Abstract
A cloud of small supercooled water droplets was subjected to shock waves of reproducible intensity in the laboratory. Nucleation of freezing occurred only when the gas driving the shock waves was cooled to −37C or below by adiabatic expansion and subsequently mixed with the droplet-bearing air. Passage of the shock wave did not produce nucleation of the cloud. Results of a numerical model of a lightning discharge show, in the pressure wave from a lightning discharge, that the degree of cooling by adiabatic expansion is probably too small to produce ice crystals by homogeneous nucleation.
