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<br />Winter Seeding - Sih'er Iodide <br />~ <br /> <br />2. The silver iodide <br />pdftides rise into <br />the clouds <br /> <br />3. Tht> silver iodide (cluses <br />cloud SLW to freeze <br />and (lil!'dte I(e crystals <br /> <br />1.A minute amount of silver <br /> <br />iodide is sprayed across a <br /> <br />propane flame "_______ <br />- <br /> <br />~ -- <br />~ -;..- <br /> <br />- <br /> <br />.--' <br />...... <br /> <br />~ <br />... <br /> <br />~ Ice crystals grow big <br /> <br />enough to fall as snow <br /> <br />belofe eV<lpolating <br /> <br />~downllld of m01l111..,i1l5 <br />.- "2- <br />-~ <br /> <br /> <br />- <br /> <br />" <br /> <br />- <br />- <br /> <br />., <br /> <br />. . <br /> <br />..... <br /> <br />- <br /> <br />.- <br />. <br />. <br /> <br />:.. <br /> <br />. <br /> <br />Airflow <br /> <br />Figure 4. Basic silver iodide seeding process from ground generators <br /> <br />chilling is accomplished through introduction of dry ice or expansion of liquid propane (LP) into <br />a gas. LP can begin ice crystal formation at -1 oC or colder, expanding the temperature \vindow <br />of opportunity. Water vapor and SLW generally increase with warnlcr temperatures. so SLW is <br />frequently more abundant within this expanded window from _loC to _Sec. Therefore. much <br />SLW is not converted to precipitation naturally and passes downwind of mountain crests, where <br />it evaporates_ Seeding agents that are effective in this warmer, expanded temperature ,,,,-indow <br />arc consequently attractive, as recognized in the design of a major LP seeding experiment29 in <br />the Northern Sierra Nevada. Whatever their initiation process. seeded ice crystals, like their <br />natural counterparts. grow rapidly at the expense of the SL W droplets. <br /> <br />The foregoing discussion deals with the cloud microphysics associated with seeding. Such <br />microphysical conditions depend in complicated '\lays on atmospheric dynamics and <br />thermodynamics. All these conditions arc intimately linked with atmospheric motions on a vast <br />range of scales, from planetary scale circulations to synoptic ("weather map.') scales, to storm <br />scales to small.scalc turbulent motions. Some understanding of all these phenomena is necessary <br />to develop a conCf.'!Hual model of the atmosphere. on which effective seeding approaches <br />depend. This conccptual model is continually rcvised as the atmosphere is studied and new <br />knowledge attained. <br /> <br />12 <br />