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<br />of the convective cell are interrelated and, together with the cloud <br />droplet characteristics, determine whether precipitation will D~rm at <br />all. <br /> <br />Treatment. - Although hygroscopic material can be released from the <br />ground, it is normally delivered to cloudbase by aircraft. The mate- <br />rial can be solid, such as salt particles, or liquid, such as the <br />mixture of urea, ammonium nitrate, and water. As soon as the hygro- <br />scopic particles are released into the updrafts they begin to attract <br />moisture from the air and grow larger. Their size when released from <br />the aircraft is chosen so that they will be large enough to start the <br />coalescence process about as soon as they enter the cloudbase. Hathe- <br />matical models of the hygroscopic seeding process are used to suggest <br />the most appropriate nuclei size for the cloud conditions. The models <br />not only predict the growth of the nuclei but also predict the tra- <br />jectory of the favorite droplet formed by the coalescence process. <br /> <br />Ice-pha~e seeding material, primarily silver iodide, may also be deliv- <br />ered to the cloud by aircraft. Silver iodide can be generated by <br />pyrotechnic devices burned from racks mounted on the aircraft or <br />dropped into the cloud from above. Liquid fuel generators similar to <br />those used in ground-based orographic seeding can be mounted on air- <br />craft and used to dispense silver iodide. <br /> <br />Ice-phase seeding to stimulate cloud dynamics is appropl'iate when <br />cloud growth is limited by a relatively weak stable layer that the <br />cloud cannot push through. The additional buoyancy from the release <br />of latent heat can often allow the cloud to penetrate the stable <br />layer and grow tall enough for natural precipitation processes. <br /> <br />Ground-based seeding is appropriate if the convective clouds tEmd. to <br />have a favored location for development, such as a hill or somE~ other <br />geographic feature. In this case the generators are lo(:ated SCl the <br />natural updrafts associated with convection will carry the seeding <br />material into the cloud. Either ice-phase or hygroscopic seedi.ng <br />materials can be used. Ground-based seeding can also be used to put <br />material into air that may eventually be involved in the conve(:ti ve <br />process. The natural collection patterns of convective clouds are <br />relied on to get the material to the cloud. Not all of the material <br />will be used effectively, and a great deal more material is usually <br />required to get the same amount of rain that can be obtained f]~om <br />airborne seeding. <br /> <br />Evaluation. - The moisture obtained from seeding convective clouds can <br />be evaluated directly by measuring the amount of precipitation that <br /> <br />7 <br />