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<br />BACKGROUND OF CLOUD SEEDING FOR THE SAN JUAN ECOLOGY <br />11 <br />paOJ!~ (W. Howell. U.S. BUTeau of Reclamation) <br /> <br />When wtnter anow.torllls sweep over the San Juan Houn- <br />tain., not. .11 oC the 1I\018tute. tha.t condense. In the <br />fo~ of cloud. above the mountaIns fella 08 snow. <br />Huch of it remains 1n particles too small to fall. <br />Carried beyond the mountains by the wind to where the <br />airflow 8ink. once more toward the plains, theee rar- <br />ticle. re.-evaporate:. The rate at which !lno'" reachea <br />the around, exprel.ed 88 8 propottion of the rate at <br />which DOiature condenaea In the cloud, 18 called the <br />.fflciency of precipitation. <br /> <br />It hae been found that 1n 80me storms, especially <br />tho.. having relatively deep cloud systems with ctoud- <br />.t~p...t_per&ture8 below about -27 C. the precipitation <br />'eff1eieney:tendB to be relatively high, and 1n such <br />situationl there Is little that the current knowledge <br />of weather aaodiftc.t1on could do to inCUltU the snow- <br />t.ll. In w..k .tor~ that conden8~ very l1ttle mois- <br />ture, there .1s Ukewhe little potentiAl fur'st'l...,tlll.- <br />tinn. However, it has been found that when r:h~ . <br />clouds are' (fee enou h and active enou h to e'Hld. . <br />r-el:ltively large amounts 0 moisture but clip. ~~l,,":l <br />, top temperature 18 warmer than about ..26 C. 5P.tldJlliL <br />of the c.louds 'With artificial lct nuchl (\rl~n flli':l't'ft. <br />the preCipitation eUlclency tram a rather- lOW VHJIll' <br />to one t ieal of the colder clou.4IJ.. Under thesl;!: <br />8'Cticula't conditions cloud Beed n has the oteu- <br />t a 0 su atant a y nerea. ni t e rate 0 prec ~l- <br />tatLon, probably by aa much aa II hundred peccent. <br /> <br />The Colondo River aa.in pilot rro ect Wlla deai nud <br />L11S1 a atati.tical t..t 0 t e cape tV 0 tie c au <br />seedin technolo of 1971 to brin about recJpita- <br />tion ncreases. en t e weat er orecflsters exre(~ced <br />stormcloud conditione considered favorable for ~eedinR <br />,,!thin a twenty-foul" hour period beginning ~t lJ A,H. <br />(and 'if ..tabii.hed ..fety criteria were met), an <br />f'experimental dayfl w.. declared. A randomized deci- <br />sion then w.. made whether the experimental day wuuld <br />be .eede~ or left' un..eded a. . control. The experi- <br />ment va. intended to run for four conlecutive winters <br />and accumulate 160 experi..ntal day. about equally <br />divided betwe.n .eeded and unaeeded. It actually <br />ran for fiv. winten and accumulated 71 sef!ded And 76 <br />unseeded day.. <br /> <br />It was thou ht that the snovf.ll on seeded d8 s mt ht <br />. xceed t at on unsee ed ex eriment81 da s b about 1 <br />percent a po.s t up to ~ercent. and that ,ex- <br />.[imeneal da . ml t account or 8S much as half uf <br />t e season . .now all. This would have corresponded <br />to a maximu. recl itation incre.s. of Ibout 7.S per- <br />cent for the 8"lon 4S a who e. t oug eva uat on <br />i. not yet complete. ILrellm1nary results indicate <br />that no luch sizable overall incre.se WI' realized. <br /> <br />There appear to have been III.Ilny "unleeded" days Jmr.'le- <br />dilteIy following seeded day. when silver iodlde <br />.moKe, tnpped in the valley. upwind of th~ mountnlll <br />range, affected the clouds and effectively caused <br />theae day. to be leeded. On other days Btron_ wJndA <br />cs.rrled t~e .eedlng effect over the mountain .::rest. <br />and outaid~~, .t.