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)?
Although not 100% sure, after some additional searches, I think the answer is the law of inertia. If one throws a ball from at the equator to the north pole, the ball also moves to the right even if one did not intend to do so. The same effect we see when one throws a ball from a moving car (example).
The speed is faster near the equator. Therefore, even through Coriolis effect is working, the law of inertia overrules the effect. That is, the movement (to right) of the ball is faster than the rotation of the earth.
Regardless of from north to south, or south to north, Coriolis effect and the law of inertia work in opposite directions. Stronger one determines the direction.
