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<br />naturally-occurring 1l\~ which common(..... have thresholds closer to .150C The chemical <br />formulations of Agl seeding llgent.\. may be mod~fiedfurther, so that the resulting IN <br />fUf1(.tion at e\'en warmer temperatures (DeA-folt 1991. Garvey 1975). <br /> <br />Drr Ice <br />The direct (Teation of cloud ice particles by di.~lJensinK ,b:r ice (COj pellets into <br />the cloud is another glaciogenk seeding technique u.hich modifies the nalllral ice <br />formation proct!ss by rapid(r trlm'ij(Jrming nearby mpor and cloud droplets into ice <br />(Schaefer 19-16, I/olroyd etal. 1978. VonnexUl 1981), <br />Compared with sill'er iodide complexes, Ihis sy,')/em has an admntllge in that il <br />makes use ofll natural subslllnce (frozen carbon dioxide. CO;!. which suhlimes at .780C <br />at 1,000 IIPa). 1I0wel'er. effeclive delivery oflhe C01 requires the lise ofaircrl~ft. The <br />CO.. is also dij}kulllo slore, as sublimalion (and fherefilre loss) is conlinuolH. It is <br />Ill/common for dry ice 10 be Ihe on(..... seeding agenlused in a projecI: if is sometimes used <br />in conjunction wilh Agl seeding. <br /> <br />Lietaid ProplIne <br /> <br />Liquid propane is afree=ing agent milch lih' lby ice. II produces almost the same <br />number of cryslals per gram as does CO;! (Kumai 1982). It Cllnnot be dispemedjrom <br />airaaji because if i.~ aJlammable substann.'. /-I{)\\'(.'l'er. il call be di.\jwnsedfrom the <br />ground if released al del'lltions which are frequently within supercooled cloul/\'. The <br />Uniled .)'tates Air Force has used liquid propane dispen.w:d from growul-based siles 10 <br />dear .\'IIpercooledfiJg al miJitmyairportsfiJr over thirty years. <br /> <br />Propane seeding \vas tcstcd as a cloud secding agent on a wintcr research program <br />conducted in California for .....inter snowpaek cnhancemcnt through the dcvelopment of a remotely <br />operated ground-based dispenser (Rcynolds 1991. 1992). Liquid propane seeding cxpcriments <br />were also conducted on the UtahINOAA Atmospheric Modification Project (Super. 1999). The <br />intcrest in propane sceding is due primnrily to thc fact that propanc seeding may be elTcctive at in- <br />cloud temperatures near _20 C comparcd to the effectiveness of silver iodide beginning at <br />temperatures of approximately .5oC. Research in some mountainous arcas ofthc \\cst in <br />wintertime indicate that there appears to be rather frequent occurrences of supercooled liquid water <br />at tc:mpcratures between 00 and -So C (Super. 1999). Sincc supercooled liquid watcr droplcts are <br />thc target of cloud seeding the hope is that seeding with liquid propane could expand the window <br />of seeding opportunities in tenns of in.c1oud temperatures. A rccent randomized research <br />experiment was conducted on the central Wasatch Plateau of Utah testing this agent's possible <br />usefulness in \\ intcr timc cloud seeding programs (Super and Heimbach. 2005). Results of thc <br />randomized treatments indicate seeding increases over a small arca during some stonn periods with <br />liquid propane. An update to the ASCE Manual 81 (ASCE. 1995) titled "Guidelines for Cloud <br />Seeding to Augment Precipitation" (due to be published early 2006) contains a recommendation <br />that future experimentation needs to be conducted using liquid propane seeding over a fixed targct <br />arca to dcmonstrate that increases are occurring over substantial timc pcriods. As a consequence. <br />propane seeding is not recommendcd for use in the near term as all operational cloud seeding <br />agcnt on the potential Colorado River Basin programs. It is recommended that a multi-war <br />r;scarch program be conducted to dctennine the elfecti....eness of propane seeding in ge'nerating <br />