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<br />seeding effects each cloud must be treated within a proper time intelVaI, or window of <br />opportunity, to produce the optimum ice crystal concentrations in clouds naturally deficient in <br />them. A cloud growing to maturity must be treated with enough time allowed so that the <br />generated ice nuclei can be lifted by its natural updraft action into and through the appropriate <br />temperature and moisture regime, residing there for a sufficient time to interact with the <br />supercooled cloud water. If this opportunity window is missed when attempting rainfall <br />stimulation, clouds may collapse prematurely resulting in wasted effort and resources. The <br />supercooled cloud volume "residence time" is critical to the success of both rain stimulation and <br />hail reduction efforts. <br /> <br />Weakly and moderately growing cumulifonn cloud behavior can be altered through what <br />is called the "dynamic effect." Under certain atmospheric conditions, clouds may be stimulated to <br />grow larger and rain longer than would be the case if otherwise left unseeded. This is done by <br />placing a sufficient amount of seeding agent quickly into the supercooled portion of a cloud to <br />promote the rapid conversion from water droplets to ice crystals. When done this way, the water- <br />to-ice conversion process occurs rapidly, releasing latent heat (fusion) on a massive scale making <br />the cloud slightly warmer and more buoyant. The cloud updrafts are expected to be invigorated, <br />thereby drawing in greater amounts of water vapor into the cloud supplying more moisture for <br />subsequent growth and "processing" into rainfall. This process causes a cloud to rain more and <br />rain longer than ifleft unseeded. <br /> <br />Although AgI produces greater numbers ofice nuclei than does dry ice, gram for gram, <br />'large numbers of ice nuclei can be produced more quickly by dropping comparatively larger <br />amounts dry ice directly into the moisture-laden cloud updrafts found in the new-growth cloud <br />towers. Relatively large amounts of dry ice are needed to produce the same number of ice crystals <br />from a given mass of AgI---roughly 1,000 to 2,000 grams. By homogeneous nucleation, dry ice <br />pellets makes contact with cloud water droplets as it falls through the cloud updraft, or the <br />droplets may be brought into the wake of the falling dry ice and immediately change into ice <br />crystals as well. However, AgI-based seeding agents, while rising in the cloud, only begin to <br />activate in droplets to cause ice crystals to fonn at temperatures near -4C to -5C---roughly 2,000 <br />to 2,500 feet above the freezing level. Dry ice is dispensed from a container auguring it into an <br />opening in the aircraft floor which lets it fall directly into the clouds. The container carries about <br />200 lbs of dry ice pellets and is released at a rate of SIbs per minute. <br /> <br />Most used among the techniques of cloud top seeding aircraft on many other programs is <br />the dropping of ejectable AgI flares. This type of seeding is not done on the WKWMP primarily <br />because of cost. It is.a much more expensive fonn of seeding agent than dry ice, despite the high <br />amount of sublimation occurring while kept in storage between operational periods. The seeding <br />technique for seeding with dry ice or with ejectable flares is essentially the same. <br /> <br />Over the years, the results from cloud physics research and other programs much like our <br />own have been applied to the WKWM Program whenever possible. This helps ensure that the <br />WKWMP retains in a reasonable state-of-the-art mode. We try to test and implement new things <br /> <br />7 <br />