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20 I I I I I I I I I I I I I I: I, I I I I English and Marwitz (l981), Marwitz and Stewart (1982) and Kochtubajda and Rogers (1984) reported the continuing production of small ice crystals in cumulus clouds that had been seeded with AgI containing nucleant aerosols generated by pyrotechnic combustion. They attributed this to continuing nucleation by the AgI aerosol. This seeding effect was quite different from CO2 seeded clouds which contained small ice crystals for only the first few minutes following seeding. Precipitation from AgI seeded clouds also persisted much longer than from CO2 seeded clouds (English et al., 1984). Differences like this might occur in similar situations where the ice nuclei function is not well defined, but it is assumed that the seeding effect is a constant; differences in outcome (such as radar echo features or precipitation location or characteristics), which could be caused by ice nucleation processes, could be incorrectly attributed to some other cause. Recent field studies in orographic clouds in Colorado and Montana that clearly establish that artificial IN can be transported over a target (by ground or air) in sufficient quantities to induce observable microphysical effects (Holdroyd et a1., 1988, Super and Heimbach, 1988; Super and Boe, 1988, Super et al., 1988) have also confirmed that potential ice nucleation activity is not instantly achieved at seeded temperatures. Ho1droyd et a1. made crude estimates of nucleus concentrations potentially "active" based on aircraft measurements with an NCAR ice nucleus counter above ground seeding locations. The level of quantification in the study is not justified because the actual ice nucleus counter used was not calibrated versus an absolute standard (ie., a CN counter or CSU isothermal cloud chamber) for the exact aerosols used (AgI from AgI.NH I-acetone-water combustion). A careful 4