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<br />Appendix B <br /> <br />100 times as much silver into the atmosphere in many parts of the country, and silver <br />from seeding is far exceeded by individual exposure from tooth fillings. Accumulation in <br />the soil, vegetation and surface runoff have not been large enough to measure above <br />natural background. A common societal attitude toward WxMod is that one is <br />"tampering with nature" and some mistrust has resulted over the decades. Many such <br />opponents are unaware that mankind has been inadvertently modifying the weather to <br />great extent, especially since the industrial revolution. <br /> <br />Statistical Evaluation. Assessments of seeding effects most often consist of comparisons <br />of the amount of precipitation measured in a target area with that from a control area. <br />The issue is how to determine if a change occurred, using measurements taken in a <br />watershed in a period before and during seeding. Similarly, one may take measurements <br />in adjacent "control" watersheds to account for spatial differences arising from seeding. <br />Either method has to deal with biases. A more statistically robust design, known as a <br />cross-over uses two similar fixed areas. During each test case, one area is selected for <br />treatment through a random process while the other serves as the control. <br /> <br />Replication. Since there is so much variability in the natural world, predictability and <br />consistency of the application and effectiveness of techniques is difficult to determine. <br />There are additional difficulties owing to variability in wind, temperature, and other <br />factors. There is also seasonal variation; for example, summer is more spatially and <br />temporally variable in precipitation than winter. <br /> <br />Transport and Dispersion. One of the most significant uncertainties is the transport and <br />dispersion of the seeding aerosols across project areas. Results from studies have <br />revealed that most of the precipitation faIling in the targets during seeding periods has' not <br />been impacted by the seeding process. New fully automated ground-based generators <br />can be located in often poorly accessible locations in high mountain terrain. <br /> <br />Cloud and precipitation microphvsics issues. The following are outstanding cloud and <br />precipitation microphysical issues: <br /> <br />. Background concentration, sizes, and chemical composition of aerosols that <br />participated in the cloud physics process <br /> <br />. Cloud water nucleation processes as they relate to chemical composition, sizes, and <br />concentrations of hygroscopic aerosol particles <br /> <br />. Ice nucleation (primary and secondary) <br /> <br />. Evolution of the droplet spectra in clouds and processes that contribute to spectra <br />broadening and the onset of coalescence <br /> <br />. Relative importance of drizzle in precipitation processes <br /> <br />-2- <br />