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Thomas E. Hoffer

Abstract

Laboratory experimentation was undertaken to investigate the effects of various soluble and insoluble nuclei on the freezing temperature of water droplets. The freezing temperature and size of over 15,000 droplets were obtained. Pure water was produced, and subsequently frozen in droplet form. The freezing temperatures of various sizes of pure water droplets were used as a reference standard for the other phases of the experiment.

The experiment showed that soluble salts, commonly found in the atmosphere, caused the freezing temperatures of the droplets to become colder than would be anticipated by bulk freezing point lowering calculations in all cases. Insoluble nuclei increased the freezing temperatures of the droplets. The addition of soluble salts to water droplets containing insoluble nuclei caused a marked depression of the freezing point below that originally observed. The magnitude of the depression was found to be a function of the solute concentration.

Certain aspects of the role of droplet freezing in the atmosphere are discussed by extending the experimental results to atmospheric processes.

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Thomas E. Hoffer
and
Steven C. Mallen

Abstract

Measurements of evaporation rates suggest that when water droplets are exposed to certain contaminants, a monolayer can form which inhibits evaporation. Experimental droplet evaporation rates are compared to theoretical rates; for pure water the agreement is good, while for contaminated water a wide variance is noted.

Some of the implications of these experiments in relation to the dispersion of fogs are discussed.

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Larry R. Eaton
and
Thomas E. Hoffer

Abstract

A system is described for accurately generating droplets of known size in the diameter range 300–500 μ, and measuring their terminal velocity in air. The error in terminal velocity is <0.5%.

Measurements were made using several diameters of tubing to investigate the effect of walls on the terminal velocity of droplets. These data are compared with those of other authors. The wall effect on drag coefficient falls to the 1% level at approximately 70 droplet diameters.

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Thomas E. Hoffer
and
Steven C. Mallen

Abstract

A vertical wind tunnel which will support droplets in the size range of 50–200 μ is described. It consists of three sections, a diffuser, a profile forming section, and the observational section. The construction details of all three sections are provided with emphasis on velocity profile formation. Its materials are such that experiments requiring a clean atmosphere can be performed on droplets less than 200 μ in diameter.

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Thomas E. Hoffer
and
Alice R. W. Presley

Abstract

The results of an experiment determining the collection efficiency of a cylinder at aircraft speeds are presented and compared with the theoretical computations of Langmuir and Blodgett. It was found that the agreement was good for low speed and low collection efficiency (0.1). The discrepancy between theory and experiment approached a factor of 2 as the theoretical collection efficiency increased.

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Thomas E. Hoffer
and
Alice R. W. Presley

Abstract

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Thomas E. Hoffer
and
Alice R. W. Presley

Abstract

This paper shows the necessity of determining experimentally the collection efficiency of an aerodynamic probe, before using it to collect an aerosol. The experimental techniques described herein, although applied to a particular probe, are general.

The collection efficiency of a vented probe is shown to be in exam of 1 at certain speeds and particle sizes while the experimental collection efficiency for a cylinder is close to theoretical predictions.

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Thomas E. Hoffer
and
Roscoe R. Braham Jr.

Abstract

As a means of studying ice nucleation, snow and ice pellets collected from the tops of clouds were melted and refrozen in order to determine their freezing temperatures. In all cases where a definite cloud top temperature could be assigned, the melted ice pellets froze at a temperature colder than that of the cloud top, indicating that these pellets did not originate through the heterogeneous freezing of cloud drops. Essentially no difference was indicated in the freezing temperatures of ice pellets collected on seeded and non-seeded days. A firm statement on this point could not be made as the number of observations is limited, and it is not certain that the seeding agent had been ingested into the cloud being studied.

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