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<br />and silver iodide particles are formed. The particles are expelled into <br /> <br />the atmosphere much like would be the exhaust from an automobile. The <br /> <br />atmosphere into which the particles are burned are the preferred updraft <br /> <br />areas at the bases of the clouds to be treated. <br /> <br />On the W~&M program, dry ice is dispensed through openings in the <br /> <br />floor of an aircarft capable of flying well into the supercooled region of <br /> <br />clouds. The dispensers have been designed to automatically release the <br /> <br />commercially manufactured dry ice pellets at a rate of one pound per mile <br /> <br />of flight. When dry ice is dropped into the supercooled part of a cloud, <br /> <br />water droplets are immediately transformed into ice crystals. Dry ice is' <br /> <br />more effective in producing ice crystals lower in the supercooled portion <br /> <br />.~. <br /> <br />or the cloud - approaching freezing, whereas, silver iodide only begins <br /> <br />I. <br /> <br />o 0 <br />producing substantial numbers of ice nuclei around -5 C (22 F). <br /> <br />" <br /> <br />! " <br /> <br />Ir. terms <br /> <br />of cost effectiveness, dry ice produces the greatest numbers of ice nuclei <br /> <br />for the money when compared to silver iodide. Also, there is no residual <br /> <br />effects as there can be with silver iodide ice nuclei. <br /> <br />In order to seed clouds to obtain the desired result, irrespective of <br /> <br />the seeding agent used and the method of seeding, p.ach cloud must be treated <br /> <br />at the proper time. In so doing, it is possible to produce the optimum ice <br /> <br />crystal concentrations in naturally deficient clouds so as to either increase <br /> <br />precipitation through the cold cloud process or to reduce hailstone develop- <br /> <br />ment. <br /> <br />. <br /> <br />Of further significance may be what is known as the dynamic effect due <br /> <br />to seeding. When natural environmental conditions are satisfactory, <br /> <br />cumulus' clouds may be stimulated to grow larger and persist longer than <br /> <br />would be the case if left unseeded. This is done by getting the particular <br /> <br />seeding agent into the supercooled part of the cloud to promote droplets <br /> <br />~becoming ice crystals. When this water-to-ice conversion process occurs <br /> <br />rapidly within the could, latent heat of fusion is released on such a scale <br /> <br />, <br /> <br />-8- <br />