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<br />003487 <br /> <br />Caution should be exercised when interpreting these results because <br />the designation of winters as dry, normal, or wet is rather arbitrary . <br />Instability in these results can be demonstrated by recalculating the <br />average munber of seedable events if dry winters are defined by water <br />content of 75 percent or less of long-term average rather than 80 percent: <br />no years would qualify as dry in the south sub-region, and the average <br />number of seedable events in the north would decrease to 16.0 during dry <br />winters and increase to 19.3 during normal winters. <br /> <br />Probably a more useful way to investigate the relationship between the <br />number of seedable events and recorded water content is to use <br />correlation. Correlation coefficients, given in the right-hand side of <br />Table 6, range from .33 to .44 with p-values ranging from .13 to .27. <br />These coefficients are low and indicate only a slight tendency for the <br />number of seedable events to increase with increasing water content. <br />Also, their probability of occurrence by chance alone is too high (above <br />COllI1lOnly accepted levels of statistical significance) to conclude with <br />reasonable certainty that the munber of events relates to the wetness of <br />the winter. <br /> <br />This result, although initially somewhat surprising, has a plausible <br />explanation. Dry winters typically occur in the western United States <br />when a ridge of high pressure dominates the middle levels of the <br />troposphere (Ibwell and Grant, 1972: 5) . Storms during these dry periods <br />are often shortwave troughs of low pressure that undercut the dominating <br />ridge. These storms are relatively warm and are likely to satisfy the <br />temperature requirements for seedability. It is probable, therefore, that <br />although fewer storms may occur during dry winters, a greater proportion <br />of them are seedable. <br /> <br />Seeding Potential for Individual Basins <br /> <br />Seeding potential for individual basins <br />the meteorological characteristics of storms. <br /> <br />depends <br />If our <br /> <br />upon topography and <br />assumption regarding <br /> <br />28 <br />