The Mechanism of the High Velocity of Propagation of Lightning Discharges

Observations by Schonland and Collens with Boys' camera show that the luminous tip of the leader stroke of the lightning discharge advances with a high velocity sometimes exceeding 3×109 cm per second. From the fact that electrons traveling 109 cm per second produce about 6000 new electrons per cm of actual path and that 6000 new electrons per cm in the direction of the field (i.e., α=6000) requires 4×106 volts per cm shows that to give the electrons a velocity in the direction of the field of 109 cm per second would require a field exceeding 4×106 volts per cm even at the tip of the stroke. The fields, and therefore the electron velocities, are certainly far less than this value. Accordingly the velocity of the tip greatly exceeds the velocity of the electrons in the tip. It is possible to account for this velocity as follows: Calculations based on justified assumptions concerning the conditions known to exist in a lightning stroke show that if the field just ahead of the tip averages 105 volts per cm for about one centimeter, ionization by collision by the electrons present in the gas before the stroke would be great enough to advance the tip at the observed rate. Such a field is found not too high to be consistent with the low conductivity of the leader path. In this way a propagation of the leader stroke of a velocity many times higher than the velocity of the electrons can be attained.