~~. ~_~u~r., ~~~~_' On at.l11 other day". <br /> <br />designated 88 flseeded," tile silver iodide fnlJE'd for <br />une reASQn or another to rescb the. clouds. 011 stUl <br />other occasions the forecast of favorable conditlon~ <br />was unfulfilled or ebe fulfUled for only 8 portion <br />of the 24-hour pertod, so that any seeding effect ..,a.s <br />greatly diluted. <br /> <br />Nevertheless there exists an identifiable Bubset uf <br />experimental daYI. free from thee. dilturbina influ- <br />ences. for which lubetantial snowfall increases <br />could be ldentltled with ~ hlRh degree of confidence. <br /> <br />The tentative conclusions lrom the pro ect charactcr-- <br />ize it 88 a limite succeS8. reve. e ser ous <br />weaknesses ot the IYll teChnology in the methods used <br />to identit OIseedablell occaaions and to place the <br />seeding: material 1"n the desired plllee at the esired <br />~. It fftiled also to give a reasonabl~ nccur~t~ <br />rnelUlure of the pOtentidl of cloud 6eedtng fur 111- <br />creAsing the sea80nal precipitation. On the other <br />hand, it furnished additional.evidence that the IIndt'I'- <br />lying principles are sound and that ~re8ent weakness <br />has to do 1M1nly with the practical ifficulties .It <br />effective appll~atlon. <br /> <br />During the five winter season. of the Pilot Project, <br />320 kR or sliver iodide, containing 147 kg of 5tlver. <br />were dispersed. An unkno'ffl proportion of 't't ~'Wa8 'de- <br />posited on the ground or on ves_tation near the gencr- <br />~tDT eiteei a further unknown proportion was carrl~d <br />to nile aide or the other of the study ar:ea or depo!llt('ll <br />with precipitation either upwind or downwind from tlw <br />study area, or escaped ,rec1pltat1on processes and <br />remained suspended in the atmosphere. An estimate that <br />70 kg, of sUver Ictually sceulIlulated within the 340.001) <br />hectares of the study area appears reasonable, an accu- <br />muiation of 0.04 grams per hectare season. ThE" cunent <br />preliminary estlmnte of average 8nO\l accumulation 1n <br />the study area for the five years aftlOunts to 65 em <br />water-equiwli1ent per season. If 2S percent of this <br />fell fro~ seeded .torms. thi. amounts to about 0.003 <br />8m ot silver per hectare ~r centimeter of seeded pre- <br />c1pLtation. <br /> <br />Ouring the fLve yearl of the COlor.do River 8aa1n Pilot <br />project. there was a midwinter dip in the frequency of <br />storms suitable for seeding, eorresponding to mean <br />upper-air temperatures too cold for ttlleedab11ity.1I The <br />experiment. did not eetablilh with a reasonable degr;;- <br />of confidence whether precipitation increases would <br />appear as prolongation of snowfall durations or as in- <br />creases at lntenalty. or Doth. Many SUCh queStions <br />can be answered only by further experimentation. <br /> <br />However the results.u elt sttOR 1 that the snowfall <br />climate under ...din. wil 1 or rom t e natura c <br />mate anI in vel' .ubtle ~a . that can be dtstin uished. <br />if at all onl b .0 hi.ticat. statlstic. anal Si8. <br />'That ued.d snow all wen any way Btunge. e t er <br />ae oerceived or A. it int.ract. with the natural on-- <br />vlronment. i. now cona1dered extremely unlikely. -- <br /> <br />11 The San Juan Ecology Project did not include studies of the techoique.s. of cloud seeding. but to set tllp. Mil2,€' <br />for the r...inder of thh volume. we asked the BUI:'f!8U of Rechmatio{l co provide an overv1e... of the Cl{\IUI <br />e.edins ,activit1ee. whi.ch they r.onetderntely provfded. <br /> <br />21 <br />