Corona from colliding drops as a possible mechanism for the triggering of lightning

Measurements have been made of the values of electric field E c , required to produce electrical corona from a pair of water drops, of radii R = 2.7 mm and r = 0.65 mm colliding with a relative velocity of 5.8 ms −1 . For central collisions the larger drop went into the ‘bag‐mode’ while for more glancing collisions a liquid filament, typically several times R in length, was drawn out between the separating drops, and eventually collapsed. The values of E c decreased with increasing values of L ‐ the length of the combined drop‐pair in the direction of the field at the moment of separation ‐ which itself increased as the collisions became less central. E c ranged from about 500 kV/m for head‐on collisions to about 250 kV/m for glancing collisions producing the longest filaments. Only a small number of corona pulses ‐ each carrying about 10 −10 ‐ accompanied each collision producing a discharge. Subsidiary experiments with model drops reinforced the conclusion that corona resulted initially from a point on the drop surface which had been disrupted by mechanical or electrical forces. Calculations indicate that the emission of positive corona from colliding raindrops can readily occur in thunderclouds, and thereby possibly trigger lightning, in fields below about 350 kV/m; a value considerably lower than that resulting from any other natural triggering process so far studied.