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<br />are frozen to become small ice particles. These ice embryos <br />then grow by deposition of vapor upon them. About 1011 to <br />10 12 ice embryos are formed for each gram of CO2 (dry ice). <br /> <br />The type of generating system employed and its mode of <br />operation depends upon the type of cloud systems that are <br />to be treated. It is essential that any prospective cloud <br />seeding project be carefully designed, making full use of <br />the historic meteorological data base. <br /> <br />All projects require monitoring not only of generator <br />performance, but also of local weather patterns not observable <br />in the conventional observing station/satellite data network. <br />These data are used for evaluation of results as well as for <br />operation decision making. Required sensors include raingage <br />and anemometers, some of which are telemetered to the control <br />\ <br />center. However, a weather radar is considered the key monitoring <br />system for summer cumulus seeding projects as well as for <br />many winter orographic snowmaking projects. Rawinsonde data <br />are used as inputs in making numerical conve~tion model runs <br />for guidance of operations in summer cumulus projects. Special <br />upwind rawinsoundings are often a feature of wintertime snowpack <br />enhancing projects. <br /> <br />In the evaluation of results, a comparison of precipi- <br />tation observed in the target to that in an unaffected, preselected <br />control area provides useful information. For example: the <br />target area to control area precipitation ratio can be compared <br />to those observed during the historical period. The same <br />approach may be used with seasonal streamflows or snow course <br />data. Purely research pro~ects normally randomize their treatment <br />with treated periods being compared to non-treated periods: this <br />may involve target to control area comparisons of treated <br /> <br />7 <br />