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  • Author or Editor: J. Hallett x
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J. Hallett

Abstract

The crystal fabric of ice formed by the freezing of supercooled water was examined in polarized light. Drops of radius 0.1 cm were frozen by homogeneous nucleation at −33C, by various foreign particles in suspension, or by impaction on a plane single crystal ice substrate. Bulk water, volume about 1.0 cm3, was frozen by the insertion of a single crystal of ice. Between 0 and −5C crystals invariably grew with a single orientation, identical with the nucleating crystal. With decrease of temperature increasing numbers of crystals with new orientations appeared, there being several hundred per drop when nucleation took place at −33C. Drops impacting on a substrate with vertical ‘c’ axis froze with horizontal ‘c’ axis in the temperature interval −5 to −15C. When the substrate was heated to 0C, drops always took the substrate orientation, even when supercooled to −22C.

Measurements of the growth rate component parallel to the basal plane of dendrites growing in water at supercooling (ΔT) down to 20C followed the relation: U=0.08ΔT 1.9 cm sec −1. Dendrite arm width and spacing decreased with increase of supercooling, both following the relation 10−2T cm. Below −8C the envelope of dendrite tips in the basal plane became hexagonal and by −16C growth rate perpendicular to the basal plane became comparable with that parallel to the basal plane. Results are interpreted in terms of different kinetic processes on different crystal faces.

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J. Hallett

Abstract

The temperature dependence of the habit of ice crystals growing in a water vapor diffusion chamber is related to the growth rate of ice crystals in a supercooled cloud. The rates of growth occurring between −4 and −6C as needles or between −12 and −16C as dendrites may be in excess of those at intermediate temperatures by as much as a factor of 100. The effect of seeding a supercooled cloud will therefore depend critically on its temperature.

Growth of crystals at large supersaturation between −4 and −6C takes place as spikes growing along a direction 25° to the c axis. These crystals have been observed in the diffusion chamber, and also as frost near hot springs in Yellowstone Park. The molecular processes responsible for these habit changes are discussed.

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B. J. Anderson
and
J. Hallett

Abstract

Nucleation of individual ice crystals on large (3.0 mm) cleaved crystals of solution-grown silver iodide and covellite is investigated by microscopy. The environmental vapor pressure is controlled by saturating two air streams by passage through ice labyrinths at different temperatures and mixing them in known proportion. This enables the vapor pressure to be changed over a period of about 10 s.

Ice crystals do not usually appear immediately when a supersaturation is imposed. Nucleation, defined as the appearance of crystals of 1 µm radius, is delayed between zero and 70 s near water saturation and between 20 and 400 s at a few percent ice supersaturation, the longer times occurring at higher temperature. This time decreases only marginally when the crystal is exposed to a period of higher supersaturation which ends a few seconds prior to the time crystals would appear at this higher value. The number of crystals per unit area increases with ice supersaturation at a given temperature; for CuS at −16°C, it increases by a factor of 3 between 3% and water saturation. Number concentrations on silver iodide are comparable, but increase with time when the surface is exposed to light. The absolute crystal concentration varies over the substrate surface. Large areas fail to nucleate at all; some areas give high concentrations, 500 mm−2. Crystals form at specific nucleation sites. Each requires a different critical ice supersaturation for nucleation which remains unchanged in sequential tests. This property disappears for AgI after exposure to light; then nucleation sites do not repeat. Nucleation events per unit area are fewer than on particulates which are inferred to contain a proportionately greater surface concentration

of nucleation sites.

Results are applied to crystal nucleation in the atmosphere and the characterization of ice nuclei in laboratory instruments.

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R. A. Black
and
J. Hallett

Abstract

Observations of the type and distribution 0°C isotherm in three Atlantic hurricanes are presented. Supercooled drops, graupel, columns and aggregated snowflakes were observed. The supercooled drops were found only in convective updrafts stronger than 5 m s−1, but not all updrafts > 5 m s−1 contained appreciable liquid. Graupel was found in all updrafts at temperatures < −2°C, and small columns were sometimes found in downdrafts. Nonconvective rainbands contained 15–30 L−1 of snow composed of columns and what appeared to be large aggregates. Other stratiform regions contained 1–15 L−1 of medium and large aggregates; columns were occasionally found there also but only within about 15 km of convection. Hurricane convection is almost completely glaciated at the −5°C level. It is suggested that the ice particles observed at 6.0 km inside the convection result primarily from downward mixing on both sides of the eyewall updraft of ice formed in the convective areas at higher, colder levels. The ice in the stratiform areas is believed to have fallen from the high-level (6.km and higher) eyewall outflow.

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J. Hallett
and
C. P. R. Saunders

Abstract

Laboratory studies of rime growth on a moving rod under conditions of secondary ice crystal production show that the rod acquires a positive charge, equivalent to charge associated with each ejected particle of 5 × 10−4C. Ice crystals produced by seeding also impart a positive charge to the rime, equivalent to a charge per particle of 5 × 10−16C. As the water vapor supply is cut off, the charge sign reverses. The results suggest that the sign of the charge transfer depends on the physical state of the rime surface and its vapor pressure excess or deficit relative to the environment. Charge separation in convective clouds is critically dependent on the changing proportion of graupel and small secondary ice crystals.

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B. J. Anderson
,
J. D. Sutkoff
, and
J. Hallett

Abstract

Ice crystals grow from the vapor in the presence of Eastman9 10 cement (containing methyl 2-cyanoacrylate monomer) in the form of fine fibres a few microns in diameter, in the c axis direction of the nucleating crystal. A previous suggestion that similar fibers observed using this material for the replication process occur undernatural conditions and are a source of atmospheric ice crystals appears unlikely.

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