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<br />MOISTURE SOURCE <br /> <br />Moist surface air is necessary for good cloud development <br />and rainfall. South Dakota's primary moisture source is <br />the Gulf of Mexico. Surface winds carry humid air across <br />the southern plains into South Dakota. <br /> <br />Moist air from the Pacific is lifted and cooled as it passes <br />over the Cascade and Rocky Mountains causing much of the <br />moisture to fall as rain or snow before reaching the great <br />plains. <br /> <br /> <br />CASCADES <br /> <br />ROCKIES <br /> <br />BLACK <br />HILLS <br /> <br /> <br />26 <br />