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<br />per kilometer in Project Cirrus (Langmuir, 1950; also see Havens, 1981 for <br />a review of Project Cirrus activities). Braham et al. (1957) seeded super- <br />cooled cumulus clouds in the Central United States with two different <br />seeding rates of about 5 and 13 kg/km. Better, though not statistically <br />significant, results were obtained with the higher seeding rate. There <br />were indications that not enough dry ice was dropped in some of the clouds <br />and that many of the clouds were seeded at too warm a temperature (-50C). <br />Marwitz and Stewart (1981) seeded supercooled convective clouds over the <br />Sierra Nevada with two different seeding rates also, 0.1 and 1.0 kg/km. <br />They reported that the high dry ice seeding rate produced too many ice par- <br />ticles to develop a naturally precipitating cloud. English and Marwitz <br />(1981) reported the occurrence of similar effects when an Alberta cumulus <br />cloud was seeded at a rate of 0.2 kg/km. The results of HIPLEX-1 (Cooper <br />and Lawson, 1984) also indicate that a seeding rate of 0.1 kg/km in Montana <br />cumulus congestus produced more ice crystals than desired or required to <br />enhance the IRG precipitation process. Care has been taken not to refer to <br />the effect of producing excessive ice particles by seeding as "overseeding" <br />since it does not strictly satisfy the classical definition of <br />"overseeding" as given earlier. However, the effect is an important con- <br />sequence of seeding and will be discussed in the context of the physical <br />hypothesis in a subsequent section of this review. <br /> <br />b. Silver iodide <br /> <br />Silver iodide has also been used extensively as a seeding agent in precipi- <br />tation enhancement experiments on supercooled convective clouds but the <br />state of knowledge of the various processes associated with its activity as <br /> <br />17 <br />