C O R I O L I S F
O R C E S
on a spinning globe/Coriolis Forces
If you could travel up to the star Polaris, which sits almost directly over Earth's North Pole, and look straight down, you'd see Earth rotating beneath you. "Rotation" describes spinning on an axis formed by a line joining Earth's geographic North and South Poles. (The Earth "revolves" around the Sun, creating the year. Knowing your "rotation" from your "revolution" is a very important step towards understanding what makes the seasons.)
If you could take this marvelous trip, you'd see that Earth was rotating counterclockwise beneath you. Contrariwise, as Alice in Wonderland would have it, if you could look down (or up if you insist!) on Earth's South Pole from a star high above it (and unfortunately for travelers seeking to confirm directions in the Southern Hemisphere, there's no austral counterpart to Polaris), you'd find our planet rotating clockwise.
Well, that's what Earth's surface, its mountains and lakes, and all its creatures, including we humans are doing, but the atmosphere and the winds and clouds which ride along in it, are not solid and not attached to the surface. They lag behind and move differently. What may appear to us to be a straight-line journey from high pressure to low pressure gets transformed by what's known as the "Coriolis Effect."
Did you ever ride on a round-about (merry-go-round) and try and throw a ball to your partner directly opposite you? You might throw it straight, but on the roundabout it might look as if it was curving away and be hard to catch! That's the Coriolis force in action. To get the ball caught by your partner you'd have to offset where you'd throw it, and aim ahead of where your partner is NOW to where he or she will be in a few moments.
What happens to the ball on the roundabout also happens to winds over the Earth. While they may start of moving in roughly straight lines from areas of high pressure to low pressure, Earth's rotation causes them to bend into the patterns that are characteristic of the jet streams and trade winds we can see on the maps.
And as you'd expect from what we said about the different apparent rotational direction of Earth when looking down (or up) at the North and South Poles, Coriolis forces work differently in the Northern and Southern Hemisphere. In the Northern Hemisphere, winds deflect to the right, or clockwise, and in the South, to the left, or counterclockwise.
By the way, it's NOT TRUE that water down a drain rotates differently north and South of the Equator! Local effects (the shape of the drain, how much water is in the tub, etc.) have much more effect than the Coriolis effect.