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Charles I. Davis and Roger L. Steele


A variety of devices that produce artificial nuclei have been used in weather modification experiments. These fall into two broad classifications, i.e., pyrotechnics and steady-state flame-type generators.

The effectiveness-temperature relationships for these classes were measured and are presented in graphical form. In summary, the steady-state systems are in general one order of magnitude more effective than the pyrotechnics at temperatures below −12C. Above this temperature, the effectiveness of the pryotechnics approaches and in some cases exceeds that of the steady-state systems. These different characteristics can be explained by particle size effects.

Other performance parameters are examined. Air-fuel ratio is one of these since effectiveness varies markedly near the stoichiometric air-fuel ratio for steady-state systems. A system which employs isopropylamine as the AgI carrier is more sensitive to changes in air-fuel ratio than the NaI-acetone AgI carriers. Another important performance parameter is AgI burn rate. In general, the effectiveness of a pyrotechnic decreases rapidly and then levels off as the burn rate increases. This same effect has been observed in steady-state systems.

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Hermann E. Gerber, Paul A. Allee, Ulprich Katz, Charles I. Davis, and Lewis O. Grant


The Goetz Aerosol Spectrometer, generally considered to possess only a fair ability in resolving size distributions of polydispersed aerosols, operates properly following a modification to the geometry of the entrance to the instrument's deposition channels. Its accuracy is demonstrated with an electron microscopic evaluation of a collecting surface deposit of a thermally produced polydispersed AgI aerosol with particle sizes ranging from 60 to 1000Å In diameter.

Thus calibrated, the instrument was utilized to investigate the activity of the same aerosol as freezing nuclei. The AgI particles on the hydrophobic chrome-plated collecting foil were nucleated by sorption at water saturation for temperatures of −15 and −20C. The results appear to reflect the influence of the Kelvin effect since the activity decreased at a faster rate than predicted by the “surface area rare” and since it showed a sharp cutoff corresponding to Fletcher's theoretical size temperature predictions for ideal sublimation nuclei.

Also, field measurements were conducted on 12,000-ft Chalk Mountain (Climax, Colo.) for the purpose of measuring the sizes of active AgI-NaI nuclei emanating from acetone ground generators located at least 6 mi upwind. The size distribution of the nuclei on seeding days proved similar to what might he expected from this generator type. On non-seeding days, the number of active nuclei decreased sharply while the peak of the size distributions shifted to larger sizes.

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