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W. C. Macklin

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C. J. McCappin and W. C. Macklin

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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

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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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I. H. Bailey and W. C. Macklin

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The effect of a number of impurities on the mechanical strength of ice formed by the accretion of super-cooled water droplets has been investigated experimentally. Significant reduction in the strength can be obtained but only at impurity concentrations in excess of a few per cent.

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W. C. Macklin and P. J. Rye

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The c-axis orientation distributions of ice formed by the accretion of supercooled droplets have been determined for a wide range of air and deposit temperatures. There are peaks in the distributions which may be related to the air temperature. Consequently, quantitative information on the growth environment of a hailstone can be obtained from its crystallographic structure.

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P. J. Rye and W. C. Macklin

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Measurements of c- and c′-axis orientation distributions in accreted ice deposits formed in the dry growth regime show peaks which may be related to the growth conditions. Theoretical studies of the factors governing the development of the equilibrium crystallographic orientation distributions in such deposits have been made. These show that surface roughness affects the sharpness of the peaks in both types of distributions. In addition, in the c′-axis distribution the position of the peak is shifted and hence the peak cannot be uniquely related to the growth conditions. If inferences of the growth environment of hailstones are to be made from crystal orientation measurements, the orientation distributions of the c-axes relative to the local normal to the surface of the hailstone should be used as these give the maximum possible resolution of the peaks.

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W. C. Macklin and B. F. Ryan

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The structure of ice grown in water supercooled to temperatures between −2C and −7.5C has been studied and recorded by flash photography. The ice structures formed below −3C are not co-planar with the basal plants of the seed crystals but are split into two, and occasionally more, segments. At temperatures below −5.5C secondary splitting occurs on the major growth segments, the complexity of the structure increasing with increased supercooling. A stepped growth mechanism has been suggested to explain thew observations. The three-dimensional structures so formed are sufficiently, complex to retain unfrozen liquid and so give rise to spongy ice.

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I. H. Bailey, J. R. Hulston, J. R. Stewart, and W. C. Macklin

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Theoretical considerations show that isotopic fractionation can occur during the freezing of supercooled droplets accreted by a hailstone. This is due to evaporation from the liquid during the freezing process with consequent enrichment of the deuterium and O18 content of the accreted ice. The enrichment is greatest when the temperature of the hailstone is near OC. Calculations, based on the Rayleigh distillation formula and allowing for diffusivity effects, indicate that the maximum increase is about 6 for deuterium and1.5 for O18. Analyses of samples of accreted ice formed in an icing tunnel have confirmed these predictions.The effect is sufficiently large to affect the interpretation of isotopic analyses of hailstones.

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