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<br />111e atmospheric elTects of long-tenn wanning and urban/industrial air pollution are only two <br />possible inlluences on seeding effectiveness. Since the 1950s there have been mri(ltions in <br />wee/inx materials and technologies. Some seeding generator locations have also changed. <br />111erefore records of the seeding operators must be preserved and examined for such changes and <br />an assessment made of their possible consequences. Of course other inlluences. related or <br />unrelated to the ones cited above. may playa role. <br /> <br />C. \Vhat can Current Knowledge and Future Applied Research do to <br />Optimize Cloud Seeding'? <br /> <br />1. What is cloud seeding? <br /> <br />The primary type of seeding in California has been in cold-season orographic clouds. so that type <br />will be the principal foclls here. Seeding to suppress hail and disperse log is routinely conducted <br />in other states. According to the North American Insterstate Weather Modification Council, <br />there arc presently operational programs in II of the 17 Western states. Precipitation <br />enhancement of warnl-scason convective clouds with hygroscopic materials (substances like salt <br />that take lip atmospheric water) has shown promise. but has not been used much in California. <br /> <br />Cold-season orographic clouds lornl as moist air llowing from the Pacific begins to rise rapidly <br />as it reaches the western (\vind\vard) side of the Sierra Nevada. This rise results in cooling, <br />condensation and. allen, precipilation as either rain or snow. In many instances within these <br />clouds, water droplets remain as liquid at temperatures below the freezing point (320F). Such <br />droplets make up supercooled liquid w(lter (SUI') clouds. whose presence leads to aircraft icing. <br />Only a small fraction of SL W droplets freeze into ice crystals. usually through interaction \\lith <br />tiny wind-blown particles called ice nuclei (IN). These crystals then grow rapidly at the expense <br />of the much more numerous SL W droplets. and can attain suflicient size to fall to the ground as <br />sno\....llakes. While natural IN exist in nature. their etTectjveness is limited unless SLW cloud <br />temperatures are rclatively cold. Silver iodide and other seeding agents can create ice crystals at <br />signiticantly wanner temperatures. Cloud seeding can thus initiate snowfall within this <br />"temperature window of opportunity," when nature is inefTective at doing so. <br /> <br />By far the most common seeding agent in the history of weather modification has been silver <br />iodide (AgO. released as a tine smoke. This compound has a crystalline structure nearly <br />identical to ice. effectively providing IN that interact with water vapor or SL W droplets to fonn <br />tiny ice crystals. Figure..J. illustratcs how AgJ seeding from ground generators works. The most <br />effective AgJ nuc\cants can begin producing ice crystals at temperatures colder than about _50e. <br />An alternative is to chill the air sulliciently so that the SLW droplets freeze without nuclei. This <br /> <br />11 <br />