Generation of protosnowflakes in supersonic flow.

Nanocrystals of ice were generated at about 200 K by the freezing of minuscule liquid drops of water produced in supersonic flow by the condensation of supersaturated water vapor. Although the mode of generation differs from that of snowflake nuclei formed in the upper atmosphere, the temperature involved is similar and the corresponding kinetics of nucleation of ice is some 20 orders of magnitude faster than that in prior laboratory studies. Therefore, it is the temperature, not the mode of generation that is crucial. Accordingly, it is plausible to suppose that these nanocrystals with diameters of approximately 24 nm possess properties that make them resemble the precursors of snowflakes formed in the upper atmosphere. Electron diffraction patterns of the submicroscopic particles recorded in flight during and beyond the progress of their freezing showed that they are somewhat flattened particles of cubic ice slightly denser than bulk cubic ice with their 111 planes perpendicular to their thin direction. The cluster edges surrounding the hexagonal array of molecules in the 111 planes of the cubic structure can presumably serve as templates for further growth, this time of hexagonal ice, by condensation of vapor. How these observations are related to prior hypotheses about the formation of snowflakes is discussed.