The Triggering of Lightning by Corona from Ice Hydrometeors or Colliding Raindrops
The mechanism by which a corona discharge, leading to the initiation of lightning, can be produced in the intense electric fields of a thundercloud has not been revealed by previous studies. The process which has been most generally favoured, in which corona is initiated by large individual raindrops, appears to require fields which are considerably larger than those measured in thunderclouds.
Experiments have been carried out to determine the electric field, Ec, required to obtain corona from both ice particles and colliding drop-pairs. In the former case it was found that corona currents of about 0.1 µA and above could be obtained from ice particles a few millimeters in length for Ec ~4 × 105 V m-1 for values of temperature and pressure likely to occur in the mid-regions of a thundercloud. These currents were drastically reduced if the temperature was below –18 °C, owing to the reduction in surface conductivity suffered by the ice sample.
In the second set of experiments a pair of water drops, of radii R = 2.7 mm and r = 0.65 mm, collided with a relative velocity 5.8 m s-1. Values of Ec ranged from ~5 × 105 V m -1 for head-on collisions to ~2.5 × 105 V m -1 for glancing collisions which produced, momentarily, long liquid filaments. This work shows that corona may be produced in fields of the magnitude known to occur in thunderclouds by these two mechanisms, either of which may provide the means of initiation of a lightning stroke.
KeywordsQuartz Geophysics Hydro Meteor
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