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Cynthia M. Scott and Mark D. Shulman


Harmonic analysis, an objective method of analyzing precipitation seasonality, is applied to 1941–70 monthly precipitation normals for nearly 200 stations in the northeastern United States. Our analysis presents distribution of the annual precipitation as the sum of six different sine curves (harmonics). The first three harmonics account for most of the variance in the original precipitation distribution in this area. Maps are presented of percent variance reduction and phase angle, and possible meteorological factors responsible for the observed patterns are suggested. In addition, the results are compared to previous applications of harmonic analysis to monthly precipitation normals during 1921–50 and 1931–60. The greatest difference appears as a substantial increase in the third harmonic in the coastal region from Maryland to Massachusetts. Variance reductions as high as 73% occurred in the center of this area, compared to a maximum value of only 46% for the 1931–60 normal period. Several possible reasons for this phenomenon are discussed.

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