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G. G. Lala and J. E. Jiusto

Abstract

An automatic instrument for the measurement of cloud condensation nucleus (CCN) concentrations utilizing a thermal gradient diffusion chamber and light scattering has been developed. The concentration of droplets (activated CCN) in an illuminated volume is determined by the measurement of the peak light intensity scattered at 45°. The CCN concentration is linearly related to the scattered light signal at a fixed supersaturation S, but the sensitivity exhibits an S 0.55 dependence over a range of supersaturations. Calibration of the system against the photographic method verifies the linear dependence of scattered light on number concentration. The main features of the system are the automatic sampling, measurement and recording of CCN droplet concentration by means of scattered light with the capability of direct calibration by means of the photographic method. Results from four days of continuous hourly measurements of the CCN spectra show the influence of meteorological events on CCN. Included in the data are a frontal passage, fog formation and dissipation, and the occurrence of a nocturnal peak.

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Roland J. Pilié and James E. Jiusto

Abstract

Contrails were produced for laboratory study by burning aircraft fuels under controlled conditions of ambient temperature and humidity at pressure altitudes between 1000 and 300 mb. Observed critical formation temperatures differ from Appleman's theoretical data in a manner similar to that obtained on project CLOUD TRAIL flights. Laboratory experiments with these trails proved that the initial phase of the condensed moisture is liquid and produced strong evidence that, contrary to general belief, the final phase is sometimes liquid. Additional evidence was obtained indicating that Appleman's criterion for a barely visible trail (0.004 g per m3 of condensed moisture) is very nearly correct for ideal conditions of observation such as used in the laboratory, but is probably small by an order of magnitude or more for adverse conditions. By modifying Appleman's theory to allow for the production of a visible quantity of liquid water under adverse viewing conditions, agreement is reached with project CLOUD TRAIL data. Also presented is a simple interpretation of the theory which substantially reduces the labor required to compute critical temperatures for contrail formation.

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J. E. Jiusto and H. K. Weickmann

Few would question such doggerel as “no two snow crystals are alike and in infinite variety they fall.” Conversely, many might challenge the statement that individual crystals are relatively scarce by comparison with snowflake aggregates, rimed crystals, and irregular forms; but they are. The types of snow occurring in a storm are a function of many variables including updraft strength, temperature, cloud moisture, liquid water content, cloud thickness, and ice nuclei and crystal concentrations. Some of the relationships are obvious, others more obscure, still others unknown. This paper describes ground observations and calculations that attempt to clarify certain snowfall forms in terms of the cloud conditions likely to produce them.

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J. E. Jiusto, R. J. Pilié, and W. C. Kocmond

Abstract

Analytic and experimental investigations were conducted to examine the concept of modifying fog with hygroscopic material. An approximate equation was derived that is useful in estimating the feasibility of such applied problems. The combined results show that it is possible to improve visibility in warm fog by seeding with micron-size salt particles (NaCl). The visibility in laboratory fog produced in a 600-m3 chamber was increased by factors of 3–10, with as little as 1.7 mg m−3 of NaCl being effective. Only a modest reduction (<1%) in ambient relative humidity by the giant salt particles is necessary to cause substantial evaporation of the fog droplets. Extrapolation of these results suggests that clearing a suitable landing zone for aircraft should not involve prohibitive amounts of properly sized seeding material.

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atmospheric sciences and problems of society

A series of statements on the relevance of the scientific and technological areas of AMS STAC Committees to national and international problems

Earl G. Droessler, John W. Winchester, Guy A. Franceschini, O. H. Daniel, J. Doyne Sartor, James E. Jiusto, and Thomas A. Gleeson
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