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<br />The efficiency in terms of possible effect produced per gram of seeding <br />material is much lower for hygrosoopic material than for silver iodide-- <br />by a factor of over a million. NevertheleSs, hygroscopic 'seeding can <br />still be efficient. If each 20 \lm diameter particle yields a 5 mm raindrop, <br />1 gallon of material can produce 16 million gallons of rain water, over <br />50 acre feet of rain. Because of operational inefficiencies in dispensing <br />and in applying material to suitable clouds, one probably cannot approach <br />such high efficiency in actual practice, even when using smaller particles <br />where potential efficiencies are greater still. Nevertheless. hygroscopic <br />seeding obviously has an important potential for operational application. <br />Also it is obvious that the system efficiency will depend critically on the <br />use of the right particle sizes in the right conditions. <br /> <br />The corresponding amount of AgI to do the job of 1 gallon of hygroscopic <br />material would be about 1 million times smaller - - a few milligrams as <br />opposed to a few kilograms. However. it should be noted that AgI and <br />hygroscopic material never do an identical job. They are applied in <br />different portions of the cloud, and for various reasons cause somewhat <br />different effects on cloud dynamics. Figure 8 diagrams some of the <br />mechanisms of hygroscopic seeding and AgI seeding. In actual practice <br />there is a spectrum of conditions and the relative weight of factors will <br />vary. Figure 8 does hit the main fq.ctors, at least for simple clouds <br />where suppression and development of adjacent cells are not considered. <br /> <br />20 <br />