Earth is widest at the equator which means the Earth’s surface travels faster at the equator than the poles as it’s rotating. Because of this, as winds travel north and south the Earth’s rotation deflects them east or west. As air moves towards the equator, it is deflected to the west, and as air moves away from the equator it is deflected to the east. This phenomenon is known as the Coriolis effect.

The Coriolis effect is a major component in explaining why winds in areas of high-pressure blow westward and low-pressure areas blow eastward in the northern hemisphere, and vice versa in the southern hemisphere.

Figure 1: The Coriolis effect