The Crystalline Structure of Ice Formed by Droplet Accretion. II: Annealed Samples and Application

Abstract

The crystal structure of annealed samples of dry growth accreted ice grown on cylinders rotating in an icing tunnel has been determined using the techniques of McCappin and Macklin (Part I). The rate of change of the crystal structure is strongly temperature dependent at annealing temperatures within a few degrees of 0°C and is most rapid at this temperature. In particular, the substructure formed in accretions grown at high ambient temperatures is eliminated within a 20?30 min annealing at 0°C. The changes that occur during annealing may be usefully described by the standard grain growth law. A plot of the mean crystal length against mean average crystal width for data obtained from the etching technique may be used as an indicator of the degree of annealing of an accreted ice sample. It is shown that little quantitative information on the growth conditions of accreted ice can be obtained from measurements of crystal size in fresh samples. However, measurements of mean crystal length and mean average width in samples annealed for 30 min or longer give ambient temperatures accurate to ? ±5°C. While this accuracy is sufficient for qualitative purposes, it is not sufficient to permit implementation of the quantitative scheme of hailstone analysis proposed by Macklin and others.