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Nancy C. Knight
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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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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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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

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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Nancy C. Knight and Charles A. Knight

Several observations illustrate the ease with which particles can contaminate hailstones collected from the ground. There are two modes of penetration: in large hailstones, particles enter through the lobe boundaries, and in smaller ones, they enter directly into porous portions of the stones. Washing or melting the exterior layer of a hailstone will not assure that the remainder is uncontaminated.

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Charles A. Knight and Nancy C. Knight

Abstract

The mechanism of origin of conical graupel is an important problem, since this graupel form is a common stage in rain and hall formation. Previous theories are reviewed and found to be questionable as general mechanisms. Another mechanism is proposed, whereby the conical graupel grow, attached at their tips to the undersides of planar snow crystals, and break oil within the cloud.

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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

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

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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