If a cannonball were fired from Arizona to Montana it would curve toward the right because of the Coriolis effect. If it started out going due north it would end up northeast of where it started (blue arrow). If you were trying to hit a target due north of you then you would have to point your cannon toward the northwest (yellow line). Click on image for full size Map courtesy of the Perry-Castaneda Library at the University of Texas

But the Earth is rotating. This causes our cannonball to appear to curve to the right. So we'll miss our target - our cannonball will land east of the flag pole. What should we have done? We should have pointed our cannon to the northwest. Then our cannonball would curve a bit to the right and if we were careful - bang - a hole in one!

Why did we use a cannonball and not a football? Because something has to move a very long way before the Coriolis effect is noticable - at least several hundred miles. When you throw a football the path only curves a tiny bit due to the Coriolis effect - far too small for you to ever notice.

The Coriolis effect makes moving objects appear to curve toward the right in the northern hemisphere and toward the left in the southern hemisphere. In our example the cannon was pointing north but a similar thing would happen no matter which way we pointed our cannon. If our cannon was pointed eastward our cannonball would appear to veer toward the south. If our cannon was pointed southward our cannonball would appear to veer toward the west, and so on.

The Coriolis effect not only works with cannonballs, it also works on winds and ocean currents. It's what makes hurricanes spin around. It's also important on other planets, in stars, and in nebulae.

Movie of a Global Atmospheric Circulation Demonstration