Influence of air velocity on the habit of ice crystal growth from the vapor

Abstract

The influence of air velocity on ice crystal growth from the vapor has beeninvestigated over the temperature range 0 to -35°C at supersaturations ranging from 2 to 40% over ice in a laboratory chamber in which it is possible to vary each of these parameters independently. Crystal growth was most sensitive to changes in the air velocity at temperatures near -4 and -15°C, where, near water saturation, introduction of only a 5 cm s -1 air velocity induced skeletal transitions — columns to needles near -4°C and plates to dendrites near -15°C. The influence of air velocity on these transitions was found by applying step changes of velocity (increasing or decreasing) while observing crystal growth into the air stream from a fiber support. After conditions were changed, a period of time (as long as 100 s) elapsed before the linear growth rate assumed a new constant value. Under certain conditions, deviations of growth from along the α-axis (near -15°C) or c-axis (near -4°C) resulted from changes of velocity. The influence of air velocity on crystal growth is compared and contrasted with that induced by changing the thermal conductivity and vapor diffusivity of the carrier gas. The results are interpreted in terms of diffusion through a local boundary layer and interaction with different nucleation and growth kinetic processes on the prism and basal surfaces.