<Tr> <Td_colspan="2"> Problems playing this file? See media help . </Td> </Tr> <P> The cause of thunder has been the subject of centuries of speculation and scientific inquiry . The first recorded theory is attributed to the Greek philosopher Aristotle in the fourth century BC, and an early speculation was that it was caused by the collision of clouds . Subsequently, numerous other theories were proposed . By the mid-19th century, the accepted theory was that lightning produced a vacuum . </P> <P> In the 20th century a consensus evolved that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel . The temperature inside the lightning channel, measured by spectral analysis, varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K . The average is about 20,400 K (20,100 ° C; 36,300 ° F). This heating causes a rapid outward expansion, impacting the surrounding cooler air at a speed faster than sound would otherwise travel . The resultant outward - moving pulse is a shock wave, similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft . </P> <P> Experimental studies of simulated lightning have produced results largely consistent with this model, though there is continued debate about the precise physical mechanisms of the process . Other causes have also been proposed, relying on electrodynamic effects of the massive current acting on the plasma in the bolt of lightning . The shockwave in thunder is sufficient to cause injury, such as internal contusion, to individuals nearby . </P>

Where does the sound of thunder come from