The Coriolis effect is caused by a combination of the inertia of moving air and the rotation of the Earth. Air tends to move from high pressure to low pressure in a straight line, but the rotation of the Earth means that, to an observer at one spot on its surface, the moving air appears to turn. This effect increases as the air moves faster.
The Coriolis effect is an observer effect. It is akin to a passenger on a moving train watching another person standing still outside the train. The person outside the train appears to be moving. In the same way, air currents appear to be moving off to one side of their original path under the Coriolis effect, but it is actually the observer that moves. In this case, the rotating Earth is like the moving train, carrying the observer along. Air is not affected as strongly, so it appears to change direction. The directional change is always toward the west.
The Coriolis effect is most evident in air moving directly north or south. It not only affects the air but also any projectile traveling through the air. For instance, a ballistic shell fired directly southward from the North Pole will land somewhat west of a target placed due south.