The rolling sound of thun­der

The Star Early Edition - - OPINION & ANALYSIS -

EV­ERY thun­der rum­ble has a dif­fer­ent fre­quency. The noise we hear as thun­der is cre­ated when light­ning su­per­heats the air it trav­els through. When air is heated it expands and, in a light­ning flash, the ex­pan­sion is so fast that a shock wave is cre­ated, caus­ing the sound wave we hear as thun­der. Es­sen­tially, the fre­quency of the sound wave de­pends on how fast the air expands, which in turn de­pends on the sta­bil­ity of the air and other com­pli­cated, ther­mo­dy­namic fac­tors.

But each roll of thun­der sounds slightly dif­fer­ent. You may have no­ticed that at the start of a roll of thun­der you will hear a sharp clap and then the rest of the thun­der rum­ble con­tin­u­ing af­ter it. The sharp clap is at a much higher fre­quency. The rea­son we hear dif­fer­ent types of thun­der – the long roll or the short crack – is due to the path the light­ning takes.

Imag­ine a flash that starts a kilo­me­tre above our heads and trav­els di­ag­o­nally to hit the Earth a kilo­me­tre from our feet. Ev­ery point on that path is (very roughly) the same dis­tance from us, so the sound from ev­ery point of the flash hits us at about the same time. This gives a loud “crack” kind of noise. Now, imag­ine a flash that starts about a kilo­me­tre up but ends close to our feet. The sound from the start of the flash’s path has to travel much fur­ther than the sound from the end and will, as a re­sult, reach us much later. This cre­ates the long, slow roll of thun­der at a lower fre­quency.

Newspapers in English

Newspapers from South Africa

© PressReader. All rights reserved.