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- Author or Editor: Nancy C. Knight x
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
Spongy hailstones have been collected by the quenching method, and their structures examined. The most common type of spongy hail in Colorado is small hail that formed as loose rime (graupel) and became soaked with liquid water during fall. Hail that grew spongily has also been observed, and the structures are illustrated. The interpretations are confirmed by experiment.
Abstract
Spongy hailstones have been collected by the quenching method, and their structures examined. The most common type of spongy hail in Colorado is small hail that formed as loose rime (graupel) and became soaked with liquid water during fall. Hail that grew spongily has also been observed, and the structures are illustrated. The interpretations are confirmed by experiment.
