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  • Author or Editor: Nancy C. Knight x
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Nancy C. Knight

Abstract

The shape factor of hailstones, defined as the ratio of their long and short axes (m′/m), has been measured for hailstones from three geographical areas: northeastern Colorado, central Oklahoma and central Alberta. The results show a general tendency toward decreasing sphericity with increasing size and are different for different areas. The results are relevant to remote hail sensing by radar techniques utilizing polarization.

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Nancy C. Knight

Abstract

Data on hailstone embryo types, using a broad classification as graupel or frozen drops, are presented from several geographical areas representing distinctly different storm “climatologies.” The relative frequency of the two embryo types varies greatly from area to area, in a Way that correlates rather well with average cloud-base temperature. The warmer based clouds produce hail with more frozen drop embryos. The correlation may be explainable either in terms of the dominant precipitation growth process—liquid coalescence or the ice process—or in terms of recycling of embryos, or both. In light of these results, the transferability of any hail suppression technology from one area to another should not be considered to be automatic.

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Charles A. Knight and Nancy C. Knight

Abstract

Small volumes of spongy ice were produced by nucleating supercooled water in balloons at temperatures from −3 to −10C. The original freezing structure is faithfully preserved by quenching the spongy ice in a dry ice-alcohol mixture, and this method is suggested as an important collection technique for freshly fallen hailstones. With slower final freezing rates, recrystallization takes place, consisting of either grain growth only, or of nucleation and growth of new grains. Free growth of ice in water at or above −10C is probably not accompanied by spontaneous nucleation of new crystals in these experiments; reports to the contrary may be a result of primary recrystallization, which does occur at normal rates of final freezing.

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Charles A. Knight and Nancy C. Knight

Abstract

Hailstones collected within or near the National Hail Research Experiment (NHRE) target area on 23 days of the randomized seeding experiment were sectioned and classified as to embryo type. No significant correlations were found between embryo type and hail size, hail amount or cloud base temperature, but a suggestive relation between seeding and embryo type does exist. The seeded storms had a substantially greater tendency to produce hail with “frozen drop” embryos than did non-seeded storms. Two simple tests give probabilities of obtaining the results by chance of 0.13 and 0.22. The result is suggestive enough to be worth investigating in a future experiment.

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Andrew G. Detwiler, Nancy C. Knight, and Andrew J. Heymsfield

Abstract

Ice particles are captured, photographed, melted, and then photographed again. Mass is estimated from the size of the melted drop. Based on a sample of 640 particles, the standard error in estimating particle mass solely from the maximum dimension of the particle is found to be a factor of 4. The standard error in estimating mass concentration M in a cloud from a sample of n well-characterized particles recorded by an optical array probe is estimated to be approximately a factor of 100.6/n^1/2.

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Petr Chýlek, B. R. D. Gupta, Nancy C. Knight, and Charles A. Knight

Abstract

A large number of photographs of thin sections of quenched hailstones have been studied to determine the most common patterns of water distribution within hailstones. The way in which water is distributed throughout the hailstone determines which of the mixing rules should be used to calculate an effective refractive index of water-ice mixture. We have concluded that the Maxwell-Garnett mixing rule for ice grains embedded in a water matrix is suitable for most graupel soaked with water. Some of the large hailstones with spongy growth suggest that a proper generalization of the Bruggeman mixing rule should be used to determine their effective index of refraction at centimeter wavelengths.

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