Ice growth from the vapor at -5 degrees C

Results are summarized and illustrated from a long series of experiments on ice growth from the vapor, nearly all in a very small range of conditions: -5°C, slightly below liquid water saturation, with minimal environmental gradients and no imposed ventilation. The temperature was chosen because c-axis ice needles grow in a narrow temperature interval there, which coincides with the temperature at which the Hallett-Mossop ice multiplication process operates most effectively, and one may suspect that this coincidence is likely to be meaningful. The ice growth habit is poorly reproducible in these conditions, dictating many runs with little change. Growth as plates can persist for hours, and two distinct types of needle growth occur, called sheath needles and sharp needles. Both are distinct from thin columns in that they taper to a point, with no discernible basal face. Both deviate slightly from parallel to the c axis. Sharp needles have been reported before, but only as occurring with an applied high DC voltage. New crystal orientations nucleate occasionally at the tips of the sharp needles; this also has been seen before in the presence of strong electric fields. There appears to be an ice multiplication mechanism in these conditions that does not involve riming.