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" . BACKGROUND OF CLOUD SEEDING FOR THE SAN JUAN ECOLOGY PROJECT11 (W. Ho~ell. U.S. Bureau of Reclamation) When winter snowstorms sweep over the San.Juan Moun- tains. not all of the moisture that condenses in the form of clouds above the mountains falls as snow. Much of it remains In particles too small to fall. Carried beyond the mountains by the wind to where the airflow sinks once more toward the plains, these par- ticles re-evaporate. The rate at which snow reaches the ground. expressed as a proportion of the rate at which moisture condenses in the cloud, Is called the efficiency of precipitation. It has been found that in some storms. especially those having relatively deep cloud systems with cloud- top temperatures below about -27 C, the precipitation efficiency tends to be relatively high, and in such situations there is little that the current knowledge of weathe~ modification could do to increase the. snow- fall. In weak storms that condense very .little mois- ture, there 1s likewise little potential for.'st'imu]a- tion. HO\Jever.~" it has been found that when t~-'.--:;-- clouds are-deep enough and active enough to conde~Q r~latively large amounts of moisture but the ~lQu~ top temperature is warmer than about -26 C. seedin2 of the clouds with artificial ice nuclei often raises the precipitation etticiency tram a rather low va~uc to one typical of the colder clo~ Under these particular conditions, cloud seed~ has the pote~- tia1 of substantially increasin~ the rate.of precipi- tation, probably by as much as a hundred percent. I I i i , The Colorado River Basin Pilot Project was d~signed as a statistical test of the capability of the cloud seeding technology of 1971 to bring about precipita- tion increases. When the weather forecasters expected stormcloud conditions considered favorable for seeding \Jithin a twenty-four hour period beginning at 11 A.M. (and.if established safety criteria were met), an "experimental day" was declared. A randomized deci- sion then was made whether the experimental day would be seeded or left" unseeded as a control. The experi- ment was intended to run for four consecutive winters and accumulate 160 experimental days about equally divided between seeded and unseeded. It actually ran for five winters and accumulated 71 seeded and 76 unseeded days. It was thought that the snowfall on seeded days mi~ht "exceed~ that on unseeded experimental days b about 15 percent an poss~ y up to 30 percent, and that.ex perimental days mi~ht account for as much as half of the season's snowfall. This would have corresponded to a maximum precipitation increase of about 7,5 per- cent for the season as a whole. Although evaluation is not yet complete, Qreliminary results indicate that no such sizable overall increase was realited. There appear to have been many "unseeded" days ilTlt:le- diately following seeded days when silver iodide smoke, trapped in the valleys up\Jind of the mountain range, affected the clouds and effectively caused these days to be seeded, On other days strong winds carried the seeding effect over the mountain 2rest and outsid~.__:~s~r,.~rea. On still other days, . designated as "seeded," the silver iodide failed for one reason or another to reach the clouds. On still other occasions the forecast of favorable conditions was unfulfilled or else fulfilled for only a portion of the 24-hour period. so that any seeding effect ~as greatly diluted. 'I Nevertheless there exists an identifiable subset of experimental days. free from these dlsturbinR influ- ences. for which substantial snowfall increases could be identitied with a hi~h deKree of confidence. ~he tentative conclusions from the project character- ize it as a limited success. It revealed serious weaknesses ot the lY/l technology in the methods used to identify "seedable" occasions and to place the seedin~ material in the desired place at the desired ~. It failed also to give a reasonabl~ accurate meaSure of the potential of cloud seeding for in- creasing the seasonal precipitation. On the other . hand, it furnished additional evidence that the under- lying principles are sound and that present weakness has to do mainly with the practical difficulties of effective application. During the five winter seasons of the Pilot Project, .32~ kg of silver iodide, containing 147 kg of silver, were dispersed. An unknown proportion .of l't ~was 'de- posited on the ground or .on vegetation near the gener- ator sites; a further unknown proportion was carried to One side or the other of the study area or deposited \Jith precipitation either upwind or downwind from the study area, or escaped precipitation processes and remained suspended 1n the atmosphere. An estimate that 70 kg of silver actually accumulated within the 340,000 hectares ofi the study area appears reasonable, an accu- mulation of 0.04 grams per hectare season. The current preliminary estimate of average snow accumulation in the study area for the five years amounts to 65 cm water-equivalent per season. If 25 percent of this fell from seeded storms, this amounts to about 0.003 gm of silver per hectare Fer centimeter of seeded pre- cipitation. During the five years. of the Colorado River 8asin Pilot project, there was a midwinter dip in the frequency of storms suitable for seeding, corresponding to mean upper-air temperatures too cold for "seedability." The exoeriments did not establish with a reasonable degree- of confidence whether precipitation increases \Jould appear as prolongation of snowfall durations or as 1n- creases or intenSity, or both. Many such questions can be answered only by further experimentation. However. the results suggest strongly that the sn~fall climate under seedinR will differ from the natural cli- mate only in very subtle ways that can be distinguished, if at all, only by sophisticated statistical analysis. That seeded snowfall \Jill be in any way strange, either as ~erceived or as it interacts \Jith the natural en- viron~ent. is now considered extremely unlikely. !/ The San Juan Ecology Project did not include studies vf the tecnniques.of cloud seeding. but to set the stage for the remainder of this volume, we asked the Bureau of Reclamation to provide an overview of the c10~,1 seeding activities, which they consider~tely provideo.