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August, 1975 and 1976, featured strongly contrasting rainfall regimes in the midwestern United States. Both the severe drought of 1976 and the record surpluses of 1975 were related to repetitive, but different, circulations in the middle troposphere which ultimately governed the amount of moist air available to disturbances crossing the Middle West. Moist air overlay the southern half of the eastern United States during much of August, 1975, and was repeatedly drawn northward and extracted by passing Middle Western disturbances, but stronger, more persistent polar anticyclones in 1976 limited the northward penetration of moist air.
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Harman et al. (1978) studied this question.
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