Runaway breakdown
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Runaway Breakdown is a theory of lightning initiation proposed by Alex Gurevich in 1992.
Electrons in air have a mean free path of ~1cm. Fast electrons which move at a large fraction of the speed of light have a mean free path up to 100 times longer. Given the long free paths, an electric field can accelerate these electrons to energies far higher than that of initially static electrons. If they strike air molecules, more relativistic electrons will be released, creating an avalanche multiplication of "runaway" electrons.
Conventional electron avalanches leading to the electrical breakdown of air at 1-atm pressure will take place at field strengths above 30 kV/cm. Since high-energy electrons have much longer free trajectories, far weaker fields can trigger avalanche breakdown, but only if a source of high-energy electrons is present to start the "runaway" process. This phenomenon was not commonly seen in the laboratory since the breakdown regions can occur only on a scale of fifty meters or more.
In the upper atmosphere, cosmic rays striking air molecules within thunderstorms can supply the relativistic electrons which trigger a breakdown in "runaway" mode. The breakdown region is a conductive plasma many tens of meters long, and it can supply the "seed" which triggers a lightning flash.
