Laboratory investigation of corona initiation by ice crystals and its importance to lightning

This article reports on a new laboratory investigation of the initiation of corona discharges by small vapour‐grown ice crystals. This study was motivated by questions regarding the initiation of lightning and the possible role played by corona discharges on ice hydrometeors. Ice crystals were grown in a static diffusion chamber on a horizontally suspended glass fibre, with environmental conditions selected to mirror those of the thunderstorm environment where initiation of lightning is believed to occur. An in situ parallel plate electrode assembly was then rotated into position, so that the crystals were at the centre. The crystals were then subjected to slowly increasing electric fields until they underwent corona discharge. The discharges were observed and recorded under dark ambient conditions using an image‐intensified video microscope, along with a synchronous record of the applied voltage. The threshold electric fields for corona discharge were determined as a function of air pressure, temperature and ice crystal length in the direction of the applied electric field. The new results are relevant to a better understanding of hydrometeor‐mediated electrical discharge activity in thunderstorms.