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agent selection, delivery and dosage rates, and quality and completeness of <br /> operational execution can alter these expectations. <br /> The potential environmental impacts of cloud seeding have been <br /> addressed in many studies. No significant adverse environmental impacts have <br /> been found due to use of silver iodide, the most commonly used seeding <br /> material, even in project areas where seeding has been conducted for fifty years <br /> or more. <br /> Fog and Stratus Dispersal <br /> The dispersal of shallow, supercooled (colder than 0° C) fog or stratus <br /> cloud decks is an established operational technology. The effects from <br /> dispersing supercooled fog and stratus are easily measured and the results <br /> highly predictable. Hence, randomized statistical verification has generally been <br /> considered unnecessary. <br /> Dispensing ice phase seeding agents, such as dry ice, liquid nitrogen, <br /> liquid propane or silver iodide into supercooled fog and stratus is effective in <br /> improving visibility. Clearings established in cloud decks embedded in strong <br /> wind fields fill in quickly unless seeding is done nearly continuously. Selection of <br /> a suitable technique is dependent upon wind, temperature and other factors. Dry <br /> ice has commonly been used in airborne delivery systems. Liquid carbon <br /> dioxide, liquid nitrogen and liquid propane have been used in ground-based <br /> delivery systems at some airports. <br /> The dispersal of warm (warmer than 0° C) fog or stratus decks over areas <br /> as large as airport runways has been operationally applied via introduction of a <br /> significant heat source. The mixing of drier air into shallow fog by helicopter <br /> downwash can create localized clearings. Various hygroscopic (water attracting) <br /> substances have also been used to improve visibility in these situations, primarily <br /> in military applications. <br /> Winter Precipitation Augmentation <br /> The capability to increase precipitation from wintertime orographic cloud <br /> systems has now been demonstrated successfully in numerous "links in the <br /> chain" research experiments. The evolution, growth and fallout of seeding- <br /> induced (and enhanced) ice particles have been documented in several <br /> mountainous regions of the western U. S. Enhanced precipitation rates in <br /> seeded cloud regions have been measured in the range of hundredths to >1 mm <br /> per hour. Although conducted over smaller temporal and spatial scales, research <br /> results tend to be consistent with evaluations of randomized experiments and a <br /> substantial and growing number of operational programs where 5% - 15% <br /> increases in seasonal precipitation have been consistently reported. Similar <br /> results have been found in both continental and coastal regions, with the <br />