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<br />D R AFT <br /> <br />effective under specific conditions. Their characteristics, however, <br />dictate the conditions under which they can be most efficiently <br />employed. Heterogeneous seeding materials, such as silver iodide <br />which act as artificial ice nuclei have the following characteristics: <br />(1) the activity spectra are temperature dependent, producing more <br />nuclei at colder temperatures; (2) are capable of reactivation; (3) <br />can travel long distances and time before activation; and (4) are <br />logistically easy to handle. Dry ice, in contrast, produces ice <br />crystals and particles by homogeneous nucleation. Its characteristics <br />are: (1) the activity spectrum is independent of temperature, (2) it <br />has a higher activity at warm temperatures, (3) it is effective over <br />a limited region in time and space, and (4) is logistically more <br />difficult to handle. The final choice of seeding materials is <br />dependent upon the seeding hypothesis and cannot be made until the <br />hypothesis is indentified. <br /> <br />Delivery techniques, ground based and airborne, will also be chosen <br />according to their characteristics after the seeding hypothesis has <br />been resolved. The ground-based technique involves prepositioning a <br />number of artificial ice nucleus generators that burn silver iodide <br />solutions upwind of the target area. When conditions are favorable <br />for seeding, a number of generators directly upwind of the target are <br />activated either by remote control or manually. The nuclei produced <br />are then carried into the cloud by transport, diffusion, and turbulent <br /> <br />~ <br /> <br />44 <br />