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<br />airmass becomes more unstable, resulting in increased lifting, and <br />consequently, in more liquid water. The amount of precipitation <br />produced by the storm begins to increase as a result of the <br />additional water. The storm then matures into an unstable phase, <br />and embedded convection develops. Liquid water contents reach <br />their highest values, and precipitation reaches its maximum rate <br />and exhibits heavy riming indicative of an accretional growth <br />process. In the final phase, airflow changes so that lifting no <br />longer occurs, the clouds begin to dissipate, and precipitation is <br />negligible. <br /> <br />The modification potential in this cloud class is believed to <br />exist during the neutral and unstable portions of the storm when <br />supercooled liquid water content is highest and natural nucleation <br />seems most inefficient. The modification potential is based on <br />increasing the efficiency of the natural precipitation processes. <br /> <br />In the second cloud class, warm moist orographic clouds form over <br />the barrier with no large-scale cyclonic features within the area. <br />These clouds form over the barrier with the onset of moist flow, <br />grow to their maximum size, and then diminish. They do not appear <br />to precipitate until they reach some critical dimension threshold <br />that allows nucleation and subsequent growth time to produce <br />precipitation sized particles naturally. In this case, the <br />modification potential is based on increasing both the probability <br />and duration of precipitation. When the cloud forms over the <br />barrier, it is small and not precipitating. It has little ice in <br />it because its top is low and warm. Therefore, artificial <br />nucleation initiates and extends precipitation. <br /> <br />This evidence provides the scientific basis for proceeding, but the <br />physical concepts must be verified and their frequency of occurrence <br />established by the physical studies. <br /> <br />Seedability Criteria. Seedability criteria are established by the <br />modification hypotheses. Some important conditions to be considered <br />are: the availability of supercooled liquid water, the presence of <br />convective instability, an appropriate balance between accretional <br />and depositional growth, the production of hydrometeors whose size <br />spectra will be sufficient to survive evaporation, and cloud <br />temperatures too warm for natural precipitation processes to be <br />efficient. Some of these conditions can be accurately and easily <br />assessed and some cannot. Therefore, it is imperative an <br />instrumentation package be assembled that not only includes the <br />best measurement of each condition, but also integrates and displays <br />the data collected from all sources for immediate analysis by <br />scientists in the field. This will not preclude the classical, <br /> <br />V-3 <br />