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61.1 kg of SF6 gas remaining on June 5, 2005. Thus, there were sufficient stores of flight hours, salt and gas to continue the SPECTRA II studies on a limited basis if the opportunity presented itself. 3.0 RESULTS OF ANALYSES The four most productive hygroscopic test cases obtained on three days (May 14, 25, and 31, 2005) were analyzed intensively. Detailed documentary information on all cases is provided in the extensive appendices (i.e., in Appendices C, D and E). Each case involved the scramble of project aircraft to clouds that appeared suitable for hygroscopic seeding followed by their base salt-powder seeding and the concurrent release of SF 6 tracer gas. Although it would have been desirable to randomize the treatment for the prospective test cases, there were not enough opportunities to warrant treatment randomization. Rather than attempt to infer the effects of seeding through case-by-case randomization, the effects of seeding were inferred within the clouds of each case by comparing the portions of the clouds containing the SFtracer gas that was released concurrently with 6 the salt-powder nucleant with those portions of the clouds that did not contain the nucleant. This proved to be an effective analysis strategy. Although everything was in place for a successful SPECTRA II effort, the tiny sample size of four test cases proved to be a major disappointment. Further, the clouds in two of the four cases became too strong for safe repeated aircraft penetration. Despite these problems, it was possible nevertheless to document strong hygroscopic seeding signatures, having the form of low concentrations of anomalously large (> 30 micron diameter) cloud drops in regions of detected SFtracer gas, while no such drops existed immediately before 6 entering and immediately after exiting the region of tracer gas. In the last of the four cases, when it was possible using the unique tracking system to return repetitively to the seeded region, raindrops (D > 100 microns) developed in the regions of large cloud drops. These findings, indicating enhanced coalescence and raindrop development in the seeded regions, are consistent with the hygroscopic seeding conceptual model. The maximization of the information content from the limited cases obtained in SPECTRA II was made possible by the use of SF tracer gas and by the powerful real-time tracking and visualization system that was 6 used in the experiments. Without this real-time capability to get precisely back to the seeded cloud and the ability to identify the seeded regions within them, little definitive would have been obtained in SPECTRA II. These must be integral components of any future hygroscopic seeding study. This unique system would have permitted the testing of the efficacy of seeding with hygroscopic flares if it had been possible to obtain the needed hygroscopic flares and if the weather had permitted it. Although such tests were not possible in SPECTRA II, they should be made in future studies having the goal of documenting the effect of hygroscopic seeding on cloud microstructure and precipitation development. 4.0 THE NUMERICAL SIMULATION OF SEEDING EFFECTS Cloud modeling and the simulation of seeding effects has always been a goal of the Texas weather modification research effort. Thus, cloud modeling and the simulation of seeding effects played a major role in SPECTRA II. Dr. Alexander Khain of the Hebrew University of Jerusalem headed-up the modeling component of SPECTRA II with input from Dr. Rosenfeld. Hygroscopic seeding was simulated using the 2-dimensional Hebrew University cloud model (HUCM). The model is based on solving an equation system for the size distribution functions of cloud hydrometeors and aerosols (both natural and seeded). The model is specially oriented forthe 7