The Coriolis force is caused by the rotation of the Earth. It arises on a rotating object because of the relationship between the rotating object and the inertia of a mass moving on the object.
The Coriolis force acts on moving air masses and water masses, and its effect is only significant on timescales longer than about a day. When viewed from the Earth, the practical effect of the Coriolis force is to accelerate an air mass to the right of its forward motion in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis force acts on an air or water parcel no matter which direction it is traveling, North, South, East, or West. The Coriolis force increases from the Equator to the poles, where it is strongest.
Illustration of the Coriolis force on a merry-go-round. The merry-go-round simulates the rotating Earth, with the outside edge of the merry-go-round being equivalent to the equator. Note that the direction the merry-go-round is rotating simulates the Southern Hemisphere, so the ball accelerates to the left of the initial path.
Video from the University of Illinois WW2010 Project. Copyright University of Illinois Board of Trustees.
In the video, the ball moves in a straight line when viewed from above the merry-go-round, a perspective where the observer is not rotating. From this perspective, there is no Coriolis force. However, when viewed from the merry-go-round, the perspective of somebody on the Earth, the ball curves to the left. The force acting on the ball is the Coriolis force.
