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<br />00349~ <br /> <br />APPENDIX A <br /> <br />Winter Cloud Seeding in Colorado <br /> <br />'theory . <br /> <br />Sene storms passing over the Colorado mountains are inefficient--they <br />do not naturally produce much snow even though they may contain high <br />arrounts of moisture. Many scientists believe that seeding these <br />inefficient storms can increase snowfall. 'the process by which <br />precipitation forms and the types of storms considered seedable are <br />discussed below. <br /> <br />Clouds (liquid water condensate) form when moist air cools and <br />condenses as it rises. Much of Colorado's precipitation results from the <br />lifting of air as it passes over mountain barriers. This is COIIUllOnly <br />referred to as orographic lifting. 'the rate at which condensate is <br />produced by this lifting is influenced by the component of the wind <br />perpendicular to the mountain barrier and the moisture content and <br />temperature of the air mass. <br /> <br />Even though temperatures in almost all clouds in the winter in <br />Colorado are below freezing, the condensate that forms when the air rises <br />often remains in a liquid phase. The condensate is said to be <br />"super-cooled." Varying concentrations of ice-forming nuclei--usually <br />clay mineral grains, biological materials, or small particles from <br />combustion or industrial processes--are also present in clouds. <br /> <br />Ice crystals form when the droplets of supercooled water freeze as <br />they come in contact with the ice-forming nuclei. 'the crystals grow as <br />vapor is deposited on them until they are heavy enough to fall. They may <br />also grow by colliding with other small cloud droplets on their way down. <br /> <br />40 <br />