It has long been recognized that there exists a disconnect between the electric fields necessary to initiate breakdown in free air and the fields that are actually measured inside thunderstorms. Early studies investigated the possible influence of various types of hydrometeors (I.e. Supercooled water drops, ice crystals, groupel) on enhancing the ambient electric field and initiating breakdown. These early investigations have been largely ignored by more recent studies that have embraced the concept of cosmic ray initiated lightning. To date there has been no conclusive data that convincingly favors either the hydrometeor or cosmic mechanism. In this presentation, published laboratory and theoretical investigations of both theories are reviewed as well recent studies of initial breakdown using multi-parameter radar, electric field change meters and VHF lightning mapping array data.
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