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<br />4. Seeding <br /> <br />Proper implementation of the static seeding hypothesis is dependent on ini- <br />tiating the precipitation process with an ice crystal concentration that is <br />optimum for the subject cloud. Therefore delivery of the seeding material <br />must be designed to produce the required ice crystal concentration at the <br />time and place in the cloud as specified by the physical hypotheses. <br />In doing so care must be taken not to "overseed" the cloud. <br /> <br />Attempts to achieve the seeding objective have involved the use of dry ice <br />and a variety of silver iodide agents, a wide range of seeding rates, <br />cloud-by-cloud and areal broadcast seeding, and ontop, in-cloud, cloud <br />base and ground-based del ivery systems. It has proven to be an extremely <br />difficult objective to achieve involving the joint solution of such complex <br />processes as seeding agent effectiveness and nucleation rates, and <br />transport and dispersion of seeding material and/or ice crystals. It is a <br />problem which has not yet been satisfactorily solved. In this reviewer's <br />opinion, the diverse results of past research and tests of the static <br />seeding hypothesis are due in large part to the inability to execute the <br />seeding operation as intended and not necessarily a reflection on the vali- <br />dity of the hypothesis. It is beyond the scope of this general review of <br />static mode seeding to thoroughly discuss this complex subject; however, a <br />few of the major seeding issues will be mentioned briefly to illustrate <br />some of the uncertainties that need to be resolved. <br /> <br />14 <br />