The mechanical shock of collision between supercooled water drops is explored as a mechanism for the origin of ice in the warm-based cumuli of the central United States. The results of laboratory experiments, using groups of 3-mm diameter hemispherical drops supported on a petroleum substrate and cooled to either −10° or −15°C, are presented to demonstrate that supercooled drops can be caused to freeze mechanically. Cavitation is examined as a mechanism closely associated with ice nucleation in supercooled water. Pressure differences extrapolated from the Weber number for collisions between precipitation-size collector drops and a wide size range of smaller drops were found to exceed the criteria for cavitation and hence, freezing. This finding suggests that collision-freezing may occur in clouds and is worthy of further attention.

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