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The simple north–south atmospheric circulation is modified by the Coriolis effect, caused by Earth's west to east rotation. Surface air near the equator moves eastward, in the same direction as Earth's rotation, with a speed of nearly 1,700 km/h. At higher latitudes, the surface and overlying air masses move eastward at slower speeds. Consequently, a parcel of air carried away from the equator by Hadley circulation moves eastward faster than the air or ground below it. An observer on the ground would see the air veer on a curved path to the east, as if moved by some force.

On the other hand, air near the poles starts off moving slowly to the east, since Earth's rotation speed is slower at high latitudes. As it moves towards the equator, a parcel of polar air travels over a faster‐moving surface. As a result, it appears to move along a curved path to the west.

In effect, winds flowing north–south are diverted to the right in the northern hemisphere and to the left in the southern hemisphere. This explains, for example, why the tropical trade winds in the northern hemisphere blow towards the southwest, whereas the trade winds in the southern hemisphere flow toward the northwest (see Figure 3.10).

The Coriolis effect also explains why winds move around pressure systems in opposite directions in different hemispheres. There is a clockwise flow around a high‐pressure area (anticyclone) in the northern hemisphere and a counterclockwise flow around an anticyclone in the southern hemisphere. Conversely, winds blow counterclockwise around low‐pressure areas (cyclones or depressions) in the northern hemisphere and clockwise in the southern hemisphere.