We come from the future
We come from the future

# The Coriolis Effect Part II: As the Hurricane Turns

The Coriolis Effect pushes objects clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. And yet hurricanes spin in exactly the opposite direction. Why?

Yesterday we saw how the movement of the earth affects certain objects. When seen from above, the objects are seen to move in a straight line. From the general human point of view – on that's stuck on a rotating planet – the objects appear to drift to the right or left as the world literally turns out from under them. In the northern hemisphere objects appear, from the thrower's point of view, to curve to the right as they are thrown. In the southern hemisphere they appear to curve to the left.

This suggests that the familiar spiral patterns of hurricanes, cyclones and tropical storms would circle clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. In fact, the opposite is true. Storms in the northern hemisphere always circle counterclockwise, or to the left. Storms in the south turn clockwise, to the right. This consistent motion is due to the Coriolis Effect, and yet it if it is, why don't the storms circle to the right?

The mechanics of this are best understood when walked through physically. You'll need some object to mark a spot on the floor, and if possible, a friend. Put a book or a plate or some object down on the floor a few feet away from you. This is a low pressure system. A low pressure system is created when air is heated enough to lift upwards, creating a relative vacuum below. Higher pressure air from around the system rushes in to fill the vacuum. That's you.

(To get into character, picture yourself wooshing through these palm trees.)

We'll pretend that you're in the northern hemisphere. This means the earth turns to the left under you, and your course appears to be diverted to the right. The low pressure system is pulling you in. Take a slow step towards the low pressure system, and have your friend pull on your right sleeve. If you're alone, imagine that someone is turning you sharply to the right. If you've done it correctly, you'll be a little closer to the low pressure system, but you'll have turned to the right, so the system will be on your left.

That low pressure system is still pulling you, so take turn and take another step towards it. Again your friend, imaginary or real, is tugging your right sleeve, turning you to the right. Repeat this step a few times and you'll notice that you're circling the low pressure system in a counterclockwise fashion. The way that the Coriolis Effect in the northern hemisphere makes objects, including winds, veer to the right of their targets causes them to circle those targets to the left – or counterclockwise. In the southern hemisphere, the way the Coriolis Effect makes objects veer to the left causes them to circle their targets to the right – or clockwise.

Be sure to pick up your low pressure system when you're done, so others don't trip on it. Imaginary friends don't have imaginary PPOs.