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♦ M <br /> Evaluations of carefully conducted hail suppression operations have <br /> demonstrated a reduction in damage caused 'by hail to agricultural crops and <br /> property. Studies of long-standing hail suppression operations in a number of <br /> locations around the world indicate a range of effects from 25% to 75% reduction <br /> in damage. Advances in radar data processing and evaluation techniques are <br /> helping to provide additional insights into the effects of seeding. Microphysical <br /> measurements from single-cloud studies and radar analyses are also providing <br /> encouraging evidence consistent with the conceptual models of hail suppression. <br /> These technological advances and research efforts continue to develop improved <br /> understanding of hail growth and hail suppression. <br /> Status of the Discipline <br /> The fundamental principles and primary cloud treatment strategies <br /> involved in weather modification are reasonably well understood and a <br /> substantial body of evidence regarding the effectiveness of cloud seeding exists.. <br /> Attainment of desirable weather modification effects depends upon several <br /> factors, including the weather regimes of a specific area and their meteorological <br /> characteristics, the design of a program to achieve a specified goal, and the <br /> execution of the program. <br /> The "level of evidence" issue regarding weather modification effectiveness <br /> remains a topic of some debate. An increasing number of cloud seeding <br /> practitioners, sponsors and investigators accept the growing body of primarily <br /> statistically expressed, but also objective physical evidence in support of cloud <br /> seeding for beneficial effects. The ranges of effects shown in this Capability <br /> Statement take into account a) the statistically significant results of some <br /> carefully controlled, randomized experiments, b) the physical evidence obtained <br /> through laboratory and atmospheric experimentation and observation and c) the <br /> results of less robust statistical evaluations of large numbers of non-randomized <br /> cloud seeding projects over decades. It remains to those considering application <br /> of cloud seeding technologies to determine what level of evidence is appropriate <br /> for their decision making. <br /> Persisting challenges in weather modification include determining and <br /> defining the conditions under which predictable and consistent effects may be <br /> achieved, and establishing and executing the most effective cloud treatment <br /> strategies. It also appears that, in some situations, air pollution effects on <br /> precipitation can confoundestimation of the effectiveness of cloud seeding, such <br /> that the potential for pollution effects should be considered in the design, <br /> execution and evaluation of cloud seeding programs. It is also important to <br /> continue the development and application of methods for estimating the <br /> effectiveness of weather modification projects, especially operational projects <br /> conducted without randomization. Continued applied research into weather <br /> modification issues is encouraged. Incremental advances in the science and <br /> technology of weather modification will lead to improvements in cloud seeding <br />