Demystifying the Coriolis force

Storm systems in the northern and southern hemispheres

Whirling winds

You have most likely seen this on weather maps. If you live in the northern hemisphere, wind circulates clockwise around high pressure centers and counterclockwise around low pressure centers.  If you live in the southern hemisphere, it's just the opposite. Why is it the opposite? For that matter, why should the air circulate in the first place? Wouldn't we expect an air flow pattern that tries to equalize the pressure by flowing directly from higher pressure regions to lower pressure regions? Something like this:

Also, global atmospheric circulation maps show winds irresistibly curving to the right in the northern hemisphere and curving to the left in the southern hemisphere. Here is a figure from noaa.gov showing the global atmospheric circulation.

Global atmospheric circulation. Click on the image for a full description at noaa.gov.

Meteorology and atmospheric science is a huge subject. People in this field spend decades learning it. So, I won't delve now into what creates the highs and lows, but just why the wind directions bend in this bizarre way.

Demystifying the Coriolis force

Google "Coriolis force demonstration" and you'll find many videos such as this one of people on spinning carousels attempting to throw a ball straight to the opposite side. If the carousel is going counterclockwise as viewed from above, the passengers will observe the ball curving to the right. Here is an animation of a passenger at 6 o'clock trying to throw a ball to the passenger at 12 o'clock. For the rotation speed shown here, the ball curves to the right and is caught by the person at 3 o'clock.

Trajectory of a ball thrown on a merry-go-round spinning counterclockwise as viewed by a passenger

Let's look at the same event from the viewpoint of someone who climbs a tree and looks down on the carousel. The climber actually sees the ball going straight:

Trajectory of a ball thrown on a merry-go-round spinning counterclockwise as viewed by an external observer

The ball's path slants to the right because its velocity is a combination of the motion towards 12 o'clock imparted by the thrower and the rightward motion from the carousel itself. Once the ball leaves the thrower's hands, it obeys Newton's First Law and travels on a straight path. But from the passenger's viewpoint, the ball acts as if there's a force constantly deflecting it to the right. This is called the Coriolis force. Strictly speaking, we shouldn't call it a force because there's nothing really pushing the ball to the right. It's actually the passengers that are on a curved trajectory.

If the carousel spins clockwise, the passengers see the ball curving to the left:

Trajectory of a ball thrown on a merry-go-round spinning clockwise as viewed by a passenger

Carousel Earth

Now the bizarre behavior of the winds is easier to explain. As shown below, the Earth turns counterclockwise as viewed from far above the North Pole and clockwise as viewed from far above the South Pole. This creates a Coriolis force that deflects winds rightward in the northern hemisphere and leftward in the southern hemisphere.

Rotation of the Earth as viewed from over the North and South Poles

Since this rotation is slow, only 1 revolution per day, the Coriolis force is a gentle one, producing large-radius curves. So it's a dominant player only for large-scale weather systems. The water draining from your bathtub can whirl either clockwise or counterclockwise.

If the Earth were not spinning, weather and global air flow patterns would be drastically different from what we observe!