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. . ! JANUARY 1978 HOLROYD, SUPER AND SILVERMAN 61 . in a complicated manner on time, particle size and temperature (Gerber, 1976), 2) Dry ice effectiveness is much less temperature dependent than AgI effectiveness. This makes it more practical to achieve a desired ice crystal concentration with dry ice, especially in a cloud with significant vertical development and corresponding large vertical temperature gradient. Garvey (1975) indicates that AgI flare effectiveness increases approximately three orders of magnitude for a temperature decrease from -8 to -160C. In contrast, Eadie and Mee (1963) and Fukuta et at. (1971) show little temperature dependence for dry ice effectiveness. 3). Nucleation is nearly instantaneous with dry ice. This is often not the case for AgI (Isaac and Douglas, 1972; Garvey et at., 1976; Gerber, 1976; and Dye et at., 1976). 4) Dry ice pellets can be dispersed continuously along a flight track, creating an initial distribution of ice crystals that approximates a vertical plane or curtain. The fractured ends of the cylindrical pellets will cause them to drift laterally from the true vertical, possibly to distances in excess of 100 m, giving that vertical plane a significant initial thickness. The resulting volume will be riddled by the numerous pellets falling through it. This head start in dispersion should result in more rapid mixing of ice crystals than accom- panies the initial vertical lines created by dropping only a few AgI flares, typically at horizontal intervals between 100 and 200 m. 5) The more rapid mixing may be enhanced by the dynamic effects of nearly instantaneous nucleation. Unpublished findings of the Allee et at. (1972) experi- ments (Weickmann, 1974, p. 335) showed a greater horizontal spreading rate of their dry ice seeded lines than with those nucleated by AgI in a stratocumulus deck. A similar experiment (Jiusto and Holroyd, 1970; Weickmann, 1974) showed that the 10 em radar echo area from a dry-ice-seeded region of a stratocumulus deck steadily increased more than three times faster than a similar volume seeded with AgI flares at cloud base, regardless of whether the time of seeding or the time of first echo was used as the starting time. 6) Dry ice does not pose and, what may be more importa.nt, it is not perceived as posing an environ- mental hazard through uptake by plants and animals. While most of the available evidence suggests that AgI as it is typically employed is not an environmental hazard, suspicion and fear abound which are difficult to dispel. Monitoring the possible environmental effects of AgI is, in general, a requirement of most large-scale seeding programs. 7) Dry-ice seeding does not involve the use of silver, a relatively scarce resource. 8) For on-top seeding dry-ice seeding is more economical than AgI. The practicability of dry-ice seeding is heavily influenced by the amount of dry ice needed to accom- plish the desired effect, and the amount needed is largely dependl~nt on its effectiveness E. The amount of dry ice M needed to generate a mean ice crystal concentra- , tion C in cloud turrets of volume V is given by M=VCjE. (8) This assumes that mixing from outside this volume can be ignored and that thorough mixing of ice crystals 10 TOTAL TREATABLE VOLUME, km3 3 ... '" EFFECTIVENESS ~rr 10. Id2g~ -~ ~- C 0 !z- BCD ILl ~8 ABC o u c:i ct 9 >- ~ ILl ~8 >- a:: o r- LL ct a:: u a:: Ci LARGE (25km~): I 1, 11.1'" 3 f !l J_ I Ip I , ,,2f 30 4pSO,. , IQO, , ,'<jKl 300 SMALL (5kn13): I 2 3 4!l 57 10 20 30 4050 100 200 300 llbo 1000 NUMBER OF TREATABLE TURRETS FIG. 7. The effectiveness of dry ice (crystals per gram) determines the amount of seeding material needed and the number of clouds treatable. The former is expressed as a function of cloud volume for several combinations of desired ice crystal concentration and dry ice effectiveness values.