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C. J. McCappin
and
W. C. Macklin

Abstract

The crystal structure of fresh samples of dry growth accreted ice grown on cylinders rotating in an icing tunnel has been determined by two methods: 1) by photographing thin sections between crossed polaroids and 2) by allowing microtomed surfaces to etch and then photographing the etched surface through a metallurgical microscope. The latter technique is capable of far higher resolution than the former. The existence of substructure in dry growth samples, previously reported by Knight and others and Ashworth and others has been confirmed. Under certain conditions, this substructure plays an important role in influencing the observed crystal structures and measured crystal sizes. The crystal sizes (mean lengths, widths and areas) are dependent on both the ambient and deposit temperatures, but are smaller than those reported previously. This is due to differences in the analytical techniques used. The possible values of mean crystal lengths and widths occurring in fresh accreted ice samples are shown to be limited to a relatively narrow set of mean length and width combinations.

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C. J. McCappin
and
W. C. Macklin

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.

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