Nucleation of Ice by Mechanical Shock

Abstract

THE frequent occurrence of heavy rainfall shortly after lightning has prompted speculation on mechanisms by which a lightning stroke might increase precipitation from clouds. One possibility is that supercooled cloud droplets are frozen by the pressure wave generated by the lightning stroke. Goyer¹ estimated that the instantaneous pressure in a lightning channel would generate a shock wave equivalent to that produced by the detonation of primacord (detonating fuse). In some field experiments², he detonated a vertical length of primacord about 100 feet from the supercooled plume of Old Faithful geyser (Yellowstone National Park, USA) and observed that a shower of ice pellets 0.5 to 1 mm in diameter fell from the plume. The calculated overpressure at the plume, being only 7 mbar, was insufficient to disrupt individual cloud droplets, so that it seemed that the supercooled cloud droplets had been induced to freeze by the passage of the shock wave. Some support for this proposal was obtained from laboratory experiments³ in which supercooled water contained in small tubes was induced to freeze by the passage of a shock wave having an overpressure of about 70 mbar.