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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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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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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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B. A. Gardiner and J. Hallett

Abstract

Airborne measurements of cloud liquid water content derived from a formvar replicator, a Johnson-Williams probe and a forward scattering spectrometer probe (FSSP) are compared. These show that in the presence of ice crystals the FSSP droplet spectra may be artificially enhanced. Typically the ice produces a flat distribution superimposed on the actual droplet distribution. The concentrations measured by the FSSP in the presence of ice are found to be 2–3 orders of magnitude greater than the actual ice concentrations as measured by the formvar replicator and a 2D-C probe. Possible explanations for the abnormal behavior of the FSSP in the presence of ice particles are discussed.

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T. C. Foster and J. Hallett

Abstract

The production of ice crystals as a result of the expansion and cooling of moist air was investigated by laboratory experiment. In particular, the warmest expanded air temperature that produces crystals was sought as a function of the initial temperature. The results fit the standard theory of homogeneous nucleation of water droplets, as long as the droplets remain at a cold enough temperature for sufficient time to from by homogeneous nucleation. Experiments were also carried out with “dry” air (dewpoint less than −40°C). Quantitatively different results were obtained, namely, that much colder expanded air temperatures were required to produce crystals with essentially no variation in numbers of crystals produced as the initial temperature varied. These results are also consistent with the same homogeneous nucleation theory. These idea were applied to the production of aircraft-produced ice particles by means of the adiabatic expansion and cooling that occur near the propeller blades of the aircraft; this mechanism is sufficient for the production of such particles in some flying situations. In particular, the difference from one aircraft to another seems a less important factor than the variations in Right conditions for a given aircraft. Situations that require large thrust from the propellers (for example, climbing, icing, or flying at very slow speed with flaps down) are most likely to produce ice particles and should be avoided in all cloud passes made when repenetration is intended. Various actual cases of aircraft produced ice were examined, and in some the same larger-than-average thrust condition was met.

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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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R. G. Oraltay and J. Hallett

Abstract

Melting, freezing, and evaporation of individual and aggregates of snow crystals are simulated in the laboratory under controlled temperature, relative humidity, and air velocity. Crystals of selected habit are grown on a vertical filament and subsequently melted or evaporated in reverse flow, with the velocity adjusted for appropriate fall speed to reproduce conditions of the melting layer. Nonequilibrium conditions are simulated for larger melting ice particles surrounded by smaller drops at a temperature up to +5°C or growth of an ice crystal surrounded by freezing ice particles down to −5°C. Initial melting of well-defined faceted crystals, as individuals or in combination, occurs as a water layer >10 μm thick. For larger (>100 μm) crystals the water becomes sequestered by capillary forces as individual drops separated by water-free ice regions, often having quasiperiodic locations along needles, columns, or arms from evaporating dendrites. Drops are also located at intersections of aggregate crystals and dendrite branches, being responsible for the maximum of the radar scatter. The drops have a finite water–ice contact angle of 37°–80°, depending on ambient conditions. Capillary forces move water from high-curvature to low-curvature regions as melting continues. Toward the end of the melting process, the ice separating the drops becomes sufficiently thin to fracture under aerodynamic forces, and mixed-phase particles are shed. Otherwise ice-free drops are shed. The melting region and the mechanism for lowering the melting layer with an increasing precipitation rate are associated with smaller ice particle production capable of being lofted in weaker updrafts.

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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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D. Lamb, K. W. Nielsen, H. E. Klieforth, and J. Hallett

Abstract

Investigations of the structure and organization of synoptic-scale storms over the Sierra Nevada Mountain Range during two successive winters (1971–73) were made with a modified B-26 aircraft. Measurements of liquid water content, temperature and dew point were made along horizontal traverses in a vertical plane oriented roughly perpendicular to the main crest and extending from Lake Tahoe to Sacramento, Calif. It is shown that the spatial distribution of liquid water is linked to the gross terrain features, as is the surface distribution of precipitation. The main centers of cloud liquid water content tend to form 40–75 km upwind of the main crest in highly convective cells.

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