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<br />Conclusions and Recommendations <br /> <br />As a result of this program and small scale research programs in <br />hygroscopic seeding, we are forced to conclude that this approach <br />to precipitation enhancement holds considerable promise for the <br />future. The costs and logistics of this method will !lot pr:eclude its <br />use in the future, especially as. it becomes engineer'ed to greater <br />efficiency. It covers an important group of seeding situations which <br />cannot be handled by silver iodide, and it should supplement silver <br />iodide in many other instances. It offers the direct opportunity to get <br />more water out of a cloud, i. e., increase the cloud efficiency. For a <br />large cloud, this can cut down on the amount of cirrus. It offers the <br />potential of helping with larger secondary effects -- aiding cloud growth <br />by lightening its water load, and stimulating adjacent cells by augmenting <br />the downcurrent and cold outflow. Its special feature is that it can initiate <br />drop growth by coalescence at cloud base - - and by adjusting particle size, <br />the growth. rate c~ be tailored to the cloud. There is an apparent lack of <br />natural giant hygroscopic nuclei, and so the artificial nuclei can be expected <br />to make major alterations in clouds. <br /> <br />With respect to the operational Oklahoma program, with both types of <br />seeding. it should be not ed that the scientific basis arises from studies <br />elsewhere on small or isolated convective clouds which are simple enough <br />to yield good answers. Probably 80% of the natural rain, and most of the <br />seeding potential, comes from the large, complex systems. Learning ho,.... <br />to seed these complex cases to get a net benefit over a large area (without <br />concurrent disbenefits) is a challenge of the magnitude of a national commit- <br />ment. Seeding for an area in summer convective conditions is based on a <br />small amount of science and a large amount of conjecture. The economic <br />stakes require that the area of this conjective be narrowed, markedly. <br /> <br />The method needs development before it is ready for operational application <br />to other than emergency programs. Of first priority is a program to develop <br />a dispensing system which puts out the material in a specified droplet range -- <br />say 900/0 of the material being in the range of 10 to 25 11m diameter (and the <br />mid-range adjustable from about 10 pm to 50 pm). The dispenser must be able <br />to release at least 1 kg/sec, and hopefully up to 3 kg/sec. <br /> <br />Secondly, there needs to be a definitive field experiment where the application <br />of this technique is made consistent with model predictions and field ex- <br />periments. This will deal primarily with simple clouds, as has been done <br />with many experiments on silver iodide seeding. <br /> <br />35 <br />