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<br />Seeding Materials. - ARIDROP used pyrotechnic silver iodide smoke <br />generators. and to a lesser degree burners employing silver- <br />ammonium-iodide complex. Both produce ice-forming nuclei free of <br />hygroscopic contaminants (Finnegan, et al, 1971). The Catalina <br />burners used silver-potassium-iodide complex. which produces ice <br />nuclei contaminated with hygroscopic substances and has been <br />found to be so inferior in the activity in the _50 to _150 C tea- <br />perature range as to have been completely superseded in modern <br />cloud-seeding practice. <br /> <br />Selection of Clouds for Seeding. - In ARIDROP. clouds were individ- <br /> <br /> <br />ually selected by the seeding pilots as being within the criteria <br /> <br />for suitability established from previous experimental work. In <br /> <br /> <br />general terms. these are actively growing cumulus clouds that have <br /> <br />not reached maturity. clouds for which the computational models of <br /> <br /> <br />cloud growth and precipitation production used for opportunity recog- <br /> <br />nition indicate the likelihood of positive response to seeding. <br /> <br />Classes of clouds for which the models indicated risk of decreased . <br /> <br /> <br />precipitation were excluded from the seeding. <br /> <br />The Catalina seeding was restricted to days when atmospheric mois- <br /> <br /> <br />ture and instability exceeded given minima. but on treatment days <br /> <br /> <br />seeding was aimed from a distance at the cloud-breeding area in <br /> <br /> <br />general and at no given cloud in particular. a hit-or-miss pro- <br /> <br />cedure as far as the individual cloud characteristics were concerned. <br /> <br />4 <br />