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Prevailing winds are generated by Coriolis effects.
I think I understand why NE winds are the prevailing winds in 0 to 30 degrees and 60 to 90 degrees latitude in the northern hemisphere (e.g., according to the picture below). But I don't understand why westeries are the prevailing winds in 30 to 60 degrees. If one throws a ball from a point at the equator to the north pole, shouldn't it deflected to the opposite direction by the direction of the rotation of the earth (i.e., creates SE winds)?