Lightning initiation-conventional and runaway-breakdown hypotheses

We evaluate two candidate mechanisms for the onset of lightning in the rela tively low electric fields measured inside thunderstorms. The first is conv entional dielectric breakdown due to local enhancement of the electric fiel d in the vicinity of hydrometeors. The second is runaway breakdown, due to extended acceleration of high-energy electrons (from cosmic rays or terrest rial sources of ionizing radiation) by the in-cloud electric field. We comp are the electric fields required for lightning onset by each mechanism with those observed inside lightning-producing clouds, and we examine the sensi tivity of the computed results to input parameters and assumptions. The con clusion of our analysis is that the conventional breakdown mechanism alone cannot trigger lightning while the runaway breakdown mechanism appears a mo re likely candidate. We identify the parameters on which each mechanism dep ends and emphasize the impact of observational uncertainties on our conclus ions.