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<br />0035fl0 <br /> <br />If their growth is sufficient, they will reach the surface as snow. If <br />their growth is not sufficient, they will be carried over the mountain <br />barrier and evaporate as they descend on the lee side. <br /> <br />Nature does not always provide optimal. concentrations of ice-forming <br />nuclei to create maximum precipitation. Clouds that contain too few <br />nuclei do not produce as much snow as available moisture would permit <br />because the nuclei grow and fall out without consuming much of the <br />moisture. In clouds that contain too many nuclei, on the other hand, <br />competition for the condensate may inhibit the growth of crystals. <br />Seeding may produce additional precipitation if artificial. ice-nucleating <br />material (such as silver iodide) is added to clouds suspected of <br />containing too few natural nuclei (Ludlam, 1955). However, the addition <br />of too much material ("overseeding") may decrease precipitation. <br /> <br />Scientists currently believe that three types of storms which commonly <br />occur over the Colorado mountains contain clouds that are "seedable." The <br />first type is caused by small-scale perturbations in the upper atIoospheric <br />flow. These perturbations usually pass through the state rather quickly, <br />forming clouds that often contain large arrounts of supercooled water and <br />relatively small arrounts of ice-forming nuclei. As a result, they produce <br />little or no precipitation. If these clouds are seeded (usually by an <br />array of ground-baSed silver iodide generators), they may produce more <br />precipitation for longer periods of time. <br /> <br />A second type of storm considered suitable for seeding contains clouds <br />that have been fonred by orographic lifting prior to major storm passage. <br />These clouds usually form over the Colorado mountains when winds are from <br />the northwest, west or southwest. They are often shallow and relatively <br />warm with moderate amounts of supercooled water and low concentrations of <br />ice-forming nuclei. <br /> <br />A third type contains clouds formed by convective lifting during and <br />after a storm' s passage. Convective lifting, which is caused by <br />differences in the vertical temperature profile of adjacent air parcels <br /> <br />41 <br />