The westerlies are prevailing winds that blow from the west to the east in the middle latitudes of both the Northern and Southern Hemispheres. They are caused by the Coriolis effect, which is a force that deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The westerlies are responsible for much of the weather in the middle latitudes, including the formation of storms and the movement of air masses.