Illustration of Earth showing how the Coriolis effect affects the motion (red arrows) of free-moving objects in the northern and southern hemispheres. This is caused by the Earth's rotation. It deflects moving objects to the right in the northern hemisphere and to the left in the southern hemisphere. Because the Earth completes only one rotation per day, the Coriolis force is quite small, and its effects generally become noticeable only for motions occurring over large distances and long periods of time, such as the large-scale movement of air in the atmosphere or water in the ocean.