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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

The range of lobe structures found in natural hailstones is illustrated and classified on the basis of form and growth conditions. Lobes that form in dry growth are cusped, and probably result from a “collection efficiency” effect, lobes growing faster than their surroundings because they collect more material. These lobes probably only form to a marked extent when a hailstone is tumbling. Lobes in wet or spongy growth, icicle lobes, are not cusped and evidently form more as icicles form, by flow of liquid water over the hailstone surfaces and preferential freezing at the tips of projections. The icicle lobes are less developed in very spongy growth than in slightly spongy growth. Distinct, radial air bubble features in hailstones are associated with the cusp-lines between the dry growth lobes. Less distinct, radial air bubble trains are found along the icicle lobe axes.

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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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Charles A. Knight and Nancy C. Knight

Abstract

The freezing of a spongy ice deposit produces a variety of distinctive internal structures caused by the expansion of the trapped water upon freezing. Cracks, certain arrangements of sub-boundaries, and some particular air bubble structures formed in this way are criteria of spongy hailstone growth, after complete freezing. Conclusions from these criteria applied to natural hail agree with conclusions from orientation fabrics.

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Charles A. Knight and Nancy C. Knight

Abstract

A detailed examination and interpretation of hailstone structure and symmetry leads to the conclusion that the normal falling behavior of moderate-to-large hailstones is rapid, symmetrical tumbling. The tumbling behavior is evidently critically sensitive to one or more of the important parameters, such as surface roughness, density distribution, etc., because similar hailstones grow with different symmetries. The concept of aerodynamic molding is found to be inapplicable to the growth and shape of natural hailstones, with the possible exception of small stones and icicle lobe structures. If, as is the conclusion, hailstones tumble while falling, previous deductions of terminal velocities and heat transfer rates may be substantially incorrect.

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Charles A. Knight and Nancy C. Knight

Abstract

The procedures required in measuring and representing hailstone orientation fabrics, and the reason for the development of one particular fabric, are discussed. Spongy growth has been found to give rise to a fabric with the c axes preferentially perpendicular to the growth direction. The origin of this fabric may be used as a spongy growth criterion. The natural hail examined did not have this fabric, and consequently did not grow spongy.

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Charles A. Knight and Nancy C. Knight

Abstract

General properties of hailstone embryos, from hailstorms mostly in the western United States, are described and discussed in terms of the conditions of growth. Most embryos are composed of large crystals, indicating growth at environmental temperatures ≳−20C. Very wet or spongy growth gives spherical embryos, that presumably tumble because of this growth character. Dry growth leads to the more common, conical embryos. The almost universal presence of a distinct embryo stage in hailstone formation, coupled with certain systematic differences between embryo growth and hailstone growth, appears to be strong evidence that “trigger particles” (Weickmann) are usually necessary in hailstone formation.

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Charles A. Knight and Nancy C. Knight

The Aurora, Nebraska, hailstorm of 22 June 2003 produced some exceptionally large hailstones, and was widely publicized. Nineteen hailstones obtained from local people have been sectioned and photographed and eight are illustrated here, recording their interior layering and external appearance. They exhibit great variability, with features that are common to large hail as well as one unusual growth form: a roughly prolate external shape indicating an approximately constant falling orientation during final growth, forming large, icicle-like projections at one end. Much of the growth is wet but not appreciably spongy, as appears to be common for large hail. While a hailstone from this storm has been called the largest recorded in the United States on the basis of its longest dimension, we suggest that the most meaningful measure of hailstone size is weight. Weight is unambiguous and easily measured, and is an excellent indicator of volume for large hail. People generally think of hail as spherical and large hail but is often far from that shape; the terms “diameter” and “circumference” are therefore inappropriate, and if records of linear dimensions are to be kept, they probably should be termed “maximum length” and “maximum perimeter.”

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