Snow Crystal Habit at Small Excesses of Vapor Density over Ice Saturation

Artificial snow crystals were grown in a diffusion chamber Of parallel-plate design which afforded accurate control of the crystal growth conditions. Observations were made at small excesses of vapor density over ice equilibrium (Δρ = 0.015 − 0.169 gm m−3) within the temperature range −8 to −24C. Crystal type was found to be primarily determined by the magnitude of the vapor density excess, in apparent contrast with the situation at supersaturations with respect to water where growth habit is a function of temperature. Transitions between crystal types were noted to be gradual rather than abrupt.