" of thunder, a thunderclap, therefore indicates that the lightning strike was very near.
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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 travels more slowly 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 feet per second or 768 mph (1,236 km/h) at 68°F (20 °C). The speed of light is high enough that it can be taken as infinite in this calculation. Therefore, the lightning is approximately one kilometer distant for every 2.9 seconds (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 kilometers (12 miles). A flash of lightning and a simultaneous sharp "clap!" of thunder, a thunderclap, therefore indicates that the lightning strike was very near.
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