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<br />If no secondary or artificial sources of ice crystals are active in a cloud, . <br /> <br />the concentration of ice crystals which actually precipitate should closely <br /> <br />agree with the concentration of natural ice nuclei activated at cloud-top. <br /> <br />If, however, an artificial source of ice crystals due to cloud seeding or <br /> <br />a secondary source due to some natural cloud process such as ice crystal <br /> <br />multiplication is present in the cloud, the two concentrations may not <br /> <br />agree. Under these conditions, the observed concentration of ice crystals <br /> <br />should be greater than that calculated strictly from cloud-top temperature <br /> <br />and the natural ice-nucleus spectrum. <br /> <br />2.2 Orographic Cloud Seeding <br /> <br />There is a natural deficiency in the concentration of ice nuclei in many <br /> <br />orographic clouds at warm temperatures. It is this natural deficiency <br /> <br /> <br />in concentration which makes the artificial modification of clouds <br /> <br /> <br />feasible. If natural ice nuclei were always available in sufficient <br /> <br /> <br />quantities, seeding clouds with artificial ice nuclei would yield no <br /> <br /> <br />benefits. <br /> <br />~he objective in orographic cloud seeding is to increase the number of <br /> <br />~e crystals in naturally deficient clouds in order that more available <br /> <br /> <br />oud water will be utilized to grow precipitating ice crystals. These <br /> <br /> <br />1 fall to the ground thus precipitating additional cloud water. <br /> <br />'ing orographic clouds with optimum concentrations of artificial ice <br /> <br /> <br />~i utilizes more of the cloud water for the diffusional growth of ice <br /> <br /> <br />lIs which results in less cloud water being available for accretional <br /> <br />The end result should be a decrease in both the size of crystals <br /> <br />, degree"of rime, but an increase in the concentration of ice <br />