Definition
Throw a ball to a friend on a spinning merry-go-round, and the ball seems to curve sideways. Earth is also a giant turntable that spins once a day. So winds and ocean currents that travel a long way across it seem to bend off course. This apparent curve, which shows up when you watch from a spinning floor, is called the Coriolis effect.
Why does a ball thrown on a merry-go-round curve?
Picture yourself sitting across from a friend on a spinning merry-go-round. You throw a ball straight at your friend. But the ball doesn't land in your friend's hands. It sails off toward the seat next to them.
The ball didn't do anything strange. It flew in a straight line, in the direction you threw it. But while it was in the air, the merry-go-round turned a little, and your friend's seat moved sideways.
To you, riding along, the scene looks different. The floor and you are spinning together, so it's hard to feel that the ride has turned at all. That's why it seems the ball curved sideways on its own.
Earth is a very big merry-go-round that turns once a day. Winds and ocean currents that travel a long way across it seem to curve for the same reason. The longer the distance and the more time it takes, the clearer the curve becomes.
Why the Northern and Southern Hemispheres spin in opposite ways
In the Northern Hemisphere, moving things bend to the right of their direction of travel. In the Southern Hemisphere, they bend to the left. That's because Earth's spin looks reversed depending on whether you view it from above the North Pole or from below the South Pole.
A typhoon or hurricane grows as winds rush in from all sides toward a low-pressure center, where the air has thinned out. In the Northern Hemisphere, winds racing toward the center get pushed a little to the right.
As a result, the winds can't head straight into the center. They circle around it instead. This is how Northern Hemisphere storms end up swirling counterclockwise.
In the Southern Hemisphere, winds get pushed to the left, so storms swirl clockwise. Ocean currents are a bit different. In the Northern Hemisphere, surface water is pushed to the right and piles up in the middle of the ocean. The mounded water then spreads back out, forming a giant clockwise whirl. A storm is a flow being sucked toward a center, so it turns the other way.
To be more precise: nothing is actually pushing
To be more precise, the Coriolis effect is not a force that actually pushes anything. It's an apparent force. It shows up only when you watch from a spinning floor, turning along with it.
An observer looking down at Earth from space wouldn't see this bending. The wind simply went the way it was already headed, while Earth turned underneath it.
The effect becomes noticeable only over long distances and plenty of time. In flows like storms and ocean currents, which travel hundreds of kilometers over several days, it grows too big to ignore.
On the other hand, in the short moment when water drains from a sink, the bend is far too small to feel. The shape of the basin and the direction the water was poured matter much more. It's also good to know that near the equator, this bending almost disappears.
Common misconceptions
In the Northern Hemisphere, water in a sink or bathtub always drains counterclockwise.
In a container that small, the Coriolis effect is too weak to notice. The direction the water turns is shaped far more by the shape of the basin, the way the water was poured, and any motion left in the water near the drain.
The Coriolis effect is a real force that Earth uses to push objects sideways.
It's an apparent force that only appears to someone turning along with the spinning Earth. Seen from outside Earth, an object keeps moving straight in its original direction.
Where you meet it
Surface currents in the Northern Hemisphere's oceans get pushed to the right, forming huge clockwise whirls that circle each ocean.
Jet streams blowing from west to east over the middle latitudes get their direction from the Coriolis effect, and long-haul airliners plan their routes with these winds in mind.
In one sentence
It's the effect where flows that travel a long way across spinning Earth seem to bend right in the Northern Hemisphere and left in the Southern Hemisphere.