Forces in the Atmosphere, Page 7.5 of 11

You've given the ball a nice steady roll, at a speed of about ten miles per hour, but even so, it takes about ten minutes for the ball to reach the other end of the runway. Meanwhile, the sun, which started out lined up with the runway, has moved during the ten minutes, so that it's now to the right of the end of the runway. And finally, as though by magic, the ball, which started off rolling toward the sun, has continued to roll directly toward the sun, so that by now it has rolled off the right-hand edge of the runway!

The sun did not exert any force on the ball to cause it to do that. In reality, the ball has rolled in a straight line, directly in line with the sun, and no forces (besides gravity) have acted on it. But if you trace the ball's path on the Earth, it's curved, and the "force" that made the ball change directions is called the coriolis force.

So the coriolis force is the tendency of objects to go in a straight line (not necessarily toward the sun), even while the Earth is turning beneath them. To us on Earth, in the Northern Hemisphere, it appears that the coriolis force is causing objects to try to turn to the right instead of following a straight line on the rotating Earth.

By the way, in the Southern Hemisphere, things are reversed. The sun moves from right to left as it passes across the southern sky, and similarly the coriolis force in the southern hemisphere causes moving objects to seem to curve toward the left.

What about at the equator? There, the coriolis force is zero, and the only force caused by the rotation of the Earth is the centrifugal force.

Questions or Comments

Technical: E-mail John Fulton < jdfult@nimbus.met.tamu.edu >
Scientific: E-mail Dr. John Nielsen-Gammon. < nielsen@ariel.met.tamu.edu >