The cause of thunder

The cause of thunder is one of the oldest riddles of recorded scientific speculation. Three centuries BC Aristotle published the first thunder theory. Many other theories were proposed until at the beginning of the present century a consensus evolved which assumed thunder must begin with a shockwave in air due to the sudden thermal expansion of the plasma in the lightning channel. The only experimental support for this theory came from spectroscopic temperature determinations up to 36000 K. Any one of the assumptions made in equating 'optical' to thermodynamic temperatures can be challenged and some have been disputed. Experiments with short atmospheric arcs of lightning strength revealed average arc pressures in excess of 400 atm and peak pressures approaching 1000 atm. These results demand much higher temperatures than those found by lightning spectroscopy. Furthermore, when the strength of the short arc explosions was plotted against the action integral of the current pulse it followed an electrodynamic law rather than a heating curve. Arc photography then proved conclusively that the plasma did not expand thermally in all directions, but preferentially at right angles to the current, as if driven by organised electrodynamic action. Possible electrodynamic forces which might drive the thunder shockwave are the Lorentz pinch force, the longitudinal Ampere force and the alpha-torque force of the Ampere-Neumann electrodynamics. The pinch force was found to be far too small and in the wrong direction to be the cause of thunder. Longitudinal and alpha-torque forces act in the correct direction but, so far, quantitative agreement has not been achieved. This may have to wait for a complete Ampere MHD.