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<br />B-12 <br /> <br />approaches significance. (In the overall analysis, days randomly assigned to be seeded were <br />analyzed as seeded, whether judged to be suitable and whether actually seeded,) When days are <br />separated by 500 mb. temperature, results show significantly less rainfall under seeding on <br />colder days (temperatures between -150C, and -200C), with an abundance of seeded days <br />totally without precipitation (rain). Results on hail suppression are inconclusive. <br /> <br />Conclusions from this study should be treated tentatively. A multiplicity of analyses is <br />performed .and effects assessed against nominal levels of significance (without allowance for <br />multiplicity) are no more than marginally significant. On each day, meteorologists, pilots, and <br />radar technicians had been informed whether the day was to be a seeded or nonseeded day. <br />Further, no effort was made to prevent rainfall observers from acquiring this information. <br />Results from the study, consequently, might be influenced by the advance day-by-day <br />knowledge of treatment copdition by operating and measuring personnel. Finally, some results <br />that approach nominal levels of statistical significance seem to be perplexing from a meteorolog- <br />ical perspective. <br /> <br />* Tasmania * <br /> <br />This was a carefully conducted study, designed to provide results separately by season. <br />Issues of multiplicity in data analysis leave conclusions unsettled. Results fail to meet <br />confirmatory standards, but some would find the results semipersuasive in supporting the <br />hypothesis that seeding increases rainfall. <br /> <br />* general summary * <br /> <br />Of the seven relatively recent randomized projects, (Alberta and South Africa omitted) <br />the present scoreboard is: <br /> <br />Possible reconfirmation (l, Israeli II) <br /> <br />Close to persuasive or almost marginal (2, Santa Barbara, Tasmania) <br /> <br />Issues of multiplicity and subjectivity (2, North Dakota, Florida) <br /> <br />Limited data and/or severe operational limitations (2, NHRE, Colorado River) <br /> <br />Overall, those who believe in the efficacy of cloud-seeding are fortunate that Israeli II may <br />prove to be a re-confirmation. We owe it to our readers to point out that at least one <br />confirmation at 5% out of 7 is to be expected almost one-third of the time by pure chance. <br />(See Section 40.) <br /> <br />13. Inadvertent modification <br /> <br />The task force was asked to giv.e some attention to the results of the largest and most <br />compete study of inadvertent weather modification by metropolitan-area emissions of heat, par- <br />ticles, and active gases -- the METROMEX study of the St. Louis area. <br /> <br />In the terms in which we have discussed experimental weather modifications, <br /> <br />METROMEX has so far clearly been in an exploratory phase, complicated by the possible ways <br />in which other geographic variables may be responsible, wholly or partially, for the observed <br />patterns. (Since it is clearly not practical to "stop St. Louis's emissions" for a day or week on a <br />randomized basis, separating these effects will remain either quite difficult or impossible,) <br /> <br />In our judgment, the status of METROMEX is neither a cause for surprise nor a cause for <br /> <br />alarm. The total quantities of heat, particles, and active gases emitted by a substantial metro- <br />politan area are so large that some weather consequences seem inevitable. The main question <br />is not "Did anything happen?" but rather "What happens?" As a consequence what we <br />