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<br />How Cloud Seeding Works <br /> <br />2. Thtosilw>riodide <br />particles rise inlo <br />Ihf"clouds <br /> <br />1. The silver iodide causes <br />cloud moisture 10 freeze <br />clnd Of'att" ice ('}'\laI5 <br /> <br />1. A minute amount of silver <br />kKiidt> is sPfayed across a <br />prop.lnerLafTW!' <br /> <br />4. Ice aystdls grow big <br />enough to fall as snow. <br /> <br />. <br /> <br />... <br /> <br />-. ..... <br />C~_ <br /> <br />--' <br /> <br />,~ <br /> <br /> <br />- <br /> <br />M <br /> <br />- <br />- <br /> <br />....~ <br />- <br /> <br /><. <br /> <br />. <br /> <br />.,. <br /> <br />Airflow <br /> <br />. <br /> <br />Figure I {;round Seeding in.. Winter Orographic Setting <br /> <br />3.0 Tn)ical Seeding Agents and ~Iodcs of Seeding <br /> <br />3.1 Typical Seeding Agents <br /> <br />The American Society of Civil Engineers. Environmental and Water Resources Institute <br />recently published a document entitled "Standard Practice for the Design and Operation of <br />Precipitation Enhancement Projects" (ASCE 2004). This Standard contains a summary of <br />dilTercnt types of cloud seeding agents. The summaries for silver iodide. dry ice and liquid <br />propane arc as follows. <br /> <br />Sill'er lot/Me <br /> <br />Si/w!r iodide. in combination with var;Oll.\' olher chemicals, most oJien sa/Is. has <br />h('en used as a ;:lacioKenic l1Ren! fin' halfa century. In spile of its relatil'I!(V hi;:h cOsl, il <br />remains afifl'Orile. especia/(J' inji)r!1l11lations which re.mlt in ice nudei (J.\~ with <br />hygroscopic lendencies. <br />Sih-er iodide has utility Wi an ice nucleant because it has tlu! fhree properties <br />required/hr field application. These are: (/J it is a nucleant. regardless of mechanism. <br />(2) it is relatil'e()' insoluble at <1(J9 g per gram ofu-ater, so Ihat the particles can <br />nucleate h.'e before they dissoh'e. and (3) it is stahle enough at high femperatures to <br />permit wlpori=ation and re-condensalio11tojhrm larKe numbers olfwl('{io11al nuclei per <br />.wam ofAgl burned (see Finnegan 1998). Tl1U.~, the ice crystallf=ation temperature <br />thresholdfiJr Ag! is abollt.50C. significantly U'arma than the thresholdfor mosl <br />