Theories of the cause of thunder

The cause of thunder has been the subject of centuries of theory and scientific inquiry. The first recorded theory is qualified to Aristotle in the third century BC, and an early speculation was that it was caused by the collision of clouds. Subsequently, numerous other theories have been planned. By the mid-19th century, the accepted theory was that lightning produced a vacuum along its path, and that thunder was caused by the consequent motion of air rushing to fill the vacuum. Later in the 19th century it was thought that thunder was caused by an explosion of steam when water along the lightning channel was heated. Another theory was that gaseous materials were created by lightning and then exploded. In the 20th century a consensus evolved that thunder must begin with a shock wave in the air due to the sudden thermal growth of the plasma in the lightning channel.
Experimental support for this theory came from spectroscopic temperature measuring up to 36000 K. Arc photography then proved that the plasma did not expand thermally in all directions, but preferentially at right angles to the electric current. According to Graneau, the mechanism for this is not yet totally understood in terms of magnetohydrodynamics.