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P. V. Hobbs

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P. V. Hobbs

Abstract

A previously derived theoretical expression, supported by experimental observations, for the rate of sintering between two ice spheres is applied to the case of adhesion between ice particles down to temperatures of −40C. Appreciable bonding is shown to take place between the particles within 10 seconds of contact even at −40C. The results predicted by the theory are found to be in quantitative agreement with the degree of sintering observed in aggregates of ice particles from natural ice fogs.

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A. L. Rangno
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P. V. Hobbs

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A. L. Rangno
and
P. V. Hobbs

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A. L. Rangno
and
P. V. Hobbs

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A. L. Rangno
and
P. V. Hobbs

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L. F. Radke
and
P. V. Hobbs

Abstract

Simultaneous measurements have been made of the concentrations of cloud condensation nuclei, sodium-containing particles, Aitken nuclei, and the magnitude of the light scattering coefficient of the air, for a period of two months in the Olympic Mountains of Washington State.

Large short-term changes in the magnitudes of these four quantities were found to be related to variations in the local meteorological conditions. The most striking changes occurred with the build up and the evaporation of cumulus clouds upwind of the measuring site. The results indicate that growing clouds absorb (and also probably generate) large numbers of particulates, and that these particulates are released when the clouds dissipate. Precipitation also caused significant reductions in the concentrations of particulates in the air.

Longer period variations in particulate concentrations were associated with the diurnal convective cycle and changes in air mass. Continental air contained higher concentrations of cloud condensation nuclei and Aitken nuclei than maritime air, but the Pacific Ocean appeared to be the principal source of sodium-containing particles. However, even in maritime air the measured concentrations of sodium-containing particles were always less than about 1% of the concentrations of cloud condensation nuclei active at 1% supersaturation.

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D. A. Burrows
and
P. V. Hobbs

Abstract

The sources of the electrical charges acquired by ice spheres rotated through natural cloud or precipitation particles are considered. In general, the steady charge that a sphere receives in such experiments derives from collisions with particles in the air and from the electrical coupling of the sphere with the ground.

The results of a number of experimental measurements on the charging of ice spheres whirled through natural snowfalls are described and analyzed. The average charge acquired by the ice sphere per ice particle collision was always negative over the range of temperatures investigated (−2 to −8C) and reached a maximum negative value of about 1.5×10−4 esu at −7C.

It is pointed out that if natural hailstones falling through cumulonimbus clouds receive charges due to collisions with ice crystals which are similar in magnitude to those measured in these experiments, this mechanism would readily explain the electrification of thunderstorms.

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P. V. Hobbs
and
A. J. Alkezweeny

Abstract

The fragmentation of water droplets during freezing has not been previously established under conditions which resemble those in natural clouds. In laboratory experiments designed to investigate this phenomenon it is important that the droplets be close to the equilibrium state with their environment prior to nucleation. In this paper theoretical expressions are derived which may be used to calculate the degree to which a droplet is in equilibrium with its environment as it falls through a gas which has a vertical gradient of temperature.

Experiments are described in which the fragmentation of freezing droplets from 50–100 μ in diameter have been observed under conditions which are close to those in natural clouds. The fragmentation appeared to be independent of the nucleation temperature over the range investigated (−20 to −32C) and it also occurred for droplets nucleated at −8C by silver iodide in supension. Droplets from 20–50 μ in diameter were not observed to fragment during freezing.

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J. E. Dye
and
P. V. Hobbs

Abstract

The fragmentation of freezing water droplets in natural clouds has been postulated by several workers, and this phenomenon has been observed in numerous laboratory investigations. However, the profound effect that environmental conditions can have on fragmentation has not been fully appreciated. In the first part of this paper the factors that might affect the freezing behavior and fragmentation of a water drop are discussed, and, where possible, are analyzed in detail.

In the second part of the paper results are presented of laboratory experiments on the freezing of suspended water drops 1 mm in diameter. Drops nucleated in air under equilibrium conditions were never observed to shatter and only one drop in ten ejected an ice splinter. The shattering and large splinter counts from suspended drops nucleated in air which have been reported by other workers are attributed to the contamination of the drops by carbon dioxide and nucleation under non-equilibrium conditions. Drops frozen in hydrogen shattered frequently if the temperature was lower than −9C. Drops frozen in helium at −10 to −12C shattered on occasions. In a mixture of air and carbon dioxide the shattering behavior was very dependent on the concentration of carbon dioxide. Large numbers of ice splinters were detected only if a drop shattered.

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