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Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part III: Analysis of Hailstone Size Distributions for Seeding and Yearly Effects

Edwin L. CrowNational Center for Atmospheric Research, Boulder, CO 80307

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Alexis B. LongNational Center for Atmospheric Research, Boulder, CO 80307

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James E. DyeNational Center for Atmospheric Research, Boulder, CO 80307

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Carlton W. UlbrichDepartment of Physics and Astronomy, Clemson University, Clemson, SC 29631

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Abstract

The hailstone size (diameter) distributions measured by hailpads during the 1972-74 randomized seeding experiment of the National Hail Research Experiment are analyzed statistically for evidence of seeding effects and differences from year to year. Two approaches are taken, one comparing the entire empirical size distributions on seed days and on control days and the other comparing the mean diameters. The latter is based on the consistency with the exponential distribution (truncated at a prescribed minimum diameter), since the exponential distribution can be characterized completely by the difference between the mean diameter and the minimum diameter. Both approaches yield statistically significant results (10% level) only for 1974, when the hailstones were larger on seed days than on control days on the average. This may have resulted from the addition of seeding by rockets in 1974 or from differences in the hailpads used in that year. However, the physical hypothesis for the experiment predicted smaller stones on seed days; that tendency did appear in 1973 (though not significantly) and the difference was negligible in 1972.

Abstract

The hailstone size (diameter) distributions measured by hailpads during the 1972-74 randomized seeding experiment of the National Hail Research Experiment are analyzed statistically for evidence of seeding effects and differences from year to year. Two approaches are taken, one comparing the entire empirical size distributions on seed days and on control days and the other comparing the mean diameters. The latter is based on the consistency with the exponential distribution (truncated at a prescribed minimum diameter), since the exponential distribution can be characterized completely by the difference between the mean diameter and the minimum diameter. Both approaches yield statistically significant results (10% level) only for 1974, when the hailstones were larger on seed days than on control days on the average. This may have resulted from the addition of seeding by rockets in 1974 or from differences in the hailpads used in that year. However, the physical hypothesis for the experiment predicted smaller stones on seed days; that tendency did appear in 1973 (though not significantly) and the difference was negligible in 1972.

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