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H. J. aufm Kampe and H. K. Weickmann

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H. K. Weickmann and H. J. aufm Kampe

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H. J. aufm Kampe and H. K. Weickmann

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H. J. aufm Kampe and H. K. Weickmann

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H. J. aufm Kampe and H. K. Weickmann

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H. J. aufm Kampe and H. K. Weickmann

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Measurements of the effectiveness of natural and artificial freezing-nuclei were carried out in a room-size cold chamber. The concentration of natural nuclei varied from approximately one per liter at about −13C to 1000 per liter at −25C. Since a concentration of 1 to 10 ice crystals per liter is sufficient to produce continuous precipitation, clouds which reach the temperature level of about −15C should precipitate. In general, this is verified by the observations of several investigators.

Investigations of the effectiveness of silver iodide, cadmium iodide and cobalt iodide indicate that cobalt iodide is almost as effective a freezing nucleus as is silver iodide. However, due to its high hygroscopicity, cobalt iodide probably can not be used to seed clouds from the ground.

The threshold temperature at which silver iodide and cobalt iodide are effective is approximately 10C higher than that at which natural aerosols of well industrialized and populated regions are effective. Consequently, whenever synoptic situations for appreciable rain exist, it will probably rain without seeding. For special cases, however, seeding may produce noteworthy amounts of rain; but this rain is local and of short duration.

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H. J. aufm Kampe and H. K. Weickmann

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With use of cloud data collected in all types of cumulus clouds, it is shown that the constant in the Trabert equation is not actually a constant but varies with the character of the droplet spectrum. Trabert's formula consequently is not suitable, in general, for the determination of liquid-water content from measurements of visibility and droplet size. It is shown how the water content can be determined from these measurements, provided that the whole droplet spectrum and not just an average droplet size is known.

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H. K. Weickmann and H. J. aufm Kampe

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Measurements of droplet size and visibility have been made at different levels above the base of all types of cumulus clouds. The evaluation of these data permits calculations of corresponding values of water content and droplet concentration. It has been found that the droplet spectrum in fair-weather cumulus is narrow, whereas the spectra in cumulus congestus and cumulonimbus are very broad with droplet radii as large as 100µ. The droplet concentration decreases with height above base, suggesting the action of an efficient process of coalescence. At a height of about 2000 m above base, minimum values of visibility have been found. It appears that coalescence of cloud droplets is an important process in the formation of droplet spectra.

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E. L. Magaziner and H. K. Weickmann

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The mesoscale, individual-hailstreak structure of hailswaths can markedly affect the analysis of hail suppression experiments that are based on protecting a given area from approaching hailswaths. Calculations reveal that such an experiment may (depending on the size of the protected area and on hailstreak length) be counted as a success even when the hail suppressing activity is ineffective. This phenomenon may have contributed to serious errors in the evaluation of the pilot test program in hail suppression conducted in the Soviet Union during 1961–63, on which consequent programs were based.

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P. M. Kuhn and H. K. Weickmann

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Based on aircraft measurements of the radiant emittance of cirrus clouds and cloudless sky, the 8-13μ infrared transmissivity of high cirrus clouds was calculated. The measurements and applied calculations were made for cirrus clouds up to 5 km thick at altitudes ranging from 25,000-42,000 ft. Results of the calculations of transmissivity ranged from 95% for high, thin cirrus to 53% for 5 km thick cirrus.

The transmissivity results were applied to radiative transfer calculations in an attempt to reduce discrepancies between in situ measurements and calculations. The application of the measured transmissivities in the calculations did, in fact, reduce the discrepancies. This should provide one interim modification in the treatment of the radiative transfer equation in the presence of cirrus while awaiting a complete solution tocomplex cirrus transmissivity and scattering.

